Holcombe:Photometry

SUPA Sydney University Perception and Action Lab Primer editing help refcard

Monitor calibration

• Ione Fine has a good explanation of monitor calibration
• OSX has built-in software that uses visual ramps to allow you to estimate gamma. It then creates an .icc monitor calibration file:

1. Open System Preferences->Displays->Color. Click "Calibrate" on the display you need an estimate of the gamma for.
2. Follow the Display Calibrator Assistant's instructions. Take note of the Native Gamma and Native White Point and don't set a target for either of these settings, simply keep the native settings.

• Psychopy also has a pair of demos to help estimate gamma. Just open and run "gammaMotionNull.py" from the "Demos" menu. The program requires you to press the up/down cursor keys depending on the apparent direction of motion of the bars. At the end of the program, it will allow you to save the data in two files. If you then run the "gammaMotionAnalysis.py" program on the data files (command-click to select multiple files at once), it should give you an estimate of screen gamma (the number labeled "threshold" above the right hand graph). Note. try not to move the mouse after the results have loaded in the "gammaMotionAnalysis.py" program as this may cause the program to crash.

Using the Spyder2Pro or Spyder3Elite

After acquiring an estimate of the screen's gamma, the spyders can be used in conjunction with this psychopy program (Media:Test.zip) to test the screen luminance grating.

Instructions: The luminance of the first screen should equal the average of the luminances of the darker and lighter bars in the second and third screens. Press 'q' or 'escape' to quit, 'space' to progress to the next screen

Using Spyder2Pro/Spyder3Elite

Initial notes:

• It's best to measure screen luminance in a completely dark room.
• If you need the software code for the Spyder3Elite installation, it's on a post-it note on the front of the installation CD sleeve.
• The third screen in Test.py (with full contrast black and white bars) may give slightly inaccurate luminance measurements for the black bars, so take this into account when calculating the average.
• When removing the Spyder3Elite from a CRT, gently pull the small tab on the suction pad to release the suction.

1. If calibrating a CRT and using Spyder2Pro, remove the plastic filter from the face of the device, making the suction pads accessible to the screen. If calibrating a LCD, replace the filter on the Spyder2Pro, remove the suction pad from the Spyder3Elite and use the counterbalance to position the Spyder on the screen.
2. Plug the spyder into a USB port directly (not through a keyboard or USB hub).
3. Start the Spyder2Pro (Spyder2Pro 2.2) or Spyder3Elite software as appropriate.
4. Ignore the wizard which starts up and instead open Tools->Colorimeter (Shortcut: command-M). If using the Spyder2Pro, place the spyder on the screen and click initialise.
5. Load the Test.py program code into the psychopy coder window and set the "screen=" parameter of the "win" object to the appropriate screen (0 for the Primary screen (the one with the OSX menu bar on it), 1 for the secondary screen etc.)
6. Before you begin, create/modify the profile for the monitor being calibrated in the Monitor Center (Tools->Monitor Center). On the right hand side fill in the gamma values under linearisation with the gamma estimate obtained above. Fill in details about any modifications you made under "Notes", and save the profile before exiting monitor center.
7. Enter the name of the appropriate monitor profile as it appears in Monitor Center between the quotes for the "monitor=" parameter of the "win" object.
8. Follow the instructions in red (also found above), measuring the luminance by placing the spyder on the appropriate part of the screen and clicking "Take Reading". For luminance in cd per meter squared, use the Y of the CIE xyY
9. The luminance of the first screen should equal the average of the luminances of the bars in the second and third screens.

Using the Photo Research Spectroradiometer (SpectraScan 670)

It should be noted that the USB cable that comes with this device is only for charging the battery, so it's unlikely you'll need to use it. All navigational terms used in this guide assume the spectroradiometer is facing you (I.e. the LCD display is not visible)

1. Plug the power adapter into the small port on the right hand side of the device, next to the mini-USB port.
2. Screw the lens into the front of the device next to the compartment that holds the battery.
3. Insert an SD card into the card slot, found beneath the eye-piece on the back right of the device.
4. Switch the spectoradiometer on by pressing the power button found at the top of the button panel on the back left of the device.
5. Wait for it to initialise. Once it's ready, the LCD screen at the back of the device will display serial number and ownership information (property of the University of Sydney).
6. The LCD screen is touch-responsive. Press the button in the top left hand corner of the screen labelled Menu.
7. The first thing you want to do is calibrate the screen. The option to do this is under utilities in the main Menu.
8. Next, under ????? in the main Menu, create a new file on the SD card within which you can store your data:

General info about photopic,scotopic

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)