Truelight Vision Test

The test image contains three rows of six coloured 'T' symbols in random orientations against a random luminance background. The colour contrast halves with each step to the right. You should see the first four symbols. You may not see the fifth as a 'T' shape, but you may be able to deduce its orientation. The sixth should not be visible. It has been made to have a colour contrast ΔE of 1.0 which would be just visible under ideal conditions, but not visible with the dark lines and the luminance signal.

The three rows test the LMS (Long, Medium, and Short) wavelength cone cell responses. If you cannot see beyond the first symbol, then you may be lacking one of these receptors. Your condition is called protanopia, deutranopia, or tritanopia depending on which row you cannot see. Protanopes and deutranopes both have difficulty resolving reds and greens. Reds will appear dark to a protanope, but light to a deutranope. Protanopes tend to confuse reds, greys, and greenish blues: deutranopes tend to confuse purples, greys, and bluish greens.

There is a less serious condition, where the eye has all three receptors, but one of the LMS signals is weak or noisy. If you can see all three rows but see more symbols in one row than another, your condition is called protanomaly, deutranomaly, or tritanomaly depending on which row you have most difficulty seeing.

Testing with an uncalibrated monitor

Your display should have the same gamma in each channel. If the light and dark background squares all look grey, then the gamma values are probably close enough for the test to work. The image was designed for display gamma of about 2.0. If your display gamma is different, then all the contrasts will scale slightly but the overall test should still work. You do not need a high contrast monitor.

Our L and M receptor spectral responses are quite similar. It is hard to distinguish protanomaly and deutranomaly with a test like this. If you were a protanope using a calibrated monitor, you would not see the first row and you would probably have difficulty with the second row too. If you have not got accurate video primaries or a D65 white point, then this may confuse L and M defects. S defects should be resolved. Unfortunately L and M defects are common, and S defects are rare.

Red-green defects are best measured using an instrument called an anomaloscope. The user matches yellow light by mixing red and green — something we cannot do on an RGB display without an independent yellow channel.