Boynton, Robert M. (1979),

Human Color Vision (New York:

Holt, Rinehart and Winston).

Wandell, Brian A. (1995),

Foundations of Vision (Sunderland, Mass.: Sinauer Associates).

Perception and

Properties of human vision are central to image system engineering. They determine how many pixels need to be provided per degree of picture angle, and how many bits are necessary to represent luminance (or tristimulus) levels. This chapter introduces the luminance discrimination and spatial properties of vision that inform image system engineering choices.

Retina

The human retina has four types of photoreceptor cells that respond to incident radiation with different spectral response curves. A retina has about 100 million rod cells, effective only at extremely low light levels; and about 5 million cone cells, of three types, that mediate colour vision. Since there is only one type of rod cell, what is loosely called night vision cannot discern colours.

The cone cells are sensitive to longwave, mediumwave, and shortwave light – roughly, light in the red, green, and blue portions of the spectrum. Because there are just three types of colour photoreceptors, three numerical components are necessary and sufficient to describe colour: Colour vision is inherently trichromatic. To arrange for three components to mimic colour vision, suitable spectral sensitivity functions must be used; this topic will be discussed in The CIE system of colorimetry, on page 265.

Adaptation

Vision operates over a remarkable range of luminance levels – about eight orders of magnitude (decades),

Diffuse white was described on page 117. This wide range of luminance levels is sometimes called dynamic range, but nothing is in motion!

Figure 23.3 A contrast sensitivity test pattern is presented to an observer in an experiment to determine the contrast sensitivity of human vision. The observer is adapted to background having luminance LB;

a bipartite patch is viewed. The experimenter adjusts ∆L; the observer reports whether he or she detects a difference in lightness between the two patches.

For image reproduction purposes, our ability to distinguish luminance differences ordinarily extends over a ratio of luminance of about three decades – 103, or 1000:1 – that is, down to about 0.1% of diffuse white as portrayed on the display. Loosely speaking, luminance levels less than 0.1% of diffuse white appear just “black”: Different luminances below that level are not ordinarily visually useful. Emergent high dynamic range (HDR) systems may increase that ratio.

Contrast sensitivity

Within the two-decade range of luminance that is useful for image reproduction, vision has a certain threshold of discrimination. It is convenient to express the discrimination capability in terms of contrast threshold, which is the ratio of a small test increment in luminance to the base luminance in a test stimulus having two adjacent patches of similar luminance.

Figure 23.3 below shows the pattern presented to an observer in an experiment to determine the contrast sensitivity of human vision. Most of the observer’s field of vision is filled by a background luminance level, LB, which fixes the observer’s state of adaptation. In the central area of the field of vision are placed two adjacent patches having slightly different luminance levels, L and L+∆L. The experimenter presents stimuli having a wide range of test values with respect to the surround, that is, a wide range of L/LB values. At each test luminance, the experimenter presents to the observer a range of luminance increments with respect to the test stimulus, that is, a range of ∆L/L values.

LB: Background luminance

L: Test luminance

∆L: Test luminance increment

LL+∆L

L

∆L log

-1.0

-1.2

-1.4

-1.6

-1.8

Schreiber, William F. (1993),

Fundamentals of Electronic

Imaging Systems, Third Edition

(Berlin: Springer-Verlag).

log 100 ≈ 463; 1.01463 ≈ 100 log 1.01

log 30 = 172 log 1.02

Fink, Donald G., ed. (1955),

Color Television Standards (New York: McGraw-Hill): 201.

When this experiment is conducted, the relationship graphed in Figure 23.4 above is found: Plotting log ∆L/L as a function of log L reveals an interval of a few decades of luminance over which the discrimination capability of vision is about 1% of the test luminance level. This experiment leads to the conclusion that – for threshold discrimination of two adjacent patches of nearly identical luminance – the discrimination capability is roughly logarithmic.

The contrast sensitivity function begins to answer this question: What is the minimum number of discrete codes required to represent relative luminance over

a particular range? In other words, what luminance codes can be thrown away without the observer noticing? On a linear luminance scale, to cover a 100:1

range with an increment of 0.01 takes 100/0.01, or about 10,000 codes, requiring about 14 bits. If codes

are spaced according to a ratio of 1.01, then only about 463 codes are required; codes can be represented in just 9 bits.(NTSC documents from the early 1950s used a contrast sensitivity of 2% and a contrast ratio of 30:1 to derive 172 steps; even today, 8 bits suffice for video distribution.)

The logarithmic relationship relates to contrast sensitivity at threshold: We are measuring the ability of the visual system to discriminate between two nearly identical luminances. If you like, call this a just noticeable