REVIEW OF SCIENTIFIC INSTRUMENTS 85, 046103 (2014)
Note: Laser beam scanning using a ferroelectric liquid crystal spatial light modulator Abhijit Das1,2 and Bosanta R. Boruah1,a) 1 2
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Department of Physics, Gauhati University, Guwahati 781014, Assam, India
(Received 30 December 2013; accepted 20 March 2014; published online 3 April 2014) In this work we describe laser beam scanning using a ferroelectric liquid crystal spatial light modulator. Commercially available ferroelectric liquid crystal spatial light modulators are capable of displaying 85 colored images in 1 s using a time dithering technique. Each colored image, in fact, comprises 24 single bit (black and white) images displayed sequentially. We have used each single bit image to write a binary phase hologram. For a collimated laser beam incident on the hologram, one of the diffracted beams can be made to travel along a user defined direction. We have constructed a beam scanner employing the above arrangement and demonstrated its use to scan a single laser beam in a laser scanning optical sectioning microscope setup. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870281] Several important applications of lasers, such as in imaging and optical trapping, involve accurate positioning of the laser beam. In beam scanning microscopes usually a galvo mirror scanner is used to scan the beam. However such a scanner suffers from limitations, for instance, short term repeatability, long term thermal drift, and non linear response of the position sensor.1 These limitations can create major issues in long exposure imaging applications, such as in monitoring a physical process in a material by collecting the fluorescence light from a sample for several hours,2 in detecting weak signals from the sample such as Raman signals,3, 4 and in visualizing slow chemical processes.5 There can be an issue even for low exposure imaging applications requiring large working distance objective lenses.6, 7 The concept of beam scanning using a hologram, which works as a diffraction grating, was introduced as early as in 1976.8 Gratings written on a liquid crystal display have been found to be extremely accurate in beam steering.9 In our previous work we showed how binary amplitude or phase hologram can be used to realize a beam scanner for an incident laser beam, which can be used in a beam scanning microscope.10 Owing to the binary nature of the amplitude or phase modulation, the steered beam has better accuracy and stability.11 Assuming that other mechanical disturbances are taken care of, the deflection angle of the beam is directly determined by the hologram pattern. Thus, such a beam scanner can provide superior beam repeatability compared to scanning devices which suffer from inherent mechanical disturbances. In our previous work we used a nematic liquid crystal spatial light modulator, which can display up to only three binary holograms per colored image. However, a ferroelectric liquid crystal spatial light modulator (FLCSLM) is based on molecules which are bistable and have a quick response time.12 It can display 24 holograms per colored image, leading to at least an 8 times increase in the scanning speed rela-
a) Electronic mail: [email protected]
0034-6748/2014/85(4)/046103/3/$30.00
tive to a nematic liquid crystal spatial light modulator based beam scanner. In this work we use a commercially available FLCSLM device that can display up to 85 RGB (red, green and blue) images per second using a time dithering technique. For each incoming RGB image, the device displays the constituent 24 single bit images in a sequential manner. We have used each of the single bit images to describe a binary phase hologram to realize a deflected beam, derived from a single laser source, along a user defined direction using a computer generated holography technique. We then employed the beam scanner to construct an optical sectioning microscope. We present here experimental results to demonstrate the working of the FLCSLM based beam scanner. We use an FLCSLM (SXGA-R3, ForthDD), having a display panel comprising 1280 × 1024 liquid crystal (LC) cells, that is compatible with an 85 Hz refresh rate video signal from a PC (personal computer). The video signal on the other hand comprises RGB images, each image having 24 bit of information per pixel. The FLCSLM device splits the incoming RGB image into 24 single bit (black and white) images and displays them sequentially. Thus, the device can display 2040 (=24 × 85) number of single bit images in 1 s. Figure 1(a) shows a schematic of 24 single bit frames which contains 8 frames, such as R0 , R1 , R2 , . . . , R7 , representing red color, 8 frames, such as G0 , G1 , G2 , . . . , G7 , representing green color, and 8 frames, such as B0 , B1 , B2 , . . . , B7 , representing blue color. However, in order to prevent any potential damage to liquid crystal molecules there is a requirement of the FLCSLM display, that display of each single bit image has to be followed by the display of its negative counterpart (a white FLCSLM pixel becoming black and vice versa). The FLCSLM control box provides an electrical signal output that becomes high or low in synchronization with the display of each of the positive or the negative single bit image. Figure 1(b) shows a snapshot of a digital oscilloscope that receives the synchronization signal from the FLCSLM in channel 1 (the bottom plot). It is noticed in the plot that the
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© 2014 AIP Publishing LLC
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A. Das and B. R. Boruah
Rev. Sci. Instrum. 85, 046103 (2014)
FIG. 1. (a) Sequential display of the 24 bit planes in an FLCSLM display panel. (b) Snapshot of the oscilloscope panel showing the plot of synchronization signal from FLCSLM (below) and the plot of camera trigger signal (above).
