LIGHT PERCEPTION Light, according to Webster's Dictionary 1, is “radiant energy which, by its action on the organs of vision, enables them to perform their function of sight.” Much iskno
Trang 12
PSYCHOPHYSICAL VISION PROPERTIES
For efficient design of imaging systems for which the output is a photograph or play to be viewed by a human observer, it is obviously beneficial to have an under-standing of the mechanism of human vision Such knowledge can be utilized todevelop conceptual models of the human visual process These models are vital inthe design of image processing systems and in the construction of measures ofimage fidelity and intelligibility
dis-2.1 LIGHT PERCEPTION
Light, according to Webster's Dictionary (1), is “radiant energy which, by its action
on the organs of vision, enables them to perform their function of sight.” Much isknown about the physical properties of light, but the mechanisms by which light
interacts with the organs of vision is not as well understood Light is known to be a
form of electromagnetic radiation lying in a relatively narrow region of the magnetic spectrum over a wavelength band of about 350 to 780 nanometers (nm) Aphysical light source may be characterized by the rate of radiant energy (radiantintensity) that it emits at a particular spectral wavelength Light entering the humanvisual system originates either from a self-luminous source or from light reflectedfrom some object or from light transmitted through some translucent object Let
represent the spectral energy distribution of light emitted from some primary
light source, and also let and denote the wavelength-dependent
transmis-sivity and reflectivity, respectively, of an object Then, for a transmissive object, the
observed light spectral energy distribution is
(2.1-1)
E( )λ
t( )λ r( )λ
C( )λ = t ( )E λλ ( )
Digital Image Processing: PIKS Inside, Third Edition William K Pratt
Copyright © 2001 John Wiley & Sons, Inc.ISBNs: 0-471-37407-5 (Hardback); 0-471-22132-5 (Electronic)
Trang 224 PSYCHOPHYSICAL VISION PROPERTIES
and for a reflective object
(2.1-2)Figure 2.1-1 shows plots of the spectral energy distribution of several commonsources of light encountered in imaging systems: sunlight, a tungsten lamp, a
FIGURE 2.1-1 Spectral energy distributions of common physical light sources.
C( )λ = r ( )E λλ ( )
Trang 3LIGHT PERCEPTION 25
light-emitting diode, a mercury arc lamp, and a helium–neon laser (2) A humanbeing viewing each of the light sources will perceive the sources differently Sun-light appears as an extremely bright yellowish-white light, while the tungsten lightbulb appears less bright and somewhat yellowish The light-emitting diode appears
to be a dim green; the mercury arc light is a highly bright bluish-white light; and thelaser produces an extremely bright and pure red beam These observations provokemany questions What are the attributes of the light sources that cause them to beperceived differently? Is the spectral energy distribution sufficient to explain the dif-ferences in perception? If not, what are adequate descriptors of visual perception?
As will be seen, answers to these questions are only partially available
There are three common perceptual descriptors of a light sensation: brightness,hue, and saturation The characteristics of these descriptors are considered below
If two light sources with the same spectral shape are observed, the source of
greater physical intensity will generally appear to be perceptually brighter
How-ever, there are numerous examples in which an object of uniform intensity appearsnot to be of uniform brightness Therefore, intensity is not an adequate quantitativemeasure of brightness
The attribute of light that distinguishes a red light from a green light or a yellow
light, for example, is called the hue of the light A prism and slit arrangement
(Figure 2.1-2) can produce narrowband wavelength light of varying color However,
it is clear that the light wavelength is not an adequate measure of color becausesome colored lights encountered in nature are not contained in the rainbow of lightproduced by a prism For example, purple light is absent Purple light can beproduced by combining equal amounts of red and blue narrowband lights Othercounterexamples exist If two light sources with the same spectral energy distribu-tion are observed under identical conditions, they will appear to possess the samehue However, it is possible to have two light sources with different spectral energy
distributions that are perceived identically Such lights are called metameric pairs The third perceptual descriptor of a colored light is its saturation, the attribute
that distinguishes a spectral light from a pastel light of the same hue In effect, ration describes the whiteness of a light source Although it is possible to speak ofthe percentage saturation of a color referenced to a spectral color on a chromaticitydiagram of the type shown in Figure 3.3-3, saturation is not usually considered to be
satu-a qusatu-antitsatu-ative mesatu-asure
FIGURE 2.1-2 Refraction of light from a prism.
