Spatial resolution and noise in organic light-emitting diode displays for medical imaging applications Asumi Yamazaki,1,2 Chih-Lei Wu,1 Wei-Chung Cheng,1 and Aldo Badano1,∗ 1 Center
for Devices and Radiological Health, Food and Drug Administration, Silver Spring, Maryland, 20993, USA 2 Graduate School of Medical Sciences, Nagoya University, Nagoya, Aichi, 4618673, Japan ∗ [email protected]
Abstract: We report on the resolution and noise characteristics of handheld and workstation organic light-emitting diode (OLED) displays in comparison with liquid crystal displays (LCDs). The results demonstrate advantages, in terms of sharpness, of handheld OLED displays with modulation transfer function (MTF) values exceeding 0.60 at the Nyquist frequencies. The OLED workstation included in this study exhibits significant signal contamination among adjacent pixels resulting in degraded resolution performance indicated by horizontal and vertical MTF values of 0.13 and 0.24 at the Nyquist frequency. On the other hand, its noise characteristics are superior to the LCD workstation tested. While the noise power spectral (NPS) values of the OLED workstation are 8.0×10−6 mm2 at 1 mm−1, the LCD workstation has NPS values of 2.6×10−5 mm2 . Although phone-size OLED displays have superior resolution and noise per pixel, the perceived resolution characteristics at appropriate viewing distances are inferior to tablet-size and workstation LCDs. In addition, our results show some degree of dependency of the resolution and noise on luminance level and viewing orientation. We also found a slightly degraded resolution and increased low-frequency noise at off-normal orientations in the handheld displays. © 2013 Optical Society of America OCIS codes: (120.2040) Displays; (110.4280) Noise in imaging systems; (230.3670) Lightemitting diodes.
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#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28111
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1.
Introduction
The use of organic light-emitting diode (OLED) displays has increasingly become widespread for applications in television, workstation, and handheld displays. Since OLED displays have emissive pixels unlike liquid crystal displays (LCDs) that need a backlight source, they can provide more effective power saving, wider viewing angle, and thinner design [1–3]. Conse#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28112
quently, OLED displays are replacing LCDs in handheld devices. OLED displays can also provide high image contrast due to lower black-level luminance, more accurate color reproduction, and faster response [4–6]. These benefits are all relevant to imaging devices. Currently, OLED displays are considered for medical applications. OLED workstation has been introduced to image-guided surgery [7], and the use is expected in endoscopy, pathology, and other clinical purposes. Besides, handheld devices positively adopting OLED displays have played important roles in remote image diagnosis for emergency cases or a situation without access to a workstation [8–12]. However, image quality is dependent on the clinical task. Generally, high resolution, low noise, and consistent contrast are desired in radiology. Since recent X-ray image detectors have accomplished small pixel pitches and improved resolution, displays with sufficiently large pixel arrays are needed to exhibit all image information. Noise in X-ray images is mainly due to quantum mottle. Display noise has to be minimal in order to maintain lower radiation doses. While consistent display contrast is needed for accurate image interpretation across devices, fairly high contrast, especially with low minimum luminance, is beneficial in specific cases such as mammography. Other medical specialties including endoscopy, pathology, and dermatology might require accurate color reproducibility with high contrast and fast temporal response. In this study, we focus on spatial resolution and noise characteristics of OLED displays. While image characteristics in LCDs have been extensively assessed in terms of resolution, noise, luminance, and reflectance [13–18], no report has been published regarding spatial image characteristics in OLED displays although luminance and contrast characteristics have been documented [2, 4–6]. This study demonstrates the characterization of spatial resolution and noise for OLED displays with a range of display sizes and pixel features in comparison with LCDs. This comparison with dedicated medical LCDs would contribute to the understanding of the applicability of OLED displays in radiology. Additionally, the image characteristics are estimated for various luminance levels and viewing angles. Previous studies have reported on the angular response of luminance and contrast in LCDs [19–23] and improved angular performance in OLED displays [2, 4, 6]. However, the angular dependency in terms of resolution and noise and its luminance dependency has not been reported in the literature. Angular dependency is an important factor to ensure displays perform consistently under different viewing conditions. Since the viewing orientation for handheld displays could vary, the resolution and noise variations with angle need to be characterized. In addition, we evaluate sharpness considering the appropriate viewing distance for each display format and compare perceived resolution characteristics. 2.
Materials and methods
2.1. Photometric camera We used a photometric charge-coupled device (CCD) camera (P199F, Westboro Photonics Inc., Ottawa, Canada) to capture the display screen for the resolution and noise measurements. The camera equips a macro lens (NIKON AF Micro-Nikkor 60 mm f/2.8D, Nikon Inc., Tokyo, Japan) and CCD sensors consisted of 1624×1224 elements with 0.0044-mm pixel pitch. The camera is calibrated at the pixel level to luminance in a range from 0.02 to 50,000 cd/m2 with a 12-bit analog-to-digital conversion. Since the measurements were performed at various camera orientations in the following sections, we first measured a modulation transfer function (MTF) of the camera system to examine the angular dependency of the resolution characteristics. We used an optic resolution pattern chart (1951 USAF resolution target, Edmund Optics Inc., Barrington, USA). We set the resolution chart, which was placed on an illuminated film viewer (KLV-5700, HAKUBA Inc., Tokyo, Japan), at a distance of 300 mm away from the camera lens. We captured the resolution chart with the camera at horizontally varying orientations from #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28113
-10 to 10 degrees with respect to the perpendicular direction as shown in Fig. 1(a). The camera pixel pitch corresponded to about 0.0010 mm on the resolution chart plane. We acquired luminance profiles on the edge of 2×2-mm square cut from the captured images as indicated by a square with red broken line in Fig. 1(b). We localized the edge line at the horizontal center of the camera field of view (FOV) and set the focus on the edge line. The edge profiles were differentiated and fast Fourier transformed to calculate the MTFs at the respective angles.
Fig. 1. Modulation transfer function (MTF) measurement of a photometric camera at various oriented angles. (a) Experiment layout of the camera and a resolution chart. The camera was oriented at varying angles from -10 to 10 degrees with respect to the perpendicular direction to the resolution chart. (b) The captured pattern image with the camera. The edge line of 2×2-mm square cut indicated by a square with red broken line was used for the MTF calculation.
