Adelson’s checker shadow illusion

More optical illusions…

From Wikipedia:

The same color illusion — also known as Adelson’s checker shadow illusion, checker shadow illusion and checker shadow — is an optical illusion published by Edward H. Adelson, Professor of Vision Science at MIT in 1995. The squares A and B on the illusion are the same color (or shade), although they seem to be different. This can be proven by copying the image into an art program and sampling the color of A and then of B, which will show that they are in fact the same color.

“When interpreted as a 3-dimensional scene, our visual system immediately estimates a lighting vector and uses this to judge the property of the material.”

The image shows what appears to be a black and white checker-board with a green cylinder resting on it that casts a shadow diagonally across the middle of the board. The black and white squares are actually different shades of gray. The image has been constructed so that “white” squares in the shadow, one of which is labeled “B,” are actually the exact same gray value as “black” squares outside the shadow, one of which is labeled “A.” The two squares A and B appear very different as a result of the illusion. A second version of the same picture includes a rectangular bridge connecting square A and B to show they are the same shade of gray.

~ by metousiosis on January 13, 2009.

8 Responses to “Adelson’s checker shadow illusion”

  1. Let me put it this way: If you switched the lights off, would that make B black? Would you consider yourself corrected about B’s status as a white square if your light meter gave you a zero from the site?

  2. Of course you can get the same reading with a colour picker, but how would that make B the same colour as A? And why is it such a big surprise to get a dark reading off a whitish square when it is in shade? I think you can only be amazed by this if you’re not an artist. Any artist painting a scene like this would skip the white on his palette and pick up a dark shade of grey to paint B. If his palette were limited he’d pick up the colour he used for A without a moment’s thought.

  3. I’m sorry to come with a bit of wet-blanket scepticism here but I don’t understand why this is an illusion. A is black-ish and B is white-ish but B is in shade so it is no surprise that it is rendered darker.

    If this is an illusion, in which respect is one deluded? Is it supposedly an illusion that B is white-ish while in fact it is not?! Like, if I told you that B was white-ish you’d be inclined to prove me wrong?

    • The squares marked A and B are the same color of grey, hence the illusion.

      • OK, so you probably think I’m just another pathetic case who doesn’t get it. Actually I was hoping to gather some material for a non-dualist theory of human perception. Most scientifically-minded people nowadays are effectively dualist and this so-called illusion brings it out very nicely. In their need to satisfy their dualist assumptions they are prepared to set aside all common sense and even basic logic.

      • Is that a typo?

      • Heehee. Well, you’d know if you’d understood any of it.

        Obviously I haven’t managed to pique your interest. I’ll leave you to it then.

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