Prof. Greg Francis
Constancy
PSY 310
Greg Francis
Lecture 19
It’s all an illusion!
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Brightness illusions
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Most people think of visual perception as a
measurement of light
As it reflects off of objects
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Object identification
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A basic task of the visual system is to figure out whether changes
in intensity are due to changes in objects (color, paint) or to
changes in illumination (light, shadows, veils)
One problem is that an edge from paint could be the same as an
edge from a shadow
Steps-movie.swf
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Ambiguity
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The process of identifying an pattern of light as a set
of objects in inherently ambiguous
 There is no way that you can always be correct!
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The visual system must use some “tricks” to try to
be right as often as possible
We can expose those tricks with illusions
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Snakes illusion
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The diamonds have the exact same scale of gray
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Snakes illusion
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We see a dark transparent veil in front of some rows
Which means the paint of the diamond behind it must be
lighter than the paint of the diamond without the veil
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Shadows
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Shadows
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Shadows
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Shadows
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Shadows
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We interpret the shadow as producing a change in
intensity
Which means the object surfaces (paint) must be
different
There are lots of brightness illusions based on this
idea
It’s a side effect of our effort to judge the
properties of objects in different kinds of lighting
 We want to be able to identify an object’s properties
regardless of lighting
 Constancy (color, brightness)

Let’s look at one more
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Perceiving light
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Perceiving light
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Perceiving light
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Perceiving light
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Perceiving light
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Perceiving light
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Demonstration
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Pick up a copy of the image and randomly select a set of
instructions.
Follow the instructions and come back to the classroom
within 10 minutes
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Light levels
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People can see objects across an amazing range of light
levels
Some is due to pupil size, rod vision
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Light levels

Even within indoor lighting ranges, we see things in a wide
variety of levels of light
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Light levels

Consider a photograph of an office
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The light from outside is so bright you cannot see anything

The light on the floor is so dark you cannot see anything
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Light levels

You can change exposure settings, but nothing really works
well
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Light levels
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You can change exposure settings, but nothing really works
well
You have to go through a complicated process to take a
really good picture
 Lots of light (like a photographer studio)
 Or process and combine different pieces of information
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Photoshop
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Take the over-exposed image and exaggerate the shadows
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The makes the details in the dark places of the floor visible
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Photoshop
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Take the under-exposed image and exaggerate the bright
regions
The makes the details in the bright places of the window
visible
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Photoshop
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Combine everything in the right way, and you get a better
picture
Your brain is doing something similar, but in a much more
sophisticated way
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PSY 310: Sensory and Perceptual Processes
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Prof. Greg Francis
Conclusions
Constancy

 We want to know the properties of objects
Ambiguity

 It is not always possible to know if changes are due to
object properties of illumination properties
Visual system

 Uses clever tricks to choose
 Usually correct
 Sometimes easy to fool
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Next time
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Depth perception
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Monocular cues
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PSY 310: Sensory and Perceptual Processes
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