2.2. Display specifications We used a 3.7-inch phone-size handheld OLED display (OLEDp1: Nexus One, HTC Corp., Taoyuan, Taiwan), a 4.0-inch phone-size handheld OLED display (OLEDp2: Galaxy S, Samsung Corp., Seoul, South Korea), a 25-inch 2-mega-pixels (MP) OLED workstation (OLEDw: PVM-2551MD, Sony Corp., Tokyo, Japan), a 3.5-inch phone-size handheld LCD (LCDp: iPhone4, Apple Inc., CA, USA), a 9.7-inch tablet-size handheld LCD (LCDt: iPad 3rd generation, Apple Inc., CA, USA), and a 21-inch 3-MP LCD workstation (LCDw: R31, Eizo Nanao Corp., Ishikawa, Japan). The display specifications are listed in Table 1. LCDw is mainly used by radiologists and other physicians for primary image diagnosis. LCDt and LCDw adopt in-plane switching (IPS) technology. OLEDp1 and OLEDp2 use active-matrix organic lightemitting diode (AMOLED) displays with PenTile sub-pixel technology, which allocates green (G) sub-pixels interleaved with alternating red (R) and blue (B) sub-pixels. The R-G-B-G layout is iterated and one pixel consists of two sub-pixels of R-G or B-G. The G sub-pixels are mapped by one-to-one correspondence with input signal pixels. The R and B sub-pixels are subsampled reconstructing the chroma signal. The luminance is processed using adaptive sub-pixel rendering filters from the input image. The brightness settings of the handheld displays were fixed at the maximum for each device throughout the experiments. There are no options to change the contrast in the handheld devices. OLEDw has various on-screen display (OSD) settings for contrast, brightness, and color space. Except in noted cases, the parameters were set to contrast: 50, brightness: 50, and color space: SMPTE-C. In addition, the brightness and contrast parameters were changed at #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28114
Table 1. Display specifications with minimum and maximum luminance values and luminance ratio. The luminance values were measured at the maximum brightness setting for the handheld displays and LCDw, and the contrast and brightness parameters were set at 50 with a color space: SMTPE-C for OLEDw. Display OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
Screen size (inch) 3.7 4.0 25 3.5 9.7 21
Pixel array 480×800 480×800 1080×1920 640×960 2048×1536 1536×2048
Pixel pitch (mm) 0.101 0.109 0.283 0.0779 0.0962 0.207
Lmin (cd/m2 ) 0.138 0.174 0.135 0.770 0.750 1.05
Lmax (cd/m2 ) 259 341 277 479 467 271
LR 1877 1960 2052 622 623 258
the fixed color space as SMPTE-C. We changed the color space parameter from EBU, ITU709, SMPTE-C, and OFF for fixed contrast and brightness levels. The brightness of LCDw was fixed at maximum and the contrast was set to the digital imaging and communications in medicine (DICOM) grayscale standard display function (GSDF). Table 1 lists the minimum luminance Lmin and maximum luminance Lmax and the luminance ratio LR= Lmax /Lmin at the specific brightness and contrast for each device. 2.3. MTF We measured MTFs to characterize spatial resolution for the displays listed in Table 1 using a methodology described in [13]. The handheld displays were set at portrait orientation while the workstation displays were set at landscape orientation. We displayed a horizontal or vertical one-pixel-line pattern embedded in a uniform background, respectively for the vertical or horizontal MTF. The horizontal direction corresponds to the red-green-blue (RGB) sub-pixel direction except for LCDt in which the vertical direction corresponds to the RGB direction. The digital driving levels (DDLs) of the line and background were selected from the conditions: line/background= 15/10, 50/30, 60/50, 70/40, and 100/80%. As the exceptions, 50/30% was excluded for OLEDw and the fixed 60/50% was used for LCDt as listed in Table 2. The OSD settings for OLEDw were set at the referential settings as mentioned in 2.2. The displayed screen was captured with the photometric camera at a magnification corresponding to about 9×9 CCD pixels per display pixel as shown in Fig. 2(a). Figure 2(b) shows the captured screen images displaying the horizontal and vertical line-patterns on each display. The distance from the camera to the screen was approximately 300 mm for OLEDp1 and OLEDp2, and the distance was increased in proportion to the pixel pitch for other displays. The camera was first directed perpendicularly to the screen surface, and the screen image was captured at 0-degree viewing angle. At the 0-degree viewing angle and DDL 60/50%, various OSD settings were tested for OLEDw. The brightness and contrast parameters were changed from 50 to 25, 75, and 100 respectively at the color space of SMPTE-C. We also selected respective color space parameters: EBU, ITU-709, SMPTE-C, and OFF at the fixed contrast 50 and brightness 50. Table 2 lists the measured luminance values (cd/m2) at 0-degree angle with the photometric camera, corresponding to the line and background DDLs. The listed luminance values of OLEDw were measured at the referential OSD settings. The luminance values were determined from the averaged pixel values on the one-pixel line and background area in the captured screen images. Next, the camera orientation was horizontally varied in the angle range from -8 to 8 degrees by 2-degree steps with respect to the perpendicular direction for OLEDp1, OLEDp2, LCDp, and LCDt, in a similar way to Fig. 1(a). The displays were rotated by 90 degrees to measure the vertical MTFs. The captured line pattern images were subtracted from the background images, which were #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28115
Table 2. Digital driving levels (DDL) and luminance values of the line (LN) and background (BG) in the displayed pattern at perpendicular viewing angle.
