Visible Light

Light & Color Color is produced by the absorption of selected wavelengths of light by an object. All the colors except the color of the object are absorbed.

Color Models The basis of the trichromatic theory of color vision is that it is possible to match an arbitrary color by superposing appropriate amounts of 3 primary colors. In additive color reproduction systems, red, green, and blue light sources are added together to reproduce a colored light. In subtractive color systems, white light is passed through cyan, magenta, and yellow filters to reproduce a colored light.

Color Additive Color Matching (RGB) Subtractive Color Matching (CYMK) cyan yellow magenta blue red green Light Beams Adding (monitors, emission) Dye Patches Subtracting (printers, absorption)

Additive Color Systems When two lights with c1(l) and c2(l) are combined, the resulting light has c(l) given by c(l) = c1(l) + c2(l). Since lights add, this is called an additive color system. By adding light sources with different wavelengths, many different colors can be generated. ex. the lighted screen of a color television tube is covered with small, glowing phosphor dots arranged in groups of three (red, green, and blue). Red, green, and blue are the primary colors of the additive color system

Subtractive Color Matching In subtractive color systems, white light is passed through cyan, magenta, and yellow filters to reproduce a colored light.

Color “subtractive color”

Subtractive Color Matching The spectral absorption of the dye filters is a function of the dye concentration. The spectral transmissivities of practical dyes change shape in a nonlinear manner with dye concentration. Yellow dye is a variable absorber of blue light. Magenta dye is a variable absorber of green light. Cyan dye is a variable absorber of red light.

Color Models We can define other color models for describing color using 3 (or more) components. Example (HSI) Hue: dominant wavelength (color) Saturation: extent to which hue dominates (vivid? dull?) Intensity: brightness Others include RGB, CMY(K), etc., etc.

Three Common Perceptual Descriptors of Light Sensation Brightness - If two light sources with the same spectral shape are observed, the source of greater physical intensity will generally appear to be perceptually brighter. Refers to how bright the light is. Hue - Hue is the attribute of light that distinguishes a red colored light from a green light or a yellow light. Refers to the color, such as red, orange, or purple. Saturation - Saturation is the attribute that distinguishes a spectral light from a pastel light of the same hue. In effect, saturation describes the "whiteness" of a light source. Refers to how vivid or dull the color is.

Classifying Colors As an aid to classify colors, it is convenient to regard colors as being points in some color solid.

HSI

Color Models What is the term for the change in color from left to right? Hue What is the term for the change in color from top to bottom? Saturation & Brightness

Color Models Three “tristimulus” values T1, T2, T3 can be considered to form the three axes of a color space. A particular color is described by its location in the color space independent of its vector magnitude. A triangle, called Maxwell’s triangle, has been drawn between the three primaries. The intersection of a color vector with the triangle gives an indication of the hue and saturation of the color.

Color Coordinate Systems There are many color coordinate systems that can be used to represent a color [C]. [C] can be matched by its tristimulus values T1(C), T2(C), T3(C) for a given set of primaries. [C] can be matched by its luminance Y(C) and its chromaticity coordinates t1(C), t2(C). [C] can be matched by some linear or nonlinear invertible function of its tristimulus or chromaticity values.

IHS Color Coordinate System Used in the image processing community to quantitatively specify intensity, hue, and saturation of a color.

Vision Capabilities and limitations of human visual system affect image processing and practical remote sensing.

Vision

Vision Our perception of color arises from the composition of light that enters the eye. The retina contains photosensitive cells (rods and cones) with pigments that absorb visible light. Rods are effective in dim light (sensitive to intensity, not l) Cones (3 types) are sensitive to l Eyes are sensitive to EM over a very narrow range of l (0.35 mm to 0.750 mm).

Eye Physiology There are two types of receptors in the retina The rods are long slender receptors The cones are generally shorter and thicker in structure The rods and cones are not distributed evenly around the retina. Rods and cones operate differently Rods are more sensitive to light than cones. 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

Vision Photopic: color vision using cone receptors 7 million cones, one per nerve fiber Fine detail, but needs bright light Scotopic: grayscale vision using rod receptors 100 million rods, several per nerve fiber Less detail, can work in low-light

Eye Physiology Rods are more sensitive to light than the cones.

