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Color Science: Understanding the Psychophysics, Physics, and Perception of Color, Slides of Multimedia Applications

Amity University - BiharMultimedia Applications

An in-depth exploration of color science, covering the psychophysical concept of color, the physics of light, and the physiology of the visual system. Topics include color perception, human vision, image formation, color systems, and gamma correction. Learn about the role of rods and cones in the eye, the sensitivity of the eye to different wavelengths, and the tristimulus values required to characterize a color.

Typology: Slides

2012/2013

Uploaded on 04/23/2013

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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
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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

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, 10 9 meters).

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.

Human Vision

  • The eye is most sensitive to light in the middle of the visible spectrum. - The sensitivity of our receptors is also a function of wavelength

Image Formation

  • Surfaces reflect different amounts of light at different wavelengths, and dark surfaces reflect less energy than light surfaces.
  • Fig. 4.4 shows the surface spectral reflectance from (1) orange sneakers and (2) faded bluejeans. The reflectance function is denoted

S (λ).

Image Formation

Image Formation

Image Formation

  • The resulting equations, taking into account reflectance are as follows

Gamma Correction

  • The light emitted is in fact roughly proportional to the voltage raised to a power ; this power is called gamma , with symbol γ.
  • (a) Thus, if the file value in the red channel is R , the screen emits light proportional to R γ, with SPD equal to that of the red phosphor paint on the screen that is the target of the red channel electron gun. The value of gamma is around 2.2.
  • (b) It is customary to append a prime to signals that are gamma-corrected by raising to the power (1 / γ) before transmission. Thus we arrive at linear signals :

Gamma Correction

  • Fig. 4.6(a) shows light output with no gamma- correction applied. We see that darker values are displayed too dark.
  • This is also shown in Fig. 4.7(a), which displays a linear ramp from left to right.
  • Fig. 4.6(b) shows the effect of pre-correcting signals by applying the power law R^1 /^ γ; it is customary to normalize voltage to the range [0,1].

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 = r (^) CR + g (^) CG + b (^) CB
  • RGB is the color model (a conceptual system for specifying colors numerically) used in computer monitors
  • This model is additive

Color-Matching Functions

  • The amounts of R, G, B to match any color have been found by experiment as shown below

Color-Matching Functions

Color-Matching Functions

  • For a general SPD E ( λ), the essential “colorimetric” information required to characterize a color is the set of tristimulus values X , Y , Z defined in analogy to (Eq. 4.2) as ( Y == luminance ):