LN/BG
DDL (%) 15/10 50/30 60/50 70/40 100/80
OLEDp1 2.38/1.69 49.1/15.7 73.5/52.9 103/29.0 223/144
OLEDp2 6.74/4.15 75.7/23.4 122/66.4 158/40.9 284/188
Luminance (cd/m2 ) OLEDw LCDp 2.03/0.825 5.69/2.21 89.5/40.2 31.0/22.4 128/92.9 33.0/13.9 176/58.8 91.7/70.7 449/286
LCDt 159/112 -
LCDw 5.97/4.82 32.9/14.1 53.8/37.3 77.5/23.0 265/135
acquired by capturing uniform patterns with the same setup. After the subtraction, 1200 line profiles in each captured image were averaged vertically (in the line direction). Line spread functions (LSFs) were obtained by normalizing the averaged profiles by the maximum luminance values. Horizontal and vertical MTFs were calculated by fast Fourier transformation of the LSFs. We represent the MTFs as a function of absolute and relative spatial frequency, with relative spatial frequency being equal to the absolute spatial frequency divided by the Nyquist frequency corresponding to the display system. The MTFs as a function of the relative spatial frequency express how much blur is present regardless of pixel pitch. If there is no resolution degradation on the display, the LSF becomes a square wave and the MTF is given by the sinc function, sin(uπ p) sinc(u/ fN ) = , (1) uπ p where u is spatial frequency (mm−1), p is display-pixel pitch (mm), and fN is Nyquist frequency (mm−1 ) = 1/2p. In addition, we correct the MTFs in terms of appropriate viewing distances and present the measurements in terms of angular spatial frequency expressed in cycles per degree. According to previous reports [24, 25], typical viewing distances V D (mm) are 600, 300, and 200 mm for workstations, tablet-size handhelds, and phone-sized handhelds respectively. The angular spatial frequency (cy/deg) is calculated by the following expression [26]: VD (cy/mm). (2) 57.3 The image capture procedures were repeated three times with the same camera positioning for each camera orientation and each DDL pattern, and the corresponding three MTFs were averaged. The standard deviations of the three MTFs were calculated for the 60/50% pattern in the perpendicular direction. Student’s t-test was applied to compare the MTFs at 0.4 cy/mm in relative spatial frequency with various DDLs and camera orientations. P-value less than 0.05 was considered statistically significant. (cy/deg) =
2.4. NPS We measured noise power spectra (NPS) to characterize spatial noise for the displays according to [13, 14, 17]. Uniform patterns with DDLs corresponding to the background levels listed in Table 2 were displayed and each screen was captured with the photometric camera. The OSD settings for OLEDw were set at the referential settings. However, only when the camera orientation was perpendicular to the display screen, various OSD settings were tested for OLEDw displaying 50% DDL pattern as well as the MTF measurements. Next, the camera orientation was changed in the angle range from -8 to 8 degrees relative to the perpendicular viewing direction for OLEDp1, OLEDp2, LCDp, and LCDt. For calculating one-dimensional (1D) horizontal #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28116
Fig. 2. (a) The experimental setting of the photometric CCD camera capturing the display screen for measuring the display MTFs. The OLEDp1, OLEDp2, LCDp, and LCDt were set by the portrait orientation while OLEDw and LCDw were set by the landscape orientation. (b) The captured screen images displaying the horizontal and vertical line-patterns. The horizontal direction corresponds to the RGB sub-pixel direction except for LCDt in which the vertical direction corresponds to the RGB direction. Orange dot-line squares showing respective one pixel regions and 0.1-mm scale bars are indicated.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28117
NPS, a region of interest (ROI) of width 512 × height 40 pixels, which positioned at the horizontal center and upper end in the captured image, was selected and the 512-point horizontal profile was acquired with numerical slit (1×40) averaging 40 pixel values to eliminate vertical noise [27, 28]. After being subtracted from the mean value of 512 data in the averaged profile, the profile was processed with a Hanning window, and fast-Fourier transformed. The window processing works to reduce spectral leakage errors [17] in Fourier space, particularly since displays have periodic pixel structures inducing spectral peaks at frequencies in accordance with the integral multiples of the inverse of the sub-pixel pitch. The 1D NPS(u) (mm2 ) calculation is expressed as follows, n m o 2 s 2 M−1 L(kx) exp −2π j k NPS(u) = x ∑ , M m=0 L M
(3)
m n o 2 n xy M−1 N−1 L(kx, ly) exp −2π j k , l NPS(u, v) = ∑ ∑ , MN m=0 M N L n=0
(4)
with k= 0, 1, 2, · · ·, M-1, where M is noise profile data points 512, s is numerical-slit length points 40, x is effective camera-pixel pitch (mm) on captured plane, u= k/(Mx) is spatial frequency (mm−1), L is average luminance (cd/m2 ) of 512 data, L(kx) is luminance difference (cd/m2 ) at (kx) from L. The 512×40 ROI was moved vertically without overlaps to repeat the NPS calculation as many times as possible and the NPS were averaged. Vertical NPS was calculated in the same way using the horizontal numerical-slit scanning. Furthermore, twodimensional (2D) NPS(u, v) (mm2 ) was calculated by a 2D fast Fourier transformation with a Hanning window processing as follows,
with k=0, 1, 2, · · ·, M-1, and l=0, 1, 2, · · ·, N-1, where x and y are effective camera-pixel pitches (mm) on captured plane, u= k/(Mx) and v= l/(Ny) are spatial frequencies (mm−1 ), L is average luminance (cd/m2) in ROI with M × N: 256×256 matrices, L(kx, ly) is luminance difference (cd/m2 ) at (kx, ly) (cd/m2) from L. The image capture procedures were carried out three times at the same camera location for each camera orientation and DDL pattern, and the three NPS were averaged. The standard deviations of the three NPS were calculated for the 50% DDL pattern in the perpendicular direction. The NPS variations with DDLs at 2.0 cy/mm and the variations with camera orientations at 0.2 and 2.0 cy/mm were analyzed using Student’s t-test. P-value less than 0.05 was considered statistically significant.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28118
3.
Results
3.1. Camera resolution Figure 3 shows the MTFs of the photometric camera at various orientations ranging from -10 to 10 degrees relative to the perpendicular direction. We recognize that the highest MTF at 0 degree is slightly decreased as the angle is tilted up to -10 or 10 degrees. However, since the MTF variation is too small to substantially affect display resolution measurements at least within the angle range from -8 to 8 degrees.
Modulation transer function
1
0.95
0.9
0.85
0.8
-10°° -8 -6°° -2° 0° 2 6°° 8 10° 0.1
1
10 -
Spatial frequency (mm 1) Fig. 3. Modulation transfer functions of the photometric camera at various viewing angles in the range from -10 to 10 degrees.