positive frame is displayed for a duration of ∼360 μs on the average followed by the display of the corresponding negative frame displayed for a duration of ∼140 μs. We have developed a PIC18F2550 microchip based board that receives the synchronization signal from the FLCSLM and generates another synchronization signal, as seen in the top plot of Fig. 1(b) (corresponds to the snapshot of the oscilloscope panel receiving the trigger signal in channel 2). This signal remains high during the display of all the 24 frames of a given RGB image. Such a signal can be used to trigger a CCD camera so that the CCD receives light only when a given RGB image is being displayed in the FLCSLM panel. We use a computer generated holography technique13, 14 to generate binary holograms which are written on the FLCSLM panel. When the panel is illuminated with a collimated laser beam, the display panel introduces a phase modulation of 0 or π depending on the binary value of the DC field applied across the corresponding LC cell. Thus, the FLCSLM panel as a whole acts as a binary phase hologram. This results in three prominent diffracted beams, namely, the +1, 0, and −1 order beams, of which we choose the +1 order diffracted beam. The deflection angle and the phase profile of the +1 order diffracted beam can be controlled programmably as they are associated directly with the binary hologram parameters being displayed on the FLCSLM panel. The schematic diagram of the FLCSLM based beam scanner in the beam scanning microscope setup is shown in Fig. 2. The beam from a DPSS (Diode-Pumped Solid State) laser (λ = 532 nm) is expanded and collimated by the lens combination BX. The beam is then incident on the FLCSLM panel. A program running on a PC generates sets of 24 single bit holograms in the form of RGB images which are sent to the FLCSLM as the video signal from the same PC. The diffracted beam from the FLCSLM are focused by the lens L1 . An iris diaphragm ID kept in the focal plane of L1 isolates the +1 order from the other orders (i.e., 0 and −1). The lens L2 re collimates the +1 order beam which will now work as the illumination beam of a scanning microscope. The lenses L1 and L2 constitute a 4f relay system so that the FLCSLM plane and the back focal plane of the microscope objective,
MO, are conjugate planes. The sample to be imaged is kept at the focal plane of MO. The light reflected by the sample is received by the same objective lens and is reflected by the beam splitter BS towards the lens L3 to be focused on the CCD plane. The microchip based control board sends a trigger signal to the CCD that comprises 20 pulses/s. Each pulse exposes the camera to the light coming from the sample plane for the duration of 24 holograms displayed on the FLCSLM panel. Figure 3(a) shows a representative RGB image, comprising 24 binary holograms, which sends the +1 order beam to equal number of uniformly spaced locations. Initially we put a plane mirror in the sample plane so that the +1 order focal spot is recorded by the camera for each of the 24 single bit images. Figure 3(b) shows 24 such focal spots recorded in a single image frame of the CCD camera. A plot along the line joining the centres of the focal spots is seen in Fig. 3(c) showing the equal spacing of the 24 spots. In order to image a rectangular area of the sample plane, a sequence of RGB images are sent to the FLCSLM panel. The RGB images are constructed in such a way that after the display of the given number of RGB images, the +1 order spots describe a rectangular grid on the focal plane. To ensure that there is no overlap between adjacent +1 order spots captured in a single CCD image, each RGB image is made to generate
FIG. 2. Schematic diagram of the experimental arrangement.