Trang 426 PSYCHOPHYSICAL VISION PROPERTIES
As an aid to classifying colors, it is convenient to regard colors as being points insome color solid, as shown in Figure 2.1-3 The Munsell system of colorclassification actually has a form similar in shape to this figure (3) However, to bequantitatively useful, a color solid should possess metric significance That is, a unitdistance within the color solid should represent a constant perceptual colordifference regardless of the particular pair of colors considered The subject ofperceptually significant color solids is considered later
2.2 EYE PHYSIOLOGY
A conceptual technique for the establishment of a model of the human visual systemwould be to perform a physiological analysis of the eye, the nerve paths to the brain,and those parts of the brain involved in visual perception Such a task, of course, ispresently beyond human abilities because of the large number of infinitesimallysmall elements in the visual chain However, much has been learned from physio-logical studies of the eye that is helpful in the development of visual models (4–7)
FIGURE 2.1-3 Perceptual representation of light.
Trang 5EYE PHYSIOLOGY 27
Figure 2.2-1 shows the horizontal cross section of a human eyeball The front
of the eye is covered by a transparent surface called the cornea The remaining outer cover, called the sclera, is composed of a fibrous coat that surrounds the choroid, a layer containing blood capillaries Inside the choroid is the retina, which is com- posed of two types of receptors: rods and cones Nerves connecting to the retina leave the eyeball through the optic nerve bundle Light entering the cornea is focused on the retina surface by a lens that changes shape under muscular control to
FIGURE 2.2-1 Eye cross section.
FIGURE 2.2-2 Sensitivity of rods and cones based on measurements by Wald.
Trang 628 PSYCHOPHYSICAL VISION PROPERTIES
perform proper focusing of near and distant objects An iris acts as a diaphram to
control the amount of light entering the eye
The rods in the retina are long slender receptors; the cones are generally shorter andthicker in structure There are also important operational distinctions The rods aremore sensitive than the cones to light At low levels of illumination, the rods provide a
visual response called scotopic vision Cones respond to higher levels of illumination; their response is called photopic vision Figure 2.2-2 illustrates the relative sensitivities
of rods and cones as a function of illumination wavelength (7,8) An eye containsabout 6.5 million cones and 100 million cones distributed over the retina (4) Figure2.2-3 shows the distribution of rods and cones over a horizontal line on the retina
(4) At a point near the optic nerve called the fovea, the density of cones is greatest.
This is the region of sharpest photopic vision There are no rods or cones in the ity of the optic nerve, and hence the eye has a blind spot in this region
vicin-FIGURE 2.2-3 Distribution of rods and cones on the retina.
Trang 7VISUAL PHENOMENA 29
In recent years, it has been determined experimentally that there are three basictypes of cones in the retina (9, 10) These cones have different absorption character-istics as a function of wavelength with peak absorptions in the red, green, and blueregions of the optical spectrum Figure 2.2-4 shows curves of the measured spectralabsorption of pigments in the retina for a particular subject (10) Two major points
of note regarding the curves are that the cones, which are primarily responsiblefor blue light perception, have relatively low sensitivity, and the absorption curvesoverlap considerably The existence of the three types of cones provides a physio-logical basis for the trichromatic theory of color vision
When a light stimulus activates a rod or cone, a photochemical transition occurs,producing a nerve impulse The manner in which nerve impulses propagate throughthe visual system is presently not well established It is known that the optic nervebundle contains on the order of 800,000 nerve fibers Because there are over100,000,000 receptors in the retina, it is obvious that in many regions of the retina,the rods and cones must be interconnected to nerve fibers on a many-to-one basis.Because neither the photochemistry of the retina nor the propagation of nerveimpulses within the eye is well understood, a deterministic characterization of thevisual process is unavailable One must be satisfied with the establishment of mod-els that characterize, and hopefully predict, human visual response The followingsection describes several visual phenomena that should be considered in the model-ing of the human visual process
2.3 VISUAL PHENOMENA
The visual phenomena described below are interrelated, in some cases only mally, but in others, to a very large extent For simplification in presentation and, insome instances, lack of knowledge, the phenomena are considered disjoint
mini-FIGURE 2.2-4 Typical spectral absorption curves of pigments of the retina.
α
Trang 830 PSYCHOPHYSICAL VISION PROPERTIES
Contrast Sensitivity The response of the eye to changes in the intensity of
illumina-tion is known to be nonlinear Consider a patch of light of intensity surrounded
by a background of intensity I (Figure 2.3-1a) The just noticeable difference is to
be determined as a function of I Over a wide range of intensities, it is found that the
ratio , called the Weber fraction, is nearly constant at a value of about 0.02
(11; 12, p 62) This result does not hold at very low or very high intensities, as
illus-trated by Figure 2.3-1a (13) Furthermore, contrast sensitivity is dependent on the intensity of the surround Consider the experiment of Figure 2.3-1b, in which two patches of light, one of intensity I and the other of intensity , are sur-rounded by light of intensity The Weber fraction for this experiment is
plotted in Figure 2.3-1b as a function of the intensity of the background In this
situation it is found that the range over which the Weber fraction remains constant is
reduced considerably compared to the experiment of Figure 2.3-1a The envelope of the lower limits of the curves of Figure 2.3-lb is equivalent to the curve of Figure 2.3-1a However, the range over which is approximately constant for a fixedbackground intensity is still comparable to the dynamic range of most electronicimaging systems
FIGURE 2.3-1 Contrast sensitivity measurements.
