This guide explains how to measure luminance (cd/m^2) of visual stimuli for reporting purposes.

Step 0: Update firmware only if needed

If your computer does not recognize the device, you may need to update the firmware. In many cases, the ColorCAL 3 is detected automatically.

1. Manual method

This method is simple and fast. If your stimulus contains only a few components, such as a background and a disk, and you only want to report luminance values in a paper, this is usually enough.

Setup

• Use the same display and settings as in the experiment.
• Present stimuli full-screen.

Connection

1. Plug in the ColorCAL 3.
2. In Terminal, list connected USB devices:

ls /dev/tty.usb*

This command will list devices connected through USB. Find the one named something like usbmodemXXXX.

1. Connect to the device:

screen /dev/tty.usbmodemXXXX

1. Type ? and press Enter to open the device menu and confirm the connection.

At that point, the device is ready to take measurements.

Measurement

1. Display the stimulus you want to measure, such as the background or the target.
2. Place the device flat at the center of the screen.
3. Type M and press Enter. The M may not be visible in the terminal.
4. The device output should look like this:

OK00, Y, x, y

Use Y as the luminance value in cd/m^2.

• Take 5 to 10 measurements per condition.
• Record the values and compute the mean.

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2. Automated method with PsychoPy

Use this method if you want to record luminance values repeatedly from code.

Setup

• Plug the ColorCAL 3 into the experiment computer.
• Use the same display settings as in the experiment.

Example code

from psychopy.hardware.crs.colorcal import ColorCAL
from psychopy import visual

win = visual.Window(fullscr=True, color=[0, 0, 0])
cc = ColorCAL()

# Example stimulus
stim = visual.Rect(win, width=2, height=2, fillColor=[0, 0, 0])

levels = [-1, -0.5, 0, 0.5, 1]

for level in levels:
    stim.fillColor = [level, level, level]
    stim.draw()
    win.flip()
    luminance = cc.getLum()
    print(level, luminance)