In order to maintain the isoluminance of the colored stimuli to that of the dimmer (gray) stimuli, measurements of the luminance of the monitor display were taken at various RGB values for each of the red, green, blue, and gray/white stimuli and plotted in
Figure 1.
The maximum nonwhite stimulus luminance of 21 cd/m
2 was limited by the maximal luminance of the blue stimulus (see the dotted line in
Fig. 1), and the approximate R (red stimulus); G (green stimulus); and RGB (gray stimulus) values were then interpolated from this graph. Spectrophotometer measurements on the stimuli were then made in experimental conditions to fine-tune the interpolated RGB values to all, given a measured luminance of 21 cd/m
2 for all nonwhite stimuli. The RGB values required to do this were as follows: red stimulus, R = 155, G = 0, B = 0; blue stimulus, R = 0, G = 73, B = 0; green stimulus, R = 0, G = 0, B = 255; gray stimulus, R = 71, G = 71, B = 71. These RGB settings displayed stimuli with
x,
y chromaticity coordinates as measured by the spectrophotometer as follows: white/gray,
x = 0.331,
y = 0.349; red,
x = 0.616,
y = 0.338; green,
x = 0.316,
y = 0.559; and blue
x = 0.161,
y = 0.081. A spectroradiometer (Radoma GS-1240; Gamma Scientific, San Diego, CA, USA) was used to measure the peak wavelength of the colored stimuli, and gave measured peak wavelength values of red, green, and blue as 615, 538, and 447 nm, respectively.
The room in which the experiment was undertaken was illuminated by an incandescent 40-W corner stand-lamp, creating a subdued ambient illumination, in order to prevent dark adaptation. All illuminance/luminance measures were taken at the plane of the patient's eyes. The monitor was left on, displaying the large bright white circle calibration stimulus for 20 minutes before any measurements or testing. The monitor displayed bright white stimuli of 250 cd/m
2 luminance, and dimmer gray or colored (red, green, or blue) stimuli of 21 cd/m
2 luminance. The brighter stimuli luminance equaled the luminance of the ETDRS lightbox charts with ambient room lighting on, at which many ACHM patients had previously reported PA anecdotally during routine VA measurements. The black background screen on the monitor (luminance 1 cd/m
2) had an illuminance of 4.2 lux in the ambient lighting.
Figure 2 shows the 20 stimuli that were randomly presented during each of the three trial runs.
The illuminance of the large bright stimuli was 36.0 lux for both the peripheral and central stimuli, and for the large dim or colored stimuli was 7.0 lux for both the peripheral and central stimuli. These values for the small stimuli were 13.2 lux (bright) and 5.4 lux (dim or colored), respectively.
The large circle stimulus subtended a visual angle of 25.4°, and the small circle 12.7°. All of the peripheral (annulus) stimuli also had an inner diameter of 12.7°, with the outer diameters subtending 28.2° (large annulus) and 18.1° (small annulus). All large stimuli (central circle or peripheral annulus) had an area four times that of the smaller stimuli (central circle or peripheral annulus).
Before testing of any subject, and after the display monitor had been left on for 20 minutes, a luminance reading was taken of a standard calibration screen (the large white circle stimulus) to verify that the luminance was 250 cd/m2, and of each of the dim, red, green, and blue stimuli to also verify that their luminance was 21 cd/m2 as expected. The patient's head rested in a chin-rest supported on a height-adjustable table, maintaining the patient's eyes 40 cm from the monitor display screen.