3.2. Device characterization All OLED displays have lower minimum luminance and noticeably higher luminance ratios than LCDs as shown in Table 1. The LSFs in Fig. 4 and MTFs as a function of absolute frequency, relative frequency, and angular frequency given by Eq. (2) in Fig. 5 were measured using the pattern with 60% DDL line and 50% DDL background for all displays. Since the LSFs and MTFs of OLEDw were consistent regardless of the OSD settings, Figs. 4 and 5 show the results at contrast 50, brightness 50 and SMPTE-C color space settings. When the vertical line is displayed in OLEDp1 with PenTile technology, the illuminated red or blue sub-pixels are located at only one side of the green sub-pixels as shown in Fig. 2(b). On the other hand, the red or blue sub-pixels on both sides of the green are illuminated in OLEDp2. The illuminated red or blue sub-pixel locations are determined by the respective sub-pixel rendering algorithms to calculate the luminance, and the algorithm of OLEDp1 causes the asymmetric horizontal LSF. However, the LSF difference between OLEDp1 and OLEDp2 does not result in noticeably different MTFs as a function of absolute frequency in Figs. 5(a) and 5(d). The MTFs in Figs.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28119
(a) Horizontal 1
OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
0.6 0.4 0.2 0 -0.2 -0.75
OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
0.8 Relative luminance
0.8 Relative luminance
(b) Vertical 1
0.6 0.4 0.2 0
0 Distance (mm)
0.75
-0.2 -0.75
0
0.75
Distance (mm)
Fig. 4. Line spread functions for the devices tested in this study.
5(a) and 5(d) show error bars indicating two standard deviations for the three-times measured MTFs. The estimated uncertainty suggests that the MTFs have sufficient accuracy for comparing sharpness among displays. The LSFs of OLEDw present the most widespread profiles among all displays in this study in both horizontal and vertical directions. Although the displays with smaller pixel pitches have the LSFs with smaller widths and higher MTFs in Figs. 5(a) and 5(d), the LSFs of OLEDw have evidently wider widths than the pixel aperture size. The LSFs exhibit non-monotonic distributions that drop once at the border between the center and the next pixels, and then increase at the next pixels before leading undershoots. The MTFs as a function of relative spatial frequency in Figs. 5(b) and 5(e) allow the spatial resolution comparisons regardless of the pixel pitch. In horizontal direction, LCDt and LCDp have MTFs close to the sinc function given by Eq. (1) expressing the ideal MTF, followed by OLEDp1, OLEDp2 and LCDw. In vertical direction, OLEDp2 and LCDt have almost equivalent MTFs to the sinc function, followed by OLEDp1 and LCDp. The MTF of LCDw is slightly decreased from the handheld displays. The MTFs of OLEDw represent the most degraded resolution characteristics in both horizontal and vertical directions. The MTFs as a function of angular frequency in Figs. 5(c) and 5(f) compare resolution characteristics at appropriate estimated viewing distances for the devices. The MTFs of OLEDw and LCDw are relatively increased because of the long viewing distance compared to the MTFs as a function of absolute frequency in Figs. 5(a) and 5(d). Even though the MTFs corrected by shorter viewing distances for the handheld displays are relatively decreased, LCDt has the best resolution characteristics followed by LCDw.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28120
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Fig. 5. Modulation transfer functions as a function of absolute frequency, relative frequency, and angular frequency expressed in cycles per degree for the devices tested in this study. (a) and (d) show error bars representing two standard deviations for measured MTFs.
1D and 2D NPS calculated using Eqs. (3) and (4) for all displays in Figs. 6 and 7 exhibit spectral peaks at the integral-multiple frequencies of (1/sub-pixel pitch) based on the prominent periodic pixel and sub-pixel structures shown in Fig. 2(b). The error bars in Fig. 6 representing two standard deviations of the three-times measured NPS suggest sufficient measurement accuracy to compare the noise characteristics among the displays. The OSD settings of OLEDw did not affect the NPS results, except for the contrast setting 25 increasing the NPS slightly. Accordingly, we show the NPS at contrast 50, brightness 50 and SMPTE-C color space settings in Figs. 6 and 7. Figure 6 represents that LCDp and LCDt have the lowest NPS values followed by OLEDp1 and OLEDp2 in horizontal and vertical directions, while OLEDw has higher NPS than the NPS of the handheld displays, and LCDw has the highest NPS in the devices tested. The 2D NPS of OLEDp1, OLEDp2, LCDp, and LCDt in Fig. 7 depict the lower NPS by the darker gray colors. In contrast, the higher NPS of LCDw and OLEDw are emphasized by the brighter gray colors in the 2D NPS.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28121
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Fig. 6. One-dimensional noise power spectra for the devices tested in this study. The error bars represent two standard deviations for measured NPS.
Fig. 7. Two-dimensional noise power spectra for the devices tested in this study.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28122
3.3. DDL dependency Figure 8 shows the horizontal LSFs by various DDLs for OLEDp1, OLEDp2, OLEDw and LCDw. The LSFs of OLEDp1 and LCDw indicate no variation with DDLs although the LSFs appear to be noisy at the DDL with LN/BG= 15/10%. In contrast, the LSF of OLEDp2 has undershoots only at the DDL with LN/BG= 70/40%. While the LSFs of OLEDw are independent of DDLs at the center pixel, the off-center peaks and undershoots exhibit subtle relative luminance differences. Subsequently, while the MTFs of OLEDp1 and LCDw have no significant differences (p>0.05) at relative frequency 0.4 cy/mm by DDLs in Fig. 9, OLEDp2 has a higher MTF at DDL 70/40% (p
for Devices and Radiological Health, Food and Drug Administration, Silver Spring, Maryland, 20993, USA 2 Graduate School of Medical Sciences, Nagoya University, Nagoya, Aichi, 4618673, Japan ∗ [email protected]
Abstract: We report on the resolution and noise characteristics of handheld and workstation organic light-emitting diode (OLED) displays in comparison with liquid crystal displays (LCDs). The results demonstrate advantages, in terms of sharpness, of handheld OLED displays with modulation transfer function (MTF) values exceeding 0.60 at the Nyquist frequencies. The OLED workstation included in this study exhibits significant signal contamination among adjacent pixels resulting in degraded resolution performance indicated by horizontal and vertical MTF values of 0.13 and 0.24 at the Nyquist frequency. On the other hand, its noise characteristics are superior to the LCD workstation tested. While the noise power spectral (NPS) values of the OLED workstation are 8.0×10−6 mm2 at 1 mm−1, the LCD workstation has NPS values of 2.6×10−5 mm2 . Although phone-size OLED displays have superior resolution and noise per pixel, the perceived resolution characteristics at appropriate viewing distances are inferior to tablet-size and workstation LCDs. In addition, our results show some degree of dependency of the resolution and noise on luminance level and viewing orientation. We also found a slightly degraded resolution and increased low-frequency noise at off-normal orientations in the handheld displays. © 2013 Optical Society of America OCIS codes: (120.2040) Displays; (110.4280) Noise in imaging systems; (230.3670) Lightemitting diodes.
