Color Physics of color Introduction Isac Newton was

Color Physics of color

Introduction • Isac Newton was the first to notice that white light is composed of a range of colors • It is in fact possible with only three colors to produce any visible color by combining them

Maxwell: • . . . I would observe that the important part of theory is not that three elements enter into our sensation of color, but that there are only three. Optically, there as many elements in the composition of a ray of light as there are different kinds of light in its spectrum. • Helmholtz and Young

Sensation • the human eye in fact does only possess the ability to distinguish three basic colors in the red, the blue and the green range • The eye contain two different types of light sensors the rods and the cones. The rods are not sensitive to color (the light receptor rhodopsin does have a sensitivity spectrum with a peak around 500 ). The cones come in three different varieties sensitive to different color. [nm], but does not produce a color sensation

Sound and color • It is worth noting the difference between the sound processing of the ears and the color processing of the eyes • If we play two tones we can still hear both tones, but if we subject the eye to two spectral lines, they are combined into one color • Sound has almost continuous range • Color only three kinds of receptor on a continuous surface

Characterization of perceived color. • any color by combining three primary colors • we need to be able to assign a well defined color to any mixture of radiation, not only limited to three stimuli • we would expect to be able to characterize any color by weighing its spectral distribution of wavelengths with sensitivity curves representing the red, the green and the blue receptors in the eye

CIE 1931 • standardized color value curves x’( )’, y’( ) and z’( ) • These curves are artificial curves not matched to the eyes color vision but related in a linear fashion. • They have been designed to be positive in the visible range.

Color value curves

spectral coordinates of the color • X = k * ( ) * x’( ) * • Y = k * ( ) * y’( ) * • Z = k * ( ) * z’( ) * • x = X/(X+Y+Z) • y = Y/(X+Y+Z) • From CIE 1931

CIE 1931 Chromaticity diagram:

Color temperature

Color temperature • Obviously there is a problem of finding the color temperature for an arbitrary radiating source. Points not coinciding with the Planckian locus are difficult to assign values of color temperature. • The Correlated Color Temperature (CCT) was originally developed to characterize incandescent lamps and is useful for sources that produce reasonable white light.

The CEI uniform Chromaticity • • u = 4 X/(X+15 Y+3 Z) v = 6 Y/(X+15 Y+3 Z) CEI 1960 In this diagram the isotemperature lines are perpendicular to the Planckian locus. This permits points not too remote from the locus to be assigned CCT values

CIELUV uniform color space 1976 • The intention of this was to create a diagram where equal distances correspond to equal differences in subjective color. The coordinates are: • u’ = u • v’ = 3/2 v = 9 Y/(X+15*Y+3*Z) • The 1960 diagram shall still be used for the CCT values

Color rendering • Even though two light sources may have been assigned the same color temperature it is not certain that colors illuminated by them looks the same. It seems understandable that illumination resulting from e. g. a few spectral lines render colors differently from a continuous spectrum and even two different continuous spectrums may render colors differently.

The RGB system: • Each primary color contribution may be characterized by one byte with a value range 0. . 255 • with three bytes R, G and B we can represent 256 x 256 different combinations of the primary colors, i. e. more than 16 million colors (often referred as "true color"). • In particular, this RGB color system is known as RGB-256

RGB • Black is the absence of color (or the absence of light) and it is represented as RGB(0, 0, 0) (Red=0, Green=0, Blue=0). White is the presence of all colors (in their maximum intensity) and it is represented as RGB(255, 255) (Red=255, Green=255, Blue=255). All shades of gray from black to white are represented with three equal values for the red, green and blue components (no color predominates), i. e. they have the form RGB(x, x, x). For example, the color defined in the jargon as "light gray" is represented as RGB(192, 192), and "dark gray" is RGB(128, 128).

visible gamut in (u', v') perceptually uniform coordinates

standard color encodings • cover only about half of the visible gamut of colors. If future display systems should be able to reproduce more information, it will be unavailable from most RGB storage formats. • Even existing display systems, since they do not all use the same primary colorants, have significantly varying gamuts, and any restriction on the stored data restricts the reproduction accuracy.

Additive and subtractive schemes • The RGB system is an additive system. The colors are added by adding the radiated power of the primary colors • In contrast colors on a surface absorbs radiation. A red color absorbs all wavelengths except the red. Mixing colors on a surface therefore produce a different result than when the colors are added.

Measurement uncertainty • In general the method of analytically finding the contributions to the uncertainties of color coordinates from the spectral lines and weighing functions is rather complicated. • An alternate method using a numerical approach to calculate the uncertainties of any color quantities, based on the GUM, is possible

Summary • The methods of defining color in a systematic way is not easy. On one hand it is a goal to have a system that correspond to physiological experience and on the other hand a well founded science that permit strong theoretical predictions to be made. The present systems are largely a mixture of these two stragtegies.

Three dimensional color spaces • introduced in 1976 by CEI • The three axis L*, u* v* also consider lightness. • L* = 116 (Y/Yn)1/3 -16 Y/Yn > 0. 008856 • u* = 13 L* (u’-u’n) • v* = 13 L* (v’-v’n) • A reference is required.

Other parameters • • • Lightness : L* Color Saturation: Suv = 13 [(u’-u’n)2 +(v’-v’n)2] ½ Croma: C’uv = L* Suv Hueangle: huv = Arc. Tan [(v’-v’n)/ (u’-u’n)] Color difference: Geometric distance in this space. A reference is required. D 65 or A or C (obsolete) • Examples of how this looks on the next slides.

Hueangle 0 degrees

Hueangle 90 degrees

Hueangle 180 degrees

Hueangle 270 Degrees