# Holcombe:Photometry

## Using the Spyder2Pro

For luminance in cd per meter squared, use the Y of the xyY

The best reference for luminance values that are photopic (cone vision) versus scotopic (rod vision) is Donald Hood's chapter in Handbook of perception and performance. We have a good table of luminance, retinal illuminance, pupil size, etc. values from that chapter in the lab.

## Alex White's experiment

In Alex White's buttonpress experiment we attempted to bring all stimuli down into the photopic range. To do this we got some neutral density filter sheets to put over the screen. We combined 3 filters: two 0.6ND (Lee Filters number 210) filters and one 0.9ND (Lee number 211) filter. This combined filter transmits 0.73% of light.

When using these filters on the Viewsonic G810 monitor, with contrast to max and brightness to min, the dimmest spinning bar (0.2 by 3 degrees visual angle) we could see well on a black background was 0.01 cd/m^2.

To find out if this luminance value is in the scotopic (rod-only) range, we tried to see how well we could perceive color. We did that by comparing that spinning bar with a green and a red spinning bar (painted with only the R and G guns), and adjusted the luminance of the red and green ones to see if those ever were indistinguishable from each other and from the gray bar. They weren't; Alex H, Alex W, and Dani could all still see color even in that range.

Actually for me (AH), as I take the red gun and slowly make it brighter, it seems that as soon as I can detect the bar I can see that it is red. This suggests that the cones are close to the most sensitive mechanism in this condition, probably because these stimuli are parafovea. When I look at them in the periphery just several degrees (after dark adaptation) the stimuli all look much brighter and it's harder to discriminate the colors. This tells us if we want to do a truly scotopic version of Alex's experiment, we'd have to use stimuli that extend farther into the periphery. This probably also would overcome the issue of the bars getting so fuzzy and occasionally invisible when very dim- they were too close to the rod scotoma of the fovea.--Alex O. Holcombe 05:41, 13 June 2008 (UTC) --Alex L. White 06:04, 13 June 2008 (UTC)