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#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28111
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1.
Introduction
The use of organic light-emitting diode (OLED) displays has increasingly become widespread for applications in television, workstation, and handheld displays. Since OLED displays have emissive pixels unlike liquid crystal displays (LCDs) that need a backlight source, they can provide more effective power saving, wider viewing angle, and thinner design [1–3]. Conse#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28112
quently, OLED displays are replacing LCDs in handheld devices. OLED displays can also provide high image contrast due to lower black-level luminance, more accurate color reproduction, and faster response [4–6]. These benefits are all relevant to imaging devices. Currently, OLED displays are considered for medical applications. OLED workstation has been introduced to image-guided surgery [7], and the use is expected in endoscopy, pathology, and other clinical purposes. Besides, handheld devices positively adopting OLED displays have played important roles in remote image diagnosis for emergency cases or a situation without access to a workstation [8–12]. However, image quality is dependent on the clinical task. Generally, high resolution, low noise, and consistent contrast are desired in radiology. Since recent X-ray image detectors have accomplished small pixel pitches and improved resolution, displays with sufficiently large pixel arrays are needed to exhibit all image information. Noise in X-ray images is mainly due to quantum mottle. Display noise has to be minimal in order to maintain lower radiation doses. While consistent display contrast is needed for accurate image interpretation across devices, fairly high contrast, especially with low minimum luminance, is beneficial in specific cases such as mammography. Other medical specialties including endoscopy, pathology, and dermatology might require accurate color reproducibility with high contrast and fast temporal response. In this study, we focus on spatial resolution and noise characteristics of OLED displays. While image characteristics in LCDs have been extensively assessed in terms of resolution, noise, luminance, and reflectance [13–18], no report has been published regarding spatial image characteristics in OLED displays although luminance and contrast characteristics have been documented [2, 4–6]. This study demonstrates the characterization of spatial resolution and noise for OLED displays with a range of display sizes and pixel features in comparison with LCDs. This comparison with dedicated medical LCDs would contribute to the understanding of the applicability of OLED displays in radiology. Additionally, the image characteristics are estimated for various luminance levels and viewing angles. Previous studies have reported on the angular response of luminance and contrast in LCDs [19–23] and improved angular performance in OLED displays [2, 4, 6]. However, the angular dependency in terms of resolution and noise and its luminance dependency has not been reported in the literature. Angular dependency is an important factor to ensure displays perform consistently under different viewing conditions. Since the viewing orientation for handheld displays could vary, the resolution and noise variations with angle need to be characterized. In addition, we evaluate sharpness considering the appropriate viewing distance for each display format and compare perceived resolution characteristics. 2.
Materials and methods
2.1. Photometric camera We used a photometric charge-coupled device (CCD) camera (P199F, Westboro Photonics Inc., Ottawa, Canada) to capture the display screen for the resolution and noise measurements. The camera equips a macro lens (NIKON AF Micro-Nikkor 60 mm f/2.8D, Nikon Inc., Tokyo, Japan) and CCD sensors consisted of 1624×1224 elements with 0.0044-mm pixel pitch. The camera is calibrated at the pixel level to luminance in a range from 0.02 to 50,000 cd/m2 with a 12-bit analog-to-digital conversion. Since the measurements were performed at various camera orientations in the following sections, we first measured a modulation transfer function (MTF) of the camera system to examine the angular dependency of the resolution characteristics. We used an optic resolution pattern chart (1951 USAF resolution target, Edmund Optics Inc., Barrington, USA). We set the resolution chart, which was placed on an illuminated film viewer (KLV-5700, HAKUBA Inc., Tokyo, Japan), at a distance of 300 mm away from the camera lens. We captured the resolution chart with the camera at horizontally varying orientations from #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28113
-10 to 10 degrees with respect to the perpendicular direction as shown in Fig. 1(a). The camera pixel pitch corresponded to about 0.0010 mm on the resolution chart plane. We acquired luminance profiles on the edge of 2×2-mm square cut from the captured images as indicated by a square with red broken line in Fig. 1(b). We localized the edge line at the horizontal center of the camera field of view (FOV) and set the focus on the edge line. The edge profiles were differentiated and fast Fourier transformed to calculate the MTFs at the respective angles.
Fig. 1. Modulation transfer function (MTF) measurement of a photometric camera at various oriented angles. (a) Experiment layout of the camera and a resolution chart. The camera was oriented at varying angles from -10 to 10 degrees with respect to the perpendicular direction to the resolution chart. (b) The captured pattern image with the camera. The edge line of 2×2-mm square cut indicated by a square with red broken line was used for the MTF calculation.
