Transcript Brightness and Lightness

Brightness and Lightness

Brightness: Describe the intensity of
the light sources such as sun, candle,



Dark, dim, bright, dazzling…
Sensation depends on adaptation. The same source
may produce different feeling at different time
Lightness: Describe the appearance of
the surfaces:


Black, dark gray, light gray and white..
Do not depends on adaptation and illumination.
Homework Set 6:
Due Friday, April 2
From Chapter 7:
P2, P8, P11, P14,
P17, P18
Lightness Constancy

The lightness is relative, does not depend
on the brightness.


During the day, a black cat is black, red flower is
red. At night, you see the same, although the light
intensity has changed a lot!
All objects appear to maintain their
familiar lightness when the lighting
condition changes.

The handkerchief always appears white even
though a light meter measure less light coming from
it at night than from a sunlit black cat!
WEBER'S LAW


The increase of stimulus necessary to produce
an increase of sensation in any sense is not a
fixed quantity but depends on the
proportion which the increase bears to the
immediately preceding stimulus. (psychophysics)
Enunciated by the German scientist, Ernst Heinrich
Weber (1795-1878), who became professor at Leipzig
(of anatomy, 1818, of physiology, 1840). He wasspecially famous for his researches into aural and
cutaneous (skin) sensations.
Weber’s Law


Equal steps in lightness arise from steps of
equal ratio in light intensity (logarithmic scale)
 1, 2, 4, 8, 16 … has equal steps in lightness
 1, 2, 3, 4, … does not have equal step (3 is
much closer to 4 than 1 is to 2.
Limitations:

Beyond certain brightness, your visual
system no longer respond to the increased
light. The same thing happens in the
opposite limit.
Why do our vision and hearing obey
Weber’s law?


Allow a maximum sensitivity to different
levels of signal.
The range of light intensity that we can
be sensitive to is enormous.

Bright sunny day vs. very dim star light,
the intensity varies by billions of times!
Tricks of the eye and wisdom of the
brain!



Most people assume that what you see is pretty
much what your eye sees and reports to your brain…
But, your brain adds very substantially to the
report it gets from your eye, so that a lot of
what you see is actually "made up" by the
brain!
Perhaps even more interestingly, the eye
actually throws away much of the information
it gets, leaving it to the rest of the brain to fill
in additional information in its own ways.
Mechanism of the lightness
constancy and weber’s law

Lateral inhibition:


Increased illumination of one region of the
retina diminishes the signal to the brain
from a neighboring region
When the overall light-intensity
increases, the lateral inhibition is
mutual, and the signal is largely ignored
by brain. (the result is the weber’s law!)


At the first processing step, each
photoreceptor generates a signal related to
the intensity of light coming from a
corresponding point of the observed object.
Photoreceptors corresponding to brighter
arrays of the object (yellow) receive more
light and generate larger signals than those
corresponding to darker areas (black).
<-light source
<- Neuron Output
response


Output neurons well to the right of the dark/light
border are excited by an overlying photoreceptor but
also inhibited by adjacent, similarly illuminated
photoreceptors. The same is true far to the left of the
dark/light border.
Equal illumination of exciting and inhibiting
photoreceptors balances out, output neurons far from
the edge in either direction will have the same output
signals
Receptive field

Refers to a region of retina which will
produce a signal to the brain,
depending on the pattern of the light
falling on it.
At the top of the figure are some
receptors. Below them are two kinds
of synapses (neural connections):
Excitation synapses are ones that
increase neural activity and inhibitory
synapses decrease neural activity.
The concentric circles represent the
neural activity recorded with the
electrode when the receptors are
stimulated with light. When one or
all of the center receptors are
stimulated, an excitatory increase in
neural activity is obtained at the
electrode. When the receptors labeled
surround are stimulated an inhibitory
decrease in neural activity is obtained.
Which one gives you a maximum response?


As one approaches the dark/light border from the
left, the signals will decrease, because inhibition
from more brightly lit photoreceptors to the right will
outweigh the excitation from the overlying dimly lit
photoreceptors.
As one approaches the dark/light border from the
right, the signals will increase because excitation
from brightly lit photoreceptors is not completely
offset by inhibition from the dimly lit photoreceptors
to the left.
Amplifying the contrast



When there is a contrast, it gets
amplified through the lateral inhibition.
The brain is made more aware of the
difference in lightness than it would be
without the lateral inhibition.
In particular, the edges, where the light
intensity changes rapidly from brighter to
darker, are made more noticeable (edge
enhancement).

Efficient in storage and transmission of
information.


The brain only need remember the edges.
Same lightness distributions: lightness
constancy
Simultaneous lightness
contrast


Lightness is affected by the
neighboring region
Illusions!



The same gray rectangle appears darker
when surrounded by white than by black.
Hermann grid illusion.
Apparent non-uniformity in the uniform
change of the lightness.
Hermann grid illusion
An example with color