1. SEMINAR
ON
“COLOR MODEL”
Submitted by:
Sadhana Singh
Shri Ram Murti Smarak College of Engg. And Tech.
Bareilly

2. Color
 Color is a psychophysical concept depending both upon the
spectral distribution of the radiant energy of the illumination
source and the visual sensations perceived by the viewer
 Color perception depends mainly upon the physics of light
and the physiology of the visual system, which results in the
following psychological color sensations:
•hue: the color sensation associated with different parts of the spectrum such as
red, yellow, or blue
•saturation: the color sensation corresponding to the degree of hue in a color
•brightness is the primary visual sensation

3. Color Science
 Light is an electromagnetic wave. Its color is characterized by
the wavelength content of the light.
 Laser light consists of a single wavelength: e.g., a ruby
laser produces a bright, scarlet-red beam.
 Most light sources produce contributions over many
wavelengths.
 However, humans cannot detect all light, just
contributions that fall in the “visible wavelengths”.
 Short wavelengths produce a blue sensation, long
wavelengths produce a red one.
 Visible light is an electromagnetic wave in the range 400
nm to 700 nm (where nm stands for nanometer, 109
meters).

4. Human Vision
 The eye works like a camera, with the lens focusing
an image onto the retina (upside-down and left-right
reversed).
 The retina consists of an array of rods and three kinds of cones.
 The rods come into play when light levels are low and produce an
image in shades of gray
 For higher light levels, the cones each produce a signal. Because of
their differing pigments, the three kinds of cones are most sensitive to
red (R), green (G), and blue (B) light.

5. Image Formation
 Surfaces reflect different amounts of light at
different wavelengths, and dark surfaces reflect
less energy than light surfaces.
 Fig. shows the surface spectral reflectance from
(1) orange sneakers and (2) faded bluejeans.
The reflectance function is denoted S().

6. Image Formation

7. Image Formation
 So image formation involves an illuminant
with SPD E() reflects off a surface with
spectral reflectance function S() and is
filtered by the eyes’ cone functions q() as
shown in figure 4.5
 The function C() is called the color signal and
is formed by the product of the illuminant and
the reflectance

8. Color Systems
 Combinations of three primary colors can match any
unknown color for observers with normal color vision
 Often, we choose red, green, and blue as the three primary
colors, and we can then represent some color C by a
mixture of red, green, and blue:
 C = rCR + gCG + bCB
 RGB is the color model (a conceptual system for
specifying colors numerically) used in computer monitors
 This model is additive

9. Color Systems
 CMYK is the color model used by printing presses
 This model is subtractive
 Light is absorbed, or subtracted by cyan, magenta, and
yellow ink
 In process-color printing, layers of translucent inks are
used, each subtracting certain colors of light
 Colors that are not absorbed pass through to the paper below which reflects all
color
 For example, magenta ink looks magenta because it allows magenta light to
pass through but absorbs all other colors

10. RGB Color Model for CRT Displays
 We expect to be able to use 8 bits per color channel
for color that is accurate enough.
 However, in fact we have to use about 12 bits per
channel to avoid an aliasing effect in dark image
areas - contour bands that result from gamma
correction.
 For images produced from computer graphics, we
store integers proportional to intensity in the frame
buffer. So should have a gamma correction LUT
between the frame buffer and the CRT.
 If gamma correction is applied to floats before
quantizing to integers, before storage in the frame
buffer, then in fact we can use only 8 bits per channel
and still avoid contouring artifacts.