2.2. Display specifications We used a 3.7-inch phone-size handheld OLED display (OLEDp1: Nexus One, HTC Corp., Taoyuan, Taiwan), a 4.0-inch phone-size handheld OLED display (OLEDp2: Galaxy S, Samsung Corp., Seoul, South Korea), a 25-inch 2-mega-pixels (MP) OLED workstation (OLEDw: PVM-2551MD, Sony Corp., Tokyo, Japan), a 3.5-inch phone-size handheld LCD (LCDp: iPhone4, Apple Inc., CA, USA), a 9.7-inch tablet-size handheld LCD (LCDt: iPad 3rd generation, Apple Inc., CA, USA), and a 21-inch 3-MP LCD workstation (LCDw: R31, Eizo Nanao Corp., Ishikawa, Japan). The display specifications are listed in Table 1. LCDw is mainly used by radiologists and other physicians for primary image diagnosis. LCDt and LCDw adopt in-plane switching (IPS) technology. OLEDp1 and OLEDp2 use active-matrix organic lightemitting diode (AMOLED) displays with PenTile sub-pixel technology, which allocates green (G) sub-pixels interleaved with alternating red (R) and blue (B) sub-pixels. The R-G-B-G layout is iterated and one pixel consists of two sub-pixels of R-G or B-G. The G sub-pixels are mapped by one-to-one correspondence with input signal pixels. The R and B sub-pixels are subsampled reconstructing the chroma signal. The luminance is processed using adaptive sub-pixel rendering filters from the input image. The brightness settings of the handheld displays were fixed at the maximum for each device throughout the experiments. There are no options to change the contrast in the handheld devices. OLEDw has various on-screen display (OSD) settings for contrast, brightness, and color space. Except in noted cases, the parameters were set to contrast: 50, brightness: 50, and color space: SMPTE-C. In addition, the brightness and contrast parameters were changed at #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28114
Table 1. Display specifications with minimum and maximum luminance values and luminance ratio. The luminance values were measured at the maximum brightness setting for the handheld displays and LCDw, and the contrast and brightness parameters were set at 50 with a color space: SMTPE-C for OLEDw. Display OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
Screen size (inch) 3.7 4.0 25 3.5 9.7 21
Pixel array 480×800 480×800 1080×1920 640×960 2048×1536 1536×2048
Pixel pitch (mm) 0.101 0.109 0.283 0.0779 0.0962 0.207
Lmin (cd/m2 ) 0.138 0.174 0.135 0.770 0.750 1.05
Lmax (cd/m2 ) 259 341 277 479 467 271
LR 1877 1960 2052 622 623 258
the fixed color space as SMPTE-C. We changed the color space parameter from EBU, ITU709, SMPTE-C, and OFF for fixed contrast and brightness levels. The brightness of LCDw was fixed at maximum and the contrast was set to the digital imaging and communications in medicine (DICOM) grayscale standard display function (GSDF). Table 1 lists the minimum luminance Lmin and maximum luminance Lmax and the luminance ratio LR= Lmax /Lmin at the specific brightness and contrast for each device. 2.3. MTF We measured MTFs to characterize spatial resolution for the displays listed in Table 1 using a methodology described in [13]. The handheld displays were set at portrait orientation while the workstation displays were set at landscape orientation. We displayed a horizontal or vertical one-pixel-line pattern embedded in a uniform background, respectively for the vertical or horizontal MTF. The horizontal direction corresponds to the red-green-blue (RGB) sub-pixel direction except for LCDt in which the vertical direction corresponds to the RGB direction. The digital driving levels (DDLs) of the line and background were selected from the conditions: line/background= 15/10, 50/30, 60/50, 70/40, and 100/80%. As the exceptions, 50/30% was excluded for OLEDw and the fixed 60/50% was used for LCDt as listed in Table 2. The OSD settings for OLEDw were set at the referential settings as mentioned in 2.2. The displayed screen was captured with the photometric camera at a magnification corresponding to about 9×9 CCD pixels per display pixel as shown in Fig. 2(a). Figure 2(b) shows the captured screen images displaying the horizontal and vertical line-patterns on each display. The distance from the camera to the screen was approximately 300 mm for OLEDp1 and OLEDp2, and the distance was increased in proportion to the pixel pitch for other displays. The camera was first directed perpendicularly to the screen surface, and the screen image was captured at 0-degree viewing angle. At the 0-degree viewing angle and DDL 60/50%, various OSD settings were tested for OLEDw. The brightness and contrast parameters were changed from 50 to 25, 75, and 100 respectively at the color space of SMPTE-C. We also selected respective color space parameters: EBU, ITU-709, SMPTE-C, and OFF at the fixed contrast 50 and brightness 50. Table 2 lists the measured luminance values (cd/m2) at 0-degree angle with the photometric camera, corresponding to the line and background DDLs. The listed luminance values of OLEDw were measured at the referential OSD settings. The luminance values were determined from the averaged pixel values on the one-pixel line and background area in the captured screen images. Next, the camera orientation was horizontally varied in the angle range from -8 to 8 degrees by 2-degree steps with respect to the perpendicular direction for OLEDp1, OLEDp2, LCDp, and LCDt, in a similar way to Fig. 1(a). The displays were rotated by 90 degrees to measure the vertical MTFs. The captured line pattern images were subtracted from the background images, which were #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28115
Table 2. Digital driving levels (DDL) and luminance values of the line (LN) and background (BG) in the displayed pattern at perpendicular viewing angle.
