Technical note A simple light meter for use in dermatology* K e y w o r d s - - D e r m a t o l o g y . Light meter
PART of the work of most dermatology departments involves photosensitivity testing in patients with skin disorders such as polymorphic light eruption and porphyria. The light sources employed in these tests are usually mercury, carbon or xenon arc lamps, used in conjunction with a monochromator or suitable optical filters (MAGNUS, 1971). TO avoid variations in exposure dose due to physical factors such as ageing of the lamp and optical components or instability in the electrical supply, it is necessary to calibrate the lamp. Calibration is best achieved with a thermopile and sensitive voltmeter. However, thermopiles in particr~lar are expensive and easily damaged by mechanical shock. To overcome the necessity of using a thermopile on a routine basis, measurements of light intensity may be made if a photoresistive cell is used in the circuit described below. The resistance of the photocell varies almost inversely with illumination, and therefore changes in the irradiance can be directly related to changes on a meter scale reading. The circuit shown in Fig. 1 uses an operational amplifier with the meter in the feedback loop. The advantages of such a configuration are twofold. First, the junction of the photocell, feedback loop and amplifier input is at earth potential, causing a fixed voltage to be applied across the photocell from the potential dividers; and, secondly, the
current flowing through the meter is of equal value to that flowing through the photocell. A simple circuit of photocell and meter in series is not practicable since, under certain conditions of light intensity, the resistance of the photocell is very much lower than that of the meter, rendering changes in photocell resistance ineffective in changing the meter current. The value of maximum current flow is limited to I00/zA to avoid selfheating of the photocell and current drain on the battery. The circuit is guarded against fluctuating battery voltage by a Zener diode. The switched attenuators are to enable five decades of light intensity to be measured, and the sixth position provides a battery level check. With the photocell disconnected, the voltages across the five resistors, 3"3kf~--0.3f~, were separately adjusted using the trimmers and set to be 3.0 V, 0.3 V, 0.03 V, 3 mV, and 0' 3 mV, respectively. This procedure produced half-scale deflection on the meter when the photocell resistance is 60 kf~, 6 k~, 600 ~, 60 f~ and 6 f~, respectively. These values of photocell resistance correspond to typical levels of illumination obtained from different light sources. It should be noted that the sensitivity of the photocell is nonlinear with wavelength. The photocell will respond to radiation in the wavelength range of approximately 290-950nm, with a maximum response at about
+
9v
270~ 1O0k.O0
@
6'2V
_ ,10
kCt
S1-a
I
o
o
Mullard
50k~
ORP12
100MA
@
0
o
33 k.Q.
Sl-b
in:
I
First received 6th December 1974 and in final form 24th January 1975
Q
Medical
Sl-c
~330
and Biological Engineering
-9V
c:~..~. I Circuit diagram o f the light meter
January 1976
101
620 rim. Consequently, the device may only be used where the spectral quality of the radiation remains unchanged during treatment. In order that the instrument may be used to give an indication of beam irradiance (in mW cm- 2), it is necessary to calibrate the light meter over a suitable range of intensities for each irradiation condition that is encountered. This is normally achieved by comparison with an absolute detector such as a thermopile, whose response is independent of wavelength over a large range. Alternatively, the light meter may be used in conjunction with a thermopile to show comparative changes in beam intensity between absolute determinations of irradiance by the thermopile. The light meter described above is simple, inexpensive and readily built by hospital technicians. It is sensitive to all light sources found in a typical dermatology department, and has proved to be useful since it is reliable,
102
robust and may be used by nontechnical personnel. B. L. DIFFEY T. R. ELKERTON
Medical Physics Department Kent and Canterbury Hospital Canterbury, Kent England M. F. DIPROSE
Medical Electronics Department University of Kent at Canterbury Canterbury, Kent England Reference
MAGNUS, I. A. (1971) in BORRIE,P. (Ed.) Modern trends in dermatology 4 (Butterworth), 150-175.
