DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Schreiber, William F., Fundamentals
of Electronic Imaging
Systems, Third Edition (Berlin:
Springer-Verlag, 1993).
log ∆L
L
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
Luminance, log cd•m-2 -1 0 1 2 3 4
Figure 19.4 Contrast sensitivity. This graph is redrawn, with permission, from Figure 3.4 of
Schreiber’s Fundamentals of Electronic Imaging Systems. Over a range of intensities of about 300:1,
the discrimination threshold of vision is approximately a constant ratio of luminance. The flat
portion of the curve shows that the perceptual response to luminance – termed lightness – is
approximately logarithmic.
lg 100
463
≈463; 1. 01 ≈100
lg 101 .
NTSC documents from the early
1950s used a contrast sensitivity
of 2% and a contrast ratio of 30:1
to derive 172 steps:
lg 30
lg 102 .
172 =
See Fink, Donald G., ed., Color
Television Standards (New York:
McGraw-Hill, 1955), p. 201.
levels, Y and Y+∆Y. The experimenter presents stimuli
having a wide range of test values with respect to the
surround, that is, a wide range of Y/Y 0 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 ∆Y /Y values.
When this experiment is conducted, the relationship
graphed in Figure 19.4 above is found: Plotting
log(∆Y/Y) as a function of log Y reveals an interval of
more than two decades of luminance over which the
discrimination capability of vision is about 1% of the
test luminance level. This leads to the conclusion that –
for threshold discrimination of two adjacent patches of
nearly identical luminance – the discrimination capability
is very nearly 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 10000 codes, or
about 14 bits. If codes are spaced according to a ratio
of 1.01, then only about 463 codes are required. This
number of codes can be represented in 9 bits. (For
video distribution, 8 bits suffice.)
CHAPTER 19 PERCEPTION AND VISUAL ACUITY 199