The document summarizes key elements of visual perception. It describes the main structures of the human eye including the cornea, iris, lens, retina, rods and cones. The retina contains two types of light receptors - rods and cones. Rods support low light vision while cones are involved in color vision and visual acuity. It also discusses brightness adaptation, discrimination thresholds, the Mach band effect and simultaneous light contrast - how the perceived brightness of an object depends on the brightness of the surrounding area.

1. Elements of Visual Perception
• Structure of human eye
Nerves & sheath
retina
scelera
choroid
Visual axis
lens
iris
cornea
fovea
anteriror
chamber

2. Structure …
• Figure shows a horizontal cross section of
human eye.
• Eye is nearly a sphere with approximate
dia of 20 mm.
• Three membranes enclose the eye;
cornea and sclera form outer cover ; the
choroid ; and the retina.
• Cornea is a tough transparent tissue that
covers anterior chamber of eye

3. Structure…
• Choroid is directly below the sclera; the
membrane contains a network of blood vessels
that serve as source of nutrition to the eye.
• The central opening of the iris (the pupil) varies
in diameter from approximately 2 to 8 mm.
• The inner most membrane of the eye is retina;
when eye is properly focused, external object is
imaged on the retina

4. Structure..
• Two types of light receptors over the surface of
retina: cones and rods.
• Cones numbering between 6 and 7 million are
located near the central portion of retina called
fovea
• Cones are highly sensitive to colour and help to
resolve fine details of image; cone vision is
called photopic.
• The number of rods is about 75million to 150
million; rods are distributed over retinal surface

5. Structure…
• Rods serve to give a general , overall
picture of the field of view ; rods are not
involved in colour vision and are sensitive
to low levels of illumination; rod vision is
called scotopic.
• The fovea which contains the cones is a
circular indentation of 1.5 mm dia. in the
retina.

6. Brightness adaptation &
discrimination
Subjective
Brightness
Log of intensity (mL)
Ba
Bb
photopic
scotopic
0-2-4-6 2 4

7. Brightness…
• Subjective brightness is a logarithmic
function of light intensity
• The range of light intensity to which
human vision can adapt is of the order of
10^ 10.
• In photopic vision alone the range is 10^6.
• But human vision cannot operate over the
above range simultaneously

8. Brightness…
• The whole range is covered through
brightness adaptation
• At brightness level Ba, the range of
subjective brightness is denoted by Bb
only; Higher levels of brightness are
covered by simply raising the adaptation
level Ba.

9. Brightness…
• Ratio of change in intensity of an object
over the intensity of the background which
human eye can discriminate 50% of the
time is called Weber ratio
• Larger Weber ratio means poor
discrimination ; smaller ratio means good
discrimination.
• Weber ratio is high for scotopic vision;
smaller for photopic vision

10. Mach band effect
• Mach bands
Due to mach band effect

11. Simultaneous contrast
• Perceived brightness of a region does not
depend simply on its intensity alone
• Squares of same intensity appear darker
as the background gets lighter.
• Optical illusion patterns are also well
known

12. Simultaneous Contrast

13. Optical Illusion