BASICS OF LIGHT AND COLOR

An Overview of Color Sensing :

Light is a narrow range of electromagnetic energy, to which the human eye is sensitive.

Electromagnetic energy travels in the form of waves, which can be described by their amplitude and frequency, or period. Normally light is described by its wavelength, in the units of nanometers (nm).

Light ranges from approximately 380nm to 780nm. Just outside this range lies ultraviolet (below 380nm) and infrared (above 780nm). Note that it is not proper to refer to ultraviolet and infrared radiation as light, since strictly speaking light is only radiation that we can see.

Most sources used for illumination emit white, or nearly, white light. Newton showed many years ago using a prism that white light is made up of contributions from all of the visible wavelengths. The light from any source can be described in terms of the relative power at each wavelength. This is known as a spectral power distribution (SPD).

Materials modify the light incident upon them in several ways. Light can be reflected from a surface. Light can be absorbed by, or transmitted through, a surface.In many cases, light is both absorbed by and reflected from a surface. The amount of absorption and reflection is often dependent on the wavelength, resulting in some wavelengths being absorbed and others reflected, to varying degrees.Reflected light from an object, perceived by the human eye, is what gives an object  its color.

Light can be transmitted, or passed through, an object in varying degrees. The amount of transmission can also be dependent on the wavelength. In the diagram on the left, light is passed through the object unchanged. In this case, the object will appear clear, such as a window. In the diagram on the right, light is partially transmitted. In this case certain wavelengths are transmitted and others are absorbed. This type of object appears translucent, and colored, such as a colored glass or plastic. This is the principal behind absorptive filters, which are used to remove or pass certain wavelengths of light.

Opaque surfaces do not pass light but rather reflect or absorb various wavelengths of light, sometimes in various degrees, as with colored surfaces. Additionally, the surface quality will affect the way in which the light is reflected.

Smooth surfaces, such as the one depicted on the left, reflect light at an angle that is 90 degrees to the incident angle. This type of reflection is known as a specular reflection. This type of surface can be described as glossy, and the specular reflection from incident light can result in what we call glare.

 If a surface is rough, or grainy. This surface reflects incident light at multiple angles, as light reflects from the various angles present in the texture of the surface. This type of reflection is known as diffuse reflection. The surface can be described as matte, or flat.

A key point ,all colors in objects result from selective absorption and reflection of light at various wavelengths , is that only wavelengths that are present can be reflected.

For example, an object will appear its “usual” color under white light because all wavelengths are present to be reflected. A “red” apple appears red under white light because red is reflected and other wavelengths are absorbed. If the apple is then illuminated with light of a single color, for example green, there are no wavelengths in the red region to be reflected. Thus, all the light is absorbed, and the apple appears dark gray instead of red.               Similarly, if we illuminate both the apple and a white sheet of paper with red light, both objects appear the same. This is because both the apple and the white paper reflect red light. The paper reflects all wavelengths, but only appears white under white light, where all wavelengths are present.

The conclusion is that the apparent color of an object is the product of both the object and the light source used for illumination of that object.

Similar to spectral power distribution curves of a light source , which describe the apparent color of the source, colored surfaces have spectral reflectance curves which show the amount of light reflected by that surface at each wavelength.

As one might expect, the red surface reflects light in the red wavelength range. Note that some colors are combinations of several wavelengths. Also note that brighter colors have a higher overall reflectance.

White paper has a fairly flat curve, reflecting light at all wavelengths.

Finally, note that black reflects almost no light at any wavelength.

The property of color of an object can be produced, when illuminated by white light, the absorption of some wavelengths and the reflection of others. Producing color in this way is known as subtractive color mixing. By contrast, colors can also be produced through the combination of light of three different colors.

The colors which, when combined, produce the widest range of other colors are red, green, and blue. These three basic colors are known as primaries. When the primaries have the right spectral content, they can be combined in various amounts to produce any other color. When combined equally, they produce white.

Subtractive color mixing works by starting with a white light source, or white medium, and removing certain wavelengths to produce a color. As with additive mixing, the amount or concentration and blend of each primary determines the resulting color.

Subtractive color mixing is used when dealing with dyes and pigments (paints). Subtractive mixing ideally uses a different set of primaries than additive mixing, in this case cyan , magenta , and yellow. These can be combined to produce any other color, just as with additive mixing. When subtractive primaries are equally combined, the resulting color is black.

Color Perception

“Indeed rays, properly expressed, are not coloured.”

- Isaac Newton, 1675

This quote from Isaac Newton is used to illustrate the fact that color is strictly a human perception; a sensation. Light can be described by wavelength and spectral distributions, but color is the result of the interaction between light and the human eye, and the operation of the brain on signals obtained from the eye.

There are two types of photoreceptors: rods and cones.

Cones are responsible for color vision and are found in three types: L, M, and S having peak sensitivities in the long, medium, and short wavelength regions.

All colors are represented by various combinations of values from these three receptors this is called trichromacy, or trichromatic color representation.

There are three types of cones, each responding differently to light of various wavelengths. Stimuli that result in different color perception have different cone signals.

The letters L, M, and S represent the three cones with their peak sensitivities in the long , middle, and short wavelength regions, respectively. The overlap of the response curves improves color discrimination. If the receptors did not have any overlap in their spectrum, we would only perceive three hues in the spectrum. When two stimuli, whether colored lights or illuminated media, produce the same cones signals, the two stimuli match in color. The cones, as any detector, integrate, or sum up, the light energy at all wavelengths incident on them.   

This summation process is weighted at each wavelength by the response of the receptor at that wavelength, according to the L, M, and S response curves. The integration process

of the three receptors reduces the entire spectrum to three signals, one for each cone type, resulting in trichromacy.

Therefore, three signals are necessary and sufficient to describe any color. When we describe color, rather than specifying three abstract values, we seek to use terms that have intuitive meaning to describe color. Color names can be used and conjure reasonably consistent perceptions. In fact, eleven basic color names have been identified: white, gray, black, red, yellow, green, blue, orange, purple, pink, and brown. However, within each color, samples may differ widely from each other. Therefore additional precision is needed in our description.

A more precise method of describing color is by hue, saturation, and lightness. Hue is the attribute of a color according to its similarity with one of the colors red, yellow, green, or blue, or a combination of adjacent pairs of these colors considered in a closed ring.

Lightness is the attribute by which a color is judged to be similar to one of a series of grays ranging from black to white. Saturation is the degree of departure of the color from a gray of the same lightness, or described another way, the intensity of the color compared to a gray or white.

HSV, or hue, saturation, and value (or lightness) is a three dimensional coordinate system useful for describing a color mathematically. In this model, in which the three values describe a cone, hue is represented by an angular position on the circle, similar in concept to that shown on the previous slide.

Saturation ranges from 0 to 1 and is represented by the radius of the hue circle, or cone. A value of 0 indicates gray and 1 is the pure primary color. Value, or lightness, is represented by the vertical height of the cone and ranges from black to white.