Eye Physiology There are three basic types of cones in the retina These cones have different absorption characteristics as a function of wavelength with peak absorptions in the red, green, and blue regions of the optical spectrum. is blue, b is green, and g is red There is a relatively low sensitivity to blue light There is a lot of overlap

Color Blindness Approximately 8% of males and 1% of females are subject to some form of color blindness. Monochromats only possess rods or rods plus one type of cone. Dichromats possess two of the three types of cones. Both monochromats and dichromats can distinguish colors insofar as they have learned to associate particular colors with particular objects.

Eye Physiology The eye contains about 6.5 million cones and 100 million rods distributed over the retina. The density of the cones is greatest at the fovea, this is the region of sharpest photopic vision.

Vision Fovea: circular indentation in the center of the retina with highest concentrations of rods and cones 1.5 mm x 1.5 mm area with receptor density of 150,000 elements per mm2 Similar density to solid-state devices

Vision Human vision in some ways is more flexible than a solid-state device! Varying focal length Brightness adaptation & discrimination

Brightness Adaptation & Discrimination Eye’s ability to discriminate discrete intensity levels is important for interpreting digital images. Human’s can adapt to ca. 1010 light levels (dynamic range)… ..but humans can only operate in a smaller sub-range at any one time ca. 103 levels

Brightness Adaptation & Discrimination Brightness adaptation – human vision system capability to change brightness sensitivity depending on conditions Weber ratio – measure of the ability to discriminate changes in intensity. Smaller ratio  better discrimination. Wr = DIc/I

Just noticeable difference (JND) at 2% Contrast Sensitivity 0% 1% 2% 3% 4% Circle constant Background constant Just noticeable difference (JND) at 2%

Just noticeable difference (JND) at 2% Contrast Sensitivity 0% 1% 2% 3% 4% Circle constant Background constant Just noticeable difference (JND) at 2%

Contrast Sensitivity 0% 1% 2% 3% 4% Background different then both halves Background same as right half Just noticeable difference (JND): 4% (top) and 2% (bottom)

Contrast Sensitivity 0% 1% 2% 3% 4% Background different then both halves Background same as right half Just noticeable difference (JND): 4% (top) and 2% (bottom)

Eye Physiology The optic nerve bundle contains on the order of 800,000 nerve fibers. There are over 100,000,000 receptors in the retina. Therefore, the rods and cones must be interconnected to nerve fibers on a many-to-one basis.

Simultaneous Contrast The simultaneous contrast phenomenon is illustrated below. The small squares in each image are the same intensity. Because the different background intensities, the small squares do not appear equally bright.

Simultaneous Contrast Perceiving the two squares on different backgrounds as different, even though they are in fact identical, is called the simultaneous contrast effect. Psychophysically, we say this effect is caused by the difference in the backgrounds, but what is the physiological mechanism behind this effect?

Simultaneous Contrast Perceiving the two squares on different backgrounds as different, even though they are in fact identical, is called the simultaneous contrast effect. Psychophysically, we say this effect is caused by the difference in the backgrounds, but what is the physiological mechanism behind this effect? Lateral Inhibition

Lateral Inhibition Record signal from nerve fiber of receptor A. Illumination of receptor A alone causes a large response. Add illumination to three nearby receptors at B causes the response at A to decrease. Increasing the illumination of B further decreases A’s response. Thus, illumination of the neighboring receptors inhibited the firing of receptor A. This inhibition is called lateral inhibition because it is transmitted laterally, across the retina, in a structure called the lateral plexus.

Lateral Inhibition

Mach Band Effect Another effect that can be explained by the lateral inhibition. The Mach band effect is illustrated in the figure below. The intensity is uniform over the width of each bar. However, the visual appearance is that each strip is darker at its right side than its left.