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A. Das and B. R. Boruah
Rev. Sci. Instrum. 85, 046103 (2014)
FIG. 3. (a) An RGB image (800 × 800 pixels) comprising 24 binary holograms sending the diffracted beams to equal number of equally spaced locations, (b) CCD image of 24 focal spots due to the display of a single RGB image in the display panel, and (c) a line plot joining centres of the focal spots shown in (b). The intensity of light is expressed in arbitrary unit (A.U.).
a sparse array of +1 order spots. In order to obtain an optically sectioned image of the sample plane, for each position of the +1 order beam in the sample plane, the net signal, in a square window in the camera image surrounding the expected location of the corresponding image of the +1 order spot, is stored in the PC. This window, termed the detector window, is moved programmably to each expected location of the +1 order spot in the camera image. The stored data at the end of the scanning is used to construct an image of the sample plane. By reducing the size of the detector window, light coming from planes other than the sample plane can be blocked, giving rise to the optical sectioning effect in the constructed image. We used a USAF 1951 resolution test target as the sample with a plane mirror kept behind only to enhance the background light. Figures 4(a) and 4(b) shows the reconstructed image of the test target using detector dimension equal to 12 pixels and 6 pixels, respectively. Both the images demonstrate the beam scanning performance by the FLCSLM based beam scanner, while a comparison between the two images shows that by decreasing the detector window dimension background light from the sample volume can be reduced. To conclude we have demonstrated the use of a ferroelectric liquid crystal spatial light modulator as a laser beam scan-
FIG. 4. Beam scanning microscope images (size = 96 × 96 pixels, MO is a 10× Olympus lens of NA = 0.3) of a USAF resolution test target, using detector window of (a) dimension = 12 pixels, (b) dimension= 6 pixels. The dimension of the scale bar at the top right corner is 78 μm.
ner. Here the display panel acts as a binary phase hologram and one of the diffracted beam plays the role of the deflected beam. Using a commercially available device, the deflected beam can be sent up to 2040 number different locations in 1 s. Compared to popular beam scanning devices such as galvo mirror scanners, the proposed beam scanner is less susceptible to mechanical vibrations and has the important advantages of repeatability and accuracy. We have assessed the performance of the beam scanner by constructing a scanning optical microscope based on the proposed beam scanner. Such a beam scanning microscope can be of particular use for imaging applications involving long exposure or repeated exposure of the target. This work is supported by a research grant, bearing the reference number SR/S2/LOP-011/2010, from the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India. 1 F.
Blais, M. Rioux, and J.-A. Beraldin, Proc. SPIE 0959, 225–246 (1988). Brouri, A. Beveratos, J. Poizat, and P. Grangier, Opt. Lett. 25, 1294 (2000). 3 F. Sarrazin, J. Salmon, D. Talaga, and L. Servant, Anal. Chem. 80, 1689 (2008). 4 M. Vishwanath, A. Nishibu, S. Saeland, B. Ward, N. Mizumoto, H. Ploegh, M. Boes, and A. Takashima, J. Investigative Dermatology 126, 2452 (2006). 5 B. Geest, N. Sanders, G. Sukhorukov, J. Demeester, and S. Smedt, Chem. Soc. Rev. 36, 636 (2007). 6 R. Webb and F. Rogomentich, Appl. Opt. 38, 4870 (1999). 7 S. Lindek, R. Pick, and E. Stelzer, Rev. Sci. Instrum. 65, 3367 (1994). 8 O. Bryngdahl and W. Lee, Appl. Opt. 15, 183 (1976). 9 X. Wang, B. Wang, J. Pouch, F. Miranda, M. Fisch, J. Anderson, V. Sergan, and P. Bos, Proc. SPIE 5162, 139–146 (2003). 10 A. Das and B. R. Boruah, Rev. Sci. Instrum. 82, 043702 (2011). 11 M. A. A. Neil, M. Booth, and T. Wilson, Opt. Lett. 23, 1849 (1998). 12 U. Efron, Spatial Light Modulator Technology: Materials, Devices and Applications (CRC Press, 1995), Vol. 47. 13 M. A. A. Neil, T. Wilson, and R. Juskaitis, J. Microsc.-Oxf. 197, 219 (2000). 14 B. R. Boruah, Am. J. Phys. 77, 331 (2009). 2 R.