LN/BG
DDL (%) 15/10 50/30 60/50 70/40 100/80
OLEDp1 2.38/1.69 49.1/15.7 73.5/52.9 103/29.0 223/144
OLEDp2 6.74/4.15 75.7/23.4 122/66.4 158/40.9 284/188
Luminance (cd/m2 ) OLEDw LCDp 2.03/0.825 5.69/2.21 89.5/40.2 31.0/22.4 128/92.9 33.0/13.9 176/58.8 91.7/70.7 449/286
LCDt 159/112 -
LCDw 5.97/4.82 32.9/14.1 53.8/37.3 77.5/23.0 265/135
acquired by capturing uniform patterns with the same setup. After the subtraction, 1200 line profiles in each captured image were averaged vertically (in the line direction). Line spread functions (LSFs) were obtained by normalizing the averaged profiles by the maximum luminance values. Horizontal and vertical MTFs were calculated by fast Fourier transformation of the LSFs. We represent the MTFs as a function of absolute and relative spatial frequency, with relative spatial frequency being equal to the absolute spatial frequency divided by the Nyquist frequency corresponding to the display system. The MTFs as a function of the relative spatial frequency express how much blur is present regardless of pixel pitch. If there is no resolution degradation on the display, the LSF becomes a square wave and the MTF is given by the sinc function, sin(uπ p) sinc(u/ fN ) = , (1) uπ p where u is spatial frequency (mm−1), p is display-pixel pitch (mm), and fN is Nyquist frequency (mm−1 ) = 1/2p. In addition, we correct the MTFs in terms of appropriate viewing distances and present the measurements in terms of angular spatial frequency expressed in cycles per degree. According to previous reports [24, 25], typical viewing distances V D (mm) are 600, 300, and 200 mm for workstations, tablet-size handhelds, and phone-sized handhelds respectively. The angular spatial frequency (cy/deg) is calculated by the following expression [26]: VD (cy/mm). (2) 57.3 The image capture procedures were repeated three times with the same camera positioning for each camera orientation and each DDL pattern, and the corresponding three MTFs were averaged. The standard deviations of the three MTFs were calculated for the 60/50% pattern in the perpendicular direction. Student’s t-test was applied to compare the MTFs at 0.4 cy/mm in relative spatial frequency with various DDLs and camera orientations. P-value less than 0.05 was considered statistically significant. (cy/deg) =
2.4. NPS We measured noise power spectra (NPS) to characterize spatial noise for the displays according to [13, 14, 17]. Uniform patterns with DDLs corresponding to the background levels listed in Table 2 were displayed and each screen was captured with the photometric camera. The OSD settings for OLEDw were set at the referential settings. However, only when the camera orientation was perpendicular to the display screen, various OSD settings were tested for OLEDw displaying 50% DDL pattern as well as the MTF measurements. Next, the camera orientation was changed in the angle range from -8 to 8 degrees relative to the perpendicular viewing direction for OLEDp1, OLEDp2, LCDp, and LCDt. For calculating one-dimensional (1D) horizontal #196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28116
Fig. 2. (a) The experimental setting of the photometric CCD camera capturing the display screen for measuring the display MTFs. The OLEDp1, OLEDp2, LCDp, and LCDt were set by the portrait orientation while OLEDw and LCDw were set by the landscape orientation. (b) The captured screen images displaying the horizontal and vertical line-patterns. The horizontal direction corresponds to the RGB sub-pixel direction except for LCDt in which the vertical direction corresponds to the RGB direction. Orange dot-line squares showing respective one pixel regions and 0.1-mm scale bars are indicated.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28117
NPS, a region of interest (ROI) of width 512 × height 40 pixels, which positioned at the horizontal center and upper end in the captured image, was selected and the 512-point horizontal profile was acquired with numerical slit (1×40) averaging 40 pixel values to eliminate vertical noise [27, 28]. After being subtracted from the mean value of 512 data in the averaged profile, the profile was processed with a Hanning window, and fast-Fourier transformed. The window processing works to reduce spectral leakage errors [17] in Fourier space, particularly since displays have periodic pixel structures inducing spectral peaks at frequencies in accordance with the integral multiples of the inverse of the sub-pixel pitch. The 1D NPS(u) (mm2 ) calculation is expressed as follows, n m o 2 s 2 M−1 L(kx) exp −2π j k NPS(u) = x ∑ , M m=0 L M
(3)
m n o 2 n xy M−1 N−1 L(kx, ly) exp −2π j k , l NPS(u, v) = ∑ ∑ , MN m=0 M N L n=0
(4)
with k= 0, 1, 2, · · ·, M-1, where M is noise profile data points 512, s is numerical-slit length points 40, x is effective camera-pixel pitch (mm) on captured plane, u= k/(Mx) is spatial frequency (mm−1), L is average luminance (cd/m2 ) of 512 data, L(kx) is luminance difference (cd/m2 ) at (kx) from L. The 512×40 ROI was moved vertically without overlaps to repeat the NPS calculation as many times as possible and the NPS were averaged. Vertical NPS was calculated in the same way using the horizontal numerical-slit scanning. Furthermore, twodimensional (2D) NPS(u, v) (mm2 ) was calculated by a 2D fast Fourier transformation with a Hanning window processing as follows,
with k=0, 1, 2, · · ·, M-1, and l=0, 1, 2, · · ·, N-1, where x and y are effective camera-pixel pitches (mm) on captured plane, u= k/(Mx) and v= l/(Ny) are spatial frequencies (mm−1 ), L is average luminance (cd/m2) in ROI with M × N: 256×256 matrices, L(kx, ly) is luminance difference (cd/m2 ) at (kx, ly) (cd/m2) from L. The image capture procedures were carried out three times at the same camera location for each camera orientation and DDL pattern, and the three NPS were averaged. The standard deviations of the three NPS were calculated for the 50% DDL pattern in the perpendicular direction. The NPS variations with DDLs at 2.0 cy/mm and the variations with camera orientations at 0.2 and 2.0 cy/mm were analyzed using Student’s t-test. P-value less than 0.05 was considered statistically significant.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28118
3.
Results
3.1. Camera resolution Figure 3 shows the MTFs of the photometric camera at various orientations ranging from -10 to 10 degrees relative to the perpendicular direction. We recognize that the highest MTF at 0 degree is slightly decreased as the angle is tilted up to -10 or 10 degrees. However, since the MTF variation is too small to substantially affect display resolution measurements at least within the angle range from -8 to 8 degrees.
Modulation transer function
1
0.95
0.9
0.85
0.8
-10°° -8 -6°° -2° 0° 2 6°° 8 10° 0.1
1
10 -
Spatial frequency (mm 1) Fig. 3. Modulation transfer functions of the photometric camera at various viewing angles in the range from -10 to 10 degrees.
3.2. Device characterization All OLED displays have lower minimum luminance and noticeably higher luminance ratios than LCDs as shown in Table 1. The LSFs in Fig. 4 and MTFs as a function of absolute frequency, relative frequency, and angular frequency given by Eq. (2) in Fig. 5 were measured using the pattern with 60% DDL line and 50% DDL background for all displays. Since the LSFs and MTFs of OLEDw were consistent regardless of the OSD settings, Figs. 4 and 5 show the results at contrast 50, brightness 50 and SMPTE-C color space settings. When the vertical line is displayed in OLEDp1 with PenTile technology, the illuminated red or blue sub-pixels are located at only one side of the green sub-pixels as shown in Fig. 2(b). On the other hand, the red or blue sub-pixels on both sides of the green are illuminated in OLEDp2. The illuminated red or blue sub-pixel locations are determined by the respective sub-pixel rendering algorithms to calculate the luminance, and the algorithm of OLEDp1 causes the asymmetric horizontal LSF. However, the LSF difference between OLEDp1 and OLEDp2 does not result in noticeably different MTFs as a function of absolute frequency in Figs. 5(a) and 5(d). The MTFs in Figs.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28119
(a) Horizontal 1
OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
0.6 0.4 0.2 0 -0.2 -0.75
OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw
0.8 Relative luminance
0.8 Relative luminance
(b) Vertical 1
0.6 0.4 0.2 0
0 Distance (mm)
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Fig. 4. Line spread functions for the devices tested in this study.