Trang 9VISUAL PHENOMENA 31
FIGURE 2.3-2 Mach band effect.
(a ) Step chart photo
(c ) Ramp chart photo
B D
(d ) Ramp chart intensity distribution
(b ) Step chart intensity distribution
Trang 1032 PSYCHOPHYSICAL VISION PROPERTIES
Because the differential of the logarithm of intensity is
(2.3-1)equal changes in the logarithm of the intensity of a light can be related to equal justnoticeable changes in its intensity over the region of intensities, for which the Weberfraction is constant For this reason, in many image processing systems, operationsare performed on the logarithm of the intensity of an image point rather than theintensity
Mach Band Consider the set of gray scale strips shown in of Figure 2.3-2a The
reflected light intensity from each strip is uniform over its width and differs from itsneighbors by a constant amount; nevertheless, the visual appearance is that each
strip is darker at its right side than at its left This is called the Mach band effect (14) Figure 2.3-2c is a photograph of the Mach band pattern of Figure 2.3-2d In the pho- tograph, a bright bar appears at position B and a dark bar appears at D Neither bar
would be predicted purely on the basis of the intensity distribution The apparentMach band overshoot in brightness is a consequence of the spatial frequencyresponse of the eye As will be seen shortly, the eye possesses a lower sensitivity tohigh and low spatial frequencies than to midfrequencies The implication for thedesigner of image processing systems is that perfect fidelity of edge contours can besacrificed to some extent because the eye has imperfect response to high-spatial-frequency brightness transitions
Simultaneous Contrast The simultaneous contrast phenomenon (7) is illustrated in
Figure 2.3-3 Each small square is actually the same intensity, but because of the ferent intensities of the surrounds, the small squares do not appear equally bright.The hue of a patch of light is also dependent on the wavelength composition of sur-rounding light A white patch on a black background will appear to be yellowish ifthe surround is a blue light
dif-Chromatic Adaption The hue of a perceived color depends on the adaption of a
viewer (15) For example, the American flag will not immediately appear red, white,and blue if the viewer has been subjected to high-intensity red light before viewing theflag The colors of the flag will appear to shift in hue toward the red complement, cyan
FIGURE 2.3-3 Simultaneous contrast.
d(logI) dI
I
-=
Trang 11MONOCHROME VISION MODEL 33
Color Blindness Approximately 8% of the males and 1% of the females in the
world population are subject to some form of color blindness (16, p 405) There are
various degrees of color blindness Some people, called monochromats, possess
only rods or rods plus one type of cone, and therefore are only capable of
monochro-matic vision Dichromats are people who possess two of the three types of cones.
Both monochromats and dichromats can distinguish colors insofar as they havelearned to associate particular colors with particular objects For example, darkroses are assumed to be red, and light roses are assumed to be yellow But if a redrose were painted yellow such that its reflectivity was maintained at the same value,
a monochromat might still call the rose red Similar examples illustrate the inability
of dichromats to distinguish hue accurately
2.4 MONOCHROME VISION MODEL
One of the modern techniques of optical system design entails the treatment of anoptical system as a two-dimensional linear system that is linear in intensity and can
be characterized by a two-dimensional transfer function (17) Consider the linearoptical system of Figure 2.4-1 The system input is a spatial light distributionobtained by passing a constant-intensity light beam through a transparency with aspatial sine-wave transmittance Because the system is linear, the spatial outputintensity distribution will also exhibit sine-wave intensity variations with possiblechanges in the amplitude and phase of the output intensity compared to the inputintensity By varying the spatial frequency (number of intensity cycles per lineardimension) of the input transparency, and recording the output intensity level and
phase, it is possible, in principle, to obtain the optical transfer function (OTF) of the
optical system
Let represent the optical transfer function of a two-dimensional linearsystem where and are angular spatial frequencies withspatial periods and in the x and y coordinate directions, respectively Then,
with denoting the input intensity distribution of the object and
FIGURE 2.4-1 Linear systems analysis of an optical system.
H(ωx,ωy)
ωx = 2π T⁄ x ωy = 2π T⁄ y
T x T y