Medical and Biological Engineering
January 1976
PART of the work of most dermatology departments involves photosensitivity testing in patients with skin disorders such as polymorphic light eruption and porphyria. The light sources employed in these tests are usually mercury, carbon or xenon arc lamps, used in conjunction with a monochromator or suitable optical filters (MAGNUS, 1971). TO avoid variations in exposure dose due to physical factors such as ageing of the lamp and optical components or instability in the electrical supply, it is necessary to calibrate the lamp. Calibration is best achieved with a thermopile and sensitive voltmeter. However, thermopiles in particr~lar are expensive and easily damaged by mechanical shock. To overcome the necessity of using a thermopile on a routine basis, measurements of light intensity may be made if a photoresistive cell is used in the circuit described below. The resistance of the photocell varies almost inversely with illumination, and therefore changes in the irradiance can be directly related to changes on a meter scale reading. The circuit shown in Fig. 1 uses an operational amplifier with the meter in the feedback loop. The advantages of such a configuration are twofold. First, the junction of the photocell, feedback loop and amplifier input is at earth potential, causing a fixed voltage to be applied across the photocell from the potential dividers; and, secondly, the
current flowing through the meter is of equal value to that flowing through the photocell. A simple circuit of photocell and meter in series is not practicable since, under certain conditions of light intensity, the resistance of the photocell is very much lower than that of the meter, rendering changes in photocell resistance ineffective in changing the meter current. The value of maximum current flow is limited to I00/zA to avoid selfheating of the photocell and current drain on the battery. The circuit is guarded against fluctuating battery voltage by a Zener diode. The switched attenuators are to enable five decades of light intensity to be measured, and the sixth position provides a battery level check. With the photocell disconnected, the voltages across the five resistors, 3"3kf~--0.3f~, were separately adjusted using the trimmers and set to be 3.0 V, 0.3 V, 0.03 V, 3 mV, and 0' 3 mV, respectively. This procedure produced half-scale deflection on the meter when the photocell resistance is 60 kf~, 6 k~, 600 ~, 60 f~ and 6 f~, respectively. These values of photocell resistance correspond to typical levels of illumination obtained from different light sources. It should be noted that the sensitivity of the photocell is nonlinear with wavelength. The photocell will respond to radiation in the wavelength range of approximately 290-950nm, with a maximum response at about
+
9v
270~ 1O0k.O0
@
6'2V
_ ,10
kCt
S1-a
I
o
o
Mullard
50k~
ORP12
100MA
@
0
o
33 k.Q.
Sl-b
in:
I
First received 6th December 1974 and in final form 24th January 1975
Q
Medical
Sl-c
~330
and Biological Engineering
-9V
c:~..~. I Circuit diagram o f the light meter
January 1976
101
620 rim. Consequently, the device may only be used where the spectral quality of the radiation remains unchanged during treatment. In order that the instrument may be used to give an indication of beam irradiance (in mW cm- 2), it is necessary to calibrate the light meter over a suitable range of intensities for each irradiation condition that is encountered. This is normally achieved by comparison with an absolute detector such as a thermopile, whose response is independent of wavelength over a large range. Alternatively, the light meter may be used in conjunction with a thermopile to show comparative changes in beam intensity between absolute determinations of irradiance by the thermopile. The light meter described above is simple, inexpensive and readily built by hospital technicians. It is sensitive to all light sources found in a typical dermatology department, and has proved to be useful since it is reliable,
102
robust and may be used by nontechnical personnel. B. L. DIFFEY T. R. ELKERTON
Medical Physics Department Kent and Canterbury Hospital Canterbury, Kent England M. F. DIPROSE
Medical Electronics Department University of Kent at Canterbury Canterbury, Kent England Reference
MAGNUS, I. A. (1971) in BORRIE,P. (Ed.) Modern trends in dermatology 4 (Butterworth), 150-175.
Medical and Biological Engineering
January 1976
