What is the checker shadow illusion test?
This checker shadow illusion test lets you find out how much a shadow changes the shade you see. Instead of just looking at the famous picture, you set one tile by eye to match another, five times over, and only then does the page lift both tiles out of the picture and show what they really were. Edward Adelson of MIT published the illusion in 1995; this version redraws his board in 3D.
This test measures how far a shadow moves your judgement of a tile’s shade. Each round is a checkerboard in a small studio, with one object casting a shadow across it. You make tile B look the same as tile A with Darker and Lighter (or by dragging), then press Same; the two tiles lift out onto a plain card and the floor shows what B really is. Round 1 has no shadow on the pair, a warm-up that measures plain matching. Rounds 2 to 5 get harder by design: a deeper shadow, a softer edge, tiles further apart, ending on Adelson’s own greys. Each round is scored in perceived lightness (CIE L*) against its own shadow: 0% means you matched the screen, 100% means you matched the paint under the shadow.
Whatever you score, it is not an eye exam or a diagnosis, and the result sheet explains what your visual system was doing once the last round is done. No published study gives a population norm for this matching version, so the result has no percentile; the tiers are the site’s own rough bands, and the order of the rounds is a design choice, not a measured difficulty scale. Screen brightness, glare and the angle you hold your phone at all change the result.
What the test measures
One thing: how much a shadow changes the shade you see on a surface.
The task
- Edward H. Adelson’s checker shadow illusion (1995), redrawn in 3D: five checkerboards, each with one object casting a shadow
- The method of adjustment: make tile B darker or lighter until it looks like tile A, then press Same
- A fixed climb: no shadow (a warm-up), a pale sharp shadow, A in the shadow, a deeper and softer shadow with the tiles further apart, then the deepest shadow on Adelson’s own greys
- Each board’s tiles share one tint and B only gets lighter or darker, so lightness is the only thing to match
Your result
- ‘Shadow erased’: how far you set B, as a share of that round’s shadow depth in CIE L*, averaged over the four shadow rounds
- Each round’s own number, and how closely you matched with no shadow at all
- A tier from ‘camera eyes’ to ‘completely fooled’, using the site’s own bands; no percentile
- A shade report with each pair lifted out of the picture and why the shadow did what it did, a share card that doesn’t give the answer away, and a challenge link for a friend
Before you start
- About two minutes, five rounds
- Turn the screen brightness up, avoid glare and look at the screen straight on
- Works on phones and computers; sound is optional
Checker shadow illusion FAQ
Is this test free? Do I need to sign up to see my result?
Yes, it’s free, and there’s nothing to sign up for: no account, no email, no payment. Your result appears on this page the moment you finish, and you can share it or play again straight away.
What is the checker shadow illusion?
The checker shadow illusion is a picture of a checkerboard with a green cylinder casting a shadow across it, published by Edward H. Adelson at MIT in 1995. Two tiles, A and B, are marked on it, one in the light and one in the shadow, and the picture asks which of the two is darker. This page doesn’t answer that here: the test lets you judge for yourself first, then lifts the tiles out of the picture so you can see.
Why does a shadow change how dark a square looks?
The result sheet explains it after the fifth round, next to each pair of tiles lifted out of the picture. Knowing the reason first would change how you set the tiles, so the page keeps it until you have finished. Until then, the test simply asks you to match by eye; the sources below link Adelson’s original 1995 page.
How is ‘shadow erased’ calculated?
‘Shadow erased’ compares how far you set tile B with how deep that round’s shadow really is, both in perceived lightness (CIE L*). 0% means you matched the grey on the screen; 100% means you matched the paint under the shadow; below 0% you went past the screen’s grey the other way. Because each round is scored against its own shadow, a pale one and a deep one count alike, and the headline is the mean of the four shadow rounds.
What is a normal score on the checker shadow illusion?
There is no published norm for this matching version. We found no study that measured the checker shadow illusion by matching and reported how people spread out, so the test gives no percentile. The tiers — under 15% of the shadow erased, under 45%, under 80% and above — are the site’s own rough bands.
Is it bad to be fooled by the checker shadow illusion?
No. Being fooled by the checker shadow illusion says nothing about your eyesight, and the test is not an eye exam. Whatever your number, the result sheet places you in one of the site’s own tiers, from ‘camera eyes’ to ‘completely fooled’, and explains what your visual system was doing while you matched the tiles.
Why did I get a different result on another screen?
Screen brightness, glare, colour modes and viewing angle all change the greys you actually see, so the same board can read differently on two devices. Later rounds also come after practice, which is why the report gives each round’s own number rather than claiming how much stronger the illusion gets from round to round.
Where the illusion comes from
The boards follow Adelson’s 1995 figure; the last round uses its greys. ‘Shadow erased’ is computed in CIE L*, each round against its own shadow. No published study gives a population norm for this matching version, so there is no percentile, and the tiers and the order of rounds are the site’s own. The site does not publish its own players’ data.