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Note: Laser beam scanning using a ferroelectric liquid crystal spatial light modulator Abhijit Das1,2 and Bosanta R. Boruah1,a) 1 2
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Department of Physics, Gauhati University, Guwahati 781014, Assam, India
(Received 30 December 2013; accepted 20 March 2014; published online 3 April 2014) In this work we describe laser beam scanning using a ferroelectric liquid crystal spatial light modulator. Commercially available ferroelectric liquid crystal spatial light modulators are capable of displaying 85 colored images in 1 s using a time dithering technique. Each colored image, in fact, comprises 24 single bit (black and white) images displayed sequentially. We have used each single bit image to write a binary phase hologram. For a collimated laser beam incident on the hologram, one of the diffracted beams can be made to travel along a user defined direction. We have constructed a beam scanner employing the above arrangement and demonstrated its use to scan a single laser beam in a laser scanning optical sectioning microscope setup. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870281] Several important applications of lasers, such as in imaging and optical trapping, involve accurate positioning of the laser beam. In beam scanning microscopes usually a galvo mirror scanner is used to scan the beam. However such a scanner suffers from limitations, for instance, short term repeatability, long term thermal drift, and non linear response of the position sensor.1 These limitations can create major issues in long exposure imaging applications, such as in monitoring a physical process in a material by collecting the fluorescence light from a sample for several hours,2 in detecting weak signals from the sample such as Raman signals,3, 4 and in visualizing slow chemical processes.5 There can be an issue even for low exposure imaging applications requiring large working distance objective lenses.6, 7 The concept of beam scanning using a hologram, which works as a diffraction grating, was introduced as early as in 1976.8 Gratings written on a liquid crystal display have been found to be extremely accurate in beam steering.9 In our previous work we showed how binary amplitude or phase hologram can be used to realize a beam scanner for an incident laser beam, which can be used in a beam scanning microscope.10 Owing to the binary nature of the amplitude or phase modulation, the steered beam has better accuracy and stability.11 Assuming that other mechanical disturbances are taken care of, the deflection angle of the beam is directly determined by the hologram pattern. Thus, such a beam scanner can provide superior beam repeatability compared to scanning devices which suffer from inherent mechanical disturbances. In our previous work we used a nematic liquid crystal spatial light modulator, which can display up to only three binary holograms per colored image. However, a ferroelectric liquid crystal spatial light modulator (FLCSLM) is based on molecules which are bistable and have a quick response time.12 It can display 24 holograms per colored image, leading to at least an 8 times increase in the scanning speed rela-
a) Electronic mail: [email protected]
0034-6748/2014/85(4)/046103/3/$30.00
tive to a nematic liquid crystal spatial light modulator based beam scanner. In this work we use a commercially available FLCSLM device that can display up to 85 RGB (red, green and blue) images per second using a time dithering technique. For each incoming RGB image, the device displays the constituent 24 single bit images in a sequential manner. We have used each of the single bit images to describe a binary phase hologram to realize a deflected beam, derived from a single laser source, along a user defined direction using a computer generated holography technique. We then employed the beam scanner to construct an optical sectioning microscope. We present here experimental results to demonstrate the working of the FLCSLM based beam scanner. We use an FLCSLM (SXGA-R3, ForthDD), having a display panel comprising 1280 × 1024 liquid crystal (LC) cells, that is compatible with an 85 Hz refresh rate video signal from a PC (personal computer). The video signal on the other hand comprises RGB images, each image having 24 bit of information per pixel. The FLCSLM device splits the incoming RGB image into 24 single bit (black and white) images and displays them sequentially. Thus, the device can display 2040 (=24 × 85) number of single bit images in 1 s. Figure 1(a) shows a schematic of 24 single bit frames which contains 8 frames, such as R0 , R1 , R2 , . . . , R7 , representing red color, 8 frames, such as G0 , G1 , G2 , . . . , G7 , representing green color, and 8 frames, such as B0 , B1 , B2 , . . . , B7 , representing blue color. However, in order to prevent any potential damage to liquid crystal molecules there is a requirement of the FLCSLM display, that display of each single bit image has to be followed by the display of its negative counterpart (a white FLCSLM pixel becoming black and vice versa). The FLCSLM control box provides an electrical signal output that becomes high or low in synchronization with the display of each of the positive or the negative single bit image. Figure 1(b) shows a snapshot of a digital oscilloscope that receives the synchronization signal from the FLCSLM in channel 1 (the bottom plot). It is noticed in the plot that the
85, 046103-1
© 2014 AIP Publishing LLC
046103-2
A. Das and B. R. Boruah
Rev. Sci. Instrum. 85, 046103 (2014)
FIG. 1. (a) Sequential display of the 24 bit planes in an FLCSLM display panel. (b) Snapshot of the oscilloscope panel showing the plot of synchronization signal from FLCSLM (below) and the plot of camera trigger signal (above).
