# Brightness and Lightness - UMD Physics

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 was-specially famous for his researches into aural and cutaneous (skin) sensations. Webers 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 Webers 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 webers law Lateral inhibition: 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 webers 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

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