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That is a very well done visual illusion. This basic
effect is well-known in visual science, but the technique
for constructing this demonstration is new, and makes a
useful contribution to the world of illusions. Note that
its creator, John Sadowski, has written a nice how-to
tutorial, so anyone with Photoshop can now make their own
version from their own colored photo (perhaps of their own
science center?):
http://www.johnsadowski.com/color_illusion_tutorial.html
Like many of the best illusions, this one actually
combines multiple visual effects, including "chromatic
adaptation", "isoluminance", and "filling-in", which
synergize to make for an especially strong illusion. This
does make it more complicated to explain than just a
simple afterimage, but I'll give it a shot:
The primary effect, as already noted, is an afterimage due
to "chromatic adaptation", in which color-sensitive
neurons in your eyes adjust their sensitivities and
neutral points based on the colors they're exposed to.
That's responsible for the complementary colors seen in
this illusion. But the illusion is stronger than just a
simple colored afterimage, such as "Bird in a Cage",
because it also taps into the way colors are contained and
accentuated by luminance edges. (You can see just the
simple afterimage part of this illusion by staring at the
colored image, then switching your gaze to a blank white
screen instead of the B&W image.)
Notice that the original colored image looks quite flat
and out-of-focus. It's actually a surprisingly detailed
photograph, as you can see for yourself just by looking at
it through a red filter. We normally use the luminance
contrast in an image to see its details and depth, but
this colored image was processed in Photoshop to remove
its luminance contrast and make it approximately
"isoluminant". (Viewing it through a red filter serves to
restore luminance contrast.)
Also, notice that the second B&W image in the illusion is
exactly superimposed on the colored image. So as long as
your eyes stay reasonably fixed on the dot, the afterimage
colors will be contained within the appropriate achromatic
edges, which stabilizes them. In general, colors "fill
in" within the edges in a picture - that's why it's okay
to color only approximately within the lines. (Aside: an
interesting related effect is the division of the
spectrum, in which superimposing thin black lines on a
continuous spectrum -as from sun through a prism- seems to
divide it up into discrete colored segments.) When your
eyes move off the dot, the colors no longer align with the
edges, in effect smearing out the afterimage - this is
also why we don't constantly see distracting afterimages
in everyday life.
To give historical credit, this type of illusion was
described and analyzed in a classic 1962 paper by
neuroscientist Nigel Daw, titled "Why After-Images are not
Seen in Normal Circumstances" (Nature 196: 1143 - 1145,
1962). And if you want to see a great new variation,
check out this demonstration created last year in a Dutch
vision lab, which vividly demonstrates how much afterimage
colors depend on the edges that contain them:
http://www.nici.kun.nl/~robvl/Filling%20in%20the%20afterimage.html
-Richard the Color Scientist
On Sun, 6 Jul 2008 19:03:51 -0700
Chuck Howarth <[log in to unmask]> wrote:
> ISEN-ASTC-L is a service of the Association of
>Science-Technology Centers
> Incorporated, a worldwide network of science museums and
>related institutions.
> *****************************************************************************
>
> An after image? Yes, but... It seems to me that there
>is something else going on here. The standard
>after-image effects last for several seconds and slowly
>fade away. The effect Dennis found seems different:
> the image appears colored, and continues to appear
> colored for quite a while as long as you continue to
>look at the dot. But turn your gaze to the side, and it
>reverts to black and white. If you then look back at
>the dot, the color does not reappear, as it would with
>most other after-image effects. So I think you guys
>got some more explaining to do.....
>
> Chuck Howarth
> Gyroscope, Inc.
> 283 Fourth Street, Suite 201
> Oakland, CA 94607
> [log in to unmask]
> 510-986-0111
>
>
> On Jul 6, 2008, at 5:43 PM, Eric Yuan wrote:
>
>>
>> I'm curious whether the afterimage effect plays a role
>>in enhancing
>> vision,
>> such as detecting motion, or widening our effective
>>field of vision
>> when we
>> scan our surroundings? Maybe straightening out our
>>vision during a
>> bumpy
>> car ride?
>>
>> -Eric
>>
>>
>> -----Original Message-----
>> From: Informal Science Education Network
>> [mailto:[log in to unmask]] On Behalf Of
>>Paul Koenig
>> Sent: Sunday, July 06, 2008 8:11 PM
>> To: [log in to unmask]
>> Subject: Re: [ISEN] Very neat illusion
>>
>>
>> It's an afterimage in the eye. I know the Exploratorium
>>has a few
>> exhibits/activities based on the same thing. Here's what
>>I found after
>> a quick search for more explanation:
>> http://www.exploratorium.edu/snacks/bird_in_cage/index.html
>> http://www.exploratorium.edu/snacks/afterimage/index.html
>>
>> It's an effect I know all too well after spending a lot
>>of time with
>> anaglyph glasses.
>>
>> - Paul Koenig
>>
>> On Jul 6, 2008, at 4:50 PM, [log in to unmask]
>>wrote:
>>
>>> Dear all,
>>>
>>> My son sent me an optical illusion unlike any I've seen
>>>before. I
>>> thought you would enjoy it. Does anyone know what is
>>>happening in
>>> the
>>> eye/brain to make this work?
>>>
>>> http://www.johnsadowski.com/big_spanish_castle.php
>>>
>>> Dennis
>>>
>>> "Work hard to find something that fascinates you. When
>>>you find it
>>> you
>>> will know your lifework" -- Richard Feynman
>>>
>>> Dennis Schatz
>>>
>>> Senior Vice President for Strategic Programs
>>>
>>> Pacific Science Center
>>>
>>> 200 Second Ave. No.
>>>
>>> Seattle, WA 98109
>>>
>>> Phone - 206-443-2867
>>>
>>> Fax - 206-443-3631
>>>
>>> www.dennisschatz.org <http://www.dennisschatz.org>
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