positive frame is displayed for a duration of ∼360 μs on the average followed by the display of the corresponding negative frame displayed for a duration of ∼140 μs. We have developed a PIC18F2550 microchip based board that receives the synchronization signal from the FLCSLM and generates another synchronization signal, as seen in the top plot of Fig. 1(b) (corresponds to the snapshot of the oscilloscope panel receiving the trigger signal in channel 2). This signal remains high during the display of all the 24 frames of a given RGB image. Such a signal can be used to trigger a CCD camera so that the CCD receives light only when a given RGB image is being displayed in the FLCSLM panel. We use a computer generated holography technique13, 14 to generate binary holograms which are written on the FLCSLM panel. When the panel is illuminated with a collimated laser beam, the display panel introduces a phase modulation of 0 or π depending on the binary value of the DC field applied across the corresponding LC cell. Thus, the FLCSLM panel as a whole acts as a binary phase hologram. This results in three prominent diffracted beams, namely, the +1, 0, and −1 order beams, of which we choose the +1 order diffracted beam. The deflection angle and the phase profile of the +1 order diffracted beam can be controlled programmably as they are associated directly with the binary hologram parameters being displayed on the FLCSLM panel. The schematic diagram of the FLCSLM based beam scanner in the beam scanning microscope setup is shown in Fig. 2. The beam from a DPSS (Diode-Pumped Solid State) laser (λ = 532 nm) is expanded and collimated by the lens combination BX. The beam is then incident on the FLCSLM panel. A program running on a PC generates sets of 24 single bit holograms in the form of RGB images which are sent to the FLCSLM as the video signal from the same PC. The diffracted beam from the FLCSLM are focused by the lens L1 . An iris diaphragm ID kept in the focal plane of L1 isolates the +1 order from the other orders (i.e., 0 and −1). The lens L2 re collimates the +1 order beam which will now work as the illumination beam of a scanning microscope. The lenses L1 and L2 constitute a 4f relay system so that the FLCSLM plane and the back focal plane of the microscope objective,
MO, are conjugate planes. The sample to be imaged is kept at the focal plane of MO. The light reflected by the sample is received by the same objective lens and is reflected by the beam splitter BS towards the lens L3 to be focused on the CCD plane. The microchip based control board sends a trigger signal to the CCD that comprises 20 pulses/s. Each pulse exposes the camera to the light coming from the sample plane for the duration of 24 holograms displayed on the FLCSLM panel. Figure 3(a) shows a representative RGB image, comprising 24 binary holograms, which sends the +1 order beam to equal number of uniformly spaced locations. Initially we put a plane mirror in the sample plane so that the +1 order focal spot is recorded by the camera for each of the 24 single bit images. Figure 3(b) shows 24 such focal spots recorded in a single image frame of the CCD camera. A plot along the line joining the centres of the focal spots is seen in Fig. 3(c) showing the equal spacing of the 24 spots. In order to image a rectangular area of the sample plane, a sequence of RGB images are sent to the FLCSLM panel. The RGB images are constructed in such a way that after the display of the given number of RGB images, the +1 order spots describe a rectangular grid on the focal plane. To ensure that there is no overlap between adjacent +1 order spots captured in a single CCD image, each RGB image is made to generate
FIG. 2. Schematic diagram of the experimental arrangement.
