This is not red!

Before you start

Tick these conditions before you start: they stay ticked if you come back later.

• 01Turn off Night Shift, True Tone, blue-light filters, reading mode or “warm colours”.
• 02Screen brightness high and fixed (turn off auto-brightness).
• 03Browser zoom at 100% and a window large enough to see the whole circle.
• 04Dim room, no direct light on the screen. Stay in the dim light for a couple of minutes before starting.
• 05Sit facing the screen at about 60–70 cm.
• 06If you wear prescription glasses, keep them on. Remove tinted or “colour-blind” lenses (EnChroma and similar).

Why they matter

This is not a medical diagnosis. A screen is not a calibrated instrument: the result is a well-informed estimate. Certification requires an eye doctor’s anomaloscope.

Calibrate your screen’s gamma

Every screen turns colours into light a little differently. This step measures yours, so the test shows exactly the intended colours. It takes about a minute.

1. Look at one row at a time. On the left is the square of fine lines: it is the reference and does not change. On the right is the solid square, which you adjust.
2. Step back a couple of paces or squint until you can no longer make out the lines in the left square.
3. Move the slider until the two squares look like a single rectangle, with no visible edge in the middle.
4. Do the same for red, green and blue, then press “Save and go to the test”. It doesn’t need to be perfect: if in doubt, use the standard value.

Zoom is not 100% or the screen uses fractional scaling. The lines may look blurry and the calibration may be less reliable. Set the zoom to 100% and reload; if nothing changes, use the standard value.

Discrimination test

Keys: arrows for the direction, space or N for “can’t see it”, P to pause. Take the time you need for each answer.

Minimum-flicker photometry

This test is optional, takes about 2 minutes and helps tell protan from deutan. It measures how bright red looks to you compared with green: people with a protan deficiency see red as much darker than normal.

1. Read the warning below and press “Start”. A disc appears that switches quickly between red and green, so it seems to flicker.
2. Look at the centre of the disc and move the slider. The green always stays the same; the slider only changes the red: to the right it gets brighter, to the left darker. As you move it, the flicker gets stronger or weaker.
3. Find the point where the flicker is weakest and press “The flicker is minimal here”. It won’t vanish completely: the colours keep alternating, but the disc almost stops pulsing.
4. Repeat 4 times. Each time the slider starts from a different point. That is deliberate, so you don’t repeat your previous answer from memory.

Warning: flashing light. Skip this part if you have photosensitive epilepsy, migraines triggered by flashing light, or if it bothers you. You can skip it: the result stays valid, with less certainty about protan vs deutan.

Your result