5(a) and 5(d) show error bars indicating two standard deviations for the three-times measured MTFs. The estimated uncertainty suggests that the MTFs have sufficient accuracy for comparing sharpness among displays. The LSFs of OLEDw present the most widespread profiles among all displays in this study in both horizontal and vertical directions. Although the displays with smaller pixel pitches have the LSFs with smaller widths and higher MTFs in Figs. 5(a) and 5(d), the LSFs of OLEDw have evidently wider widths than the pixel aperture size. The LSFs exhibit non-monotonic distributions that drop once at the border between the center and the next pixels, and then increase at the next pixels before leading undershoots. The MTFs as a function of relative spatial frequency in Figs. 5(b) and 5(e) allow the spatial resolution comparisons regardless of the pixel pitch. In horizontal direction, LCDt and LCDp have MTFs close to the sinc function given by Eq. (1) expressing the ideal MTF, followed by OLEDp1, OLEDp2 and LCDw. In vertical direction, OLEDp2 and LCDt have almost equivalent MTFs to the sinc function, followed by OLEDp1 and LCDp. The MTF of LCDw is slightly decreased from the handheld displays. The MTFs of OLEDw represent the most degraded resolution characteristics in both horizontal and vertical directions. The MTFs as a function of angular frequency in Figs. 5(c) and 5(f) compare resolution characteristics at appropriate estimated viewing distances for the devices. The MTFs of OLEDw and LCDw are relatively increased because of the long viewing distance compared to the MTFs as a function of absolute frequency in Figs. 5(a) and 5(d). Even though the MTFs corrected by shorter viewing distances for the handheld displays are relatively decreased, LCDt has the best resolution characteristics followed by LCDw.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28120
(a) Absolute frequency (horizontal)
(b) Relative frequency (horizontal)
0.8 0.7 OLEDp1 OLEDp2 OLEDw 0.6 LCDp LCDt LCDw 0.5 0.1
1
0.9 0.8 OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw Sinc function
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Modulation transfer function
0.9
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(d) Absolute frequency (vertical)
(e) Relative frequency (vertical)
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(f) Angular frequency (vertical)
0.9 0.8 OLEDp1 OLEDp2 OLEDw LCDp LCDt LCDw Sinc function
0.7 0.6 0.5
10 -
10
1 Modulation transfer function
Modulation transfer function
0.9
1
0.7 OLEDp1 OLEDp2 OLEDw 0.6 LCDp LCDt LCDw 0.5 1
Spatial frequency (cy/deg)
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Spatial frequency (mm 1)
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Spatial frequency/ Nyquist frequency (mm 1)
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0.7 OLEDp1 OLEDp2 OLEDw 0.6 LCDp LCDt LCDw 0.5 0.1
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Spatial frequency (mm 1)
Modulation transfer function
(c) Angular frequency (horizontal)
1 Modulation transfer function
Modulation transfer function
1
0.1
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Spatial frequency/ Nyquist frequency (mm 1)
0.9 0.8 0.7 OLEDp1 OLEDp2 OLEDw 0.6 LCDp LCDt LCDw 0.5 1
10
Spatial frequency (cy/deg)
Fig. 5. Modulation transfer functions as a function of absolute frequency, relative frequency, and angular frequency expressed in cycles per degree for the devices tested in this study. (a) and (d) show error bars representing two standard deviations for measured MTFs.
1D and 2D NPS calculated using Eqs. (3) and (4) for all displays in Figs. 6 and 7 exhibit spectral peaks at the integral-multiple frequencies of (1/sub-pixel pitch) based on the prominent periodic pixel and sub-pixel structures shown in Fig. 2(b). The error bars in Fig. 6 representing two standard deviations of the three-times measured NPS suggest sufficient measurement accuracy to compare the noise characteristics among the displays. The OSD settings of OLEDw did not affect the NPS results, except for the contrast setting 25 increasing the NPS slightly. Accordingly, we show the NPS at contrast 50, brightness 50 and SMPTE-C color space settings in Figs. 6 and 7. Figure 6 represents that LCDp and LCDt have the lowest NPS values followed by OLEDp1 and OLEDp2 in horizontal and vertical directions, while OLEDw has higher NPS than the NPS of the handheld displays, and LCDw has the highest NPS in the devices tested. The 2D NPS of OLEDp1, OLEDp2, LCDp, and LCDt in Fig. 7 depict the lower NPS by the darker gray colors. In contrast, the higher NPS of LCDw and OLEDw are emphasized by the brighter gray colors in the 2D NPS.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28121
10
-4
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2
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Noise power spectral value (mm )
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Noise power spectral value (mm )
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OLEDp1 OLEDp2
OLEDw LCDp
LCDt LCDw
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Spatial frequency (mm 1)
Spatial frequency (mm 1) OLEDp1 OLEDp2
OLEDw LCDp
LCDt LCDw
Fig. 6. One-dimensional noise power spectra for the devices tested in this study. The error bars represent two standard deviations for measured NPS.
Fig. 7. Two-dimensional noise power spectra for the devices tested in this study.
#196101 - $15.00 USD Received 20 Aug 2013; revised 20 Sep 2013; accepted 25 Sep 2013; published 8 Nov 2013 (C) 2013 OSA 18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028111 | OPTICS EXPRESS 28122
3.3. DDL dependency Figure 8 shows the horizontal LSFs by various DDLs for OLEDp1, OLEDp2, OLEDw and LCDw. The LSFs of OLEDp1 and LCDw indicate no variation with DDLs although the LSFs appear to be noisy at the DDL with LN/BG= 15/10%. In contrast, the LSF of OLEDp2 has undershoots only at the DDL with LN/BG= 70/40%. While the LSFs of OLEDw are independent of DDLs at the center pixel, the off-center peaks and undershoots exhibit subtle relative luminance differences. Subsequently, while the MTFs of OLEDp1 and LCDw have no significant differences (p>0.05) at relative frequency 0.4 cy/mm by DDLs in Fig. 9, OLEDp2 has a higher MTF at DDL 70/40% (p