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A. Das and B. R. Boruah
Rev. Sci. Instrum. 85, 046103 (2014)
FIG. 3. (a) An RGB image (800 × 800 pixels) comprising 24 binary holograms sending the diffracted beams to equal number of equally spaced locations, (b) CCD image of 24 focal spots due to the display of a single RGB image in the display panel, and (c) a line plot joining centres of the focal spots shown in (b). The intensity of light is expressed in arbitrary unit (A.U.).
a sparse array of +1 order spots. In order to obtain an optically sectioned image of the sample plane, for each position of the +1 order beam in the sample plane, the net signal, in a square window in the camera image surrounding the expected location of the corresponding image of the +1 order spot, is stored in the PC. This window, termed the detector window, is moved programmably to each expected location of the +1 order spot in the camera image. The stored data at the end of the scanning is used to construct an image of the sample plane. By reducing the size of the detector window, light coming from planes other than the sample plane can be blocked, giving rise to the optical sectioning effect in the constructed image. We used a USAF 1951 resolution test target as the sample with a plane mirror kept behind only to enhance the background light. Figures 4(a) and 4(b) shows the reconstructed image of the test target using detector dimension equal to 12 pixels and 6 pixels, respectively. Both the images demonstrate the beam scanning performance by the FLCSLM based beam scanner, while a comparison between the two images shows that by decreasing the detector window dimension background light from the sample volume can be reduced. To conclude we have demonstrated the use of a ferroelectric liquid crystal spatial light modulator as a laser beam scan-
FIG. 4. Beam scanning microscope images (size = 96 × 96 pixels, MO is a 10× Olympus lens of NA = 0.3) of a USAF resolution test target, using detector window of (a) dimension = 12 pixels, (b) dimension= 6 pixels. The dimension of the scale bar at the top right corner is 78 μm.
ner. Here the display panel acts as a binary phase hologram and one of the diffracted beam plays the role of the deflected beam. Using a commercially available device, the deflected beam can be sent up to 2040 number different locations in 1 s. Compared to popular beam scanning devices such as galvo mirror scanners, the proposed beam scanner is less susceptible to mechanical vibrations and has the important advantages of repeatability and accuracy. We have assessed the performance of the beam scanner by constructing a scanning optical microscope based on the proposed beam scanner. Such a beam scanning microscope can be of particular use for imaging applications involving long exposure or repeated exposure of the target. This work is supported by a research grant, bearing the reference number SR/S2/LOP-011/2010, from the Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India. 1 F.
Blais, M. Rioux, and J.-A. Beraldin, Proc. SPIE 0959, 225–246 (1988). Brouri, A. Beveratos, J. Poizat, and P. Grangier, Opt. Lett. 25, 1294 (2000). 3 F. Sarrazin, J. Salmon, D. Talaga, and L. Servant, Anal. Chem. 80, 1689 (2008). 4 M. Vishwanath, A. Nishibu, S. Saeland, B. Ward, N. Mizumoto, H. Ploegh, M. Boes, and A. Takashima, J. Investigative Dermatology 126, 2452 (2006). 5 B. Geest, N. Sanders, G. Sukhorukov, J. Demeester, and S. Smedt, Chem. Soc. Rev. 36, 636 (2007). 6 R. Webb and F. Rogomentich, Appl. Opt. 38, 4870 (1999). 7 S. Lindek, R. Pick, and E. Stelzer, Rev. Sci. Instrum. 65, 3367 (1994). 8 O. Bryngdahl and W. Lee, Appl. Opt. 15, 183 (1976). 9 X. Wang, B. Wang, J. Pouch, F. Miranda, M. Fisch, J. Anderson, V. Sergan, and P. Bos, Proc. SPIE 5162, 139–146 (2003). 10 A. Das and B. R. Boruah, Rev. Sci. Instrum. 82, 043702 (2011). 11 M. A. A. Neil, M. Booth, and T. Wilson, Opt. Lett. 23, 1849 (1998). 12 U. Efron, Spatial Light Modulator Technology: Materials, Devices and Applications (CRC Press, 1995), Vol. 47. 13 M. A. A. Neil, T. Wilson, and R. Juskaitis, J. Microsc.-Oxf. 197, 219 (2000). 14 B. R. Boruah, Am. J. Phys. 77, 331 (2009). 2 R.
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
