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# Classroom Change This Friday’s class will be at Physics 1201.

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Classroom Change This Friday’s class will be at Physics 1201

More discussions about the chromaticity diagram Spectral colors (saturated, monochromatic) lies on the boundary of the horse shoe. All colors inside are unsaturated. The straight-line between the two colors represents all the colors that one can get from the mixture of the two.

The spectral colors full outside of the triangle (representing negative mixing). The complementary of any color is found by extending a straight line from that color through white and to the opposite side of the horseshoe. The points in the triangle represent all the colors that one can make with three primary colors.

CIE Chromaticity diagram Invent three imaginary colors: x, y, z. Then using these colors, one can get all colors with positive amount of mixing.  The imaginary x consisting of 150% red, and a negative 50% green.  For any color, the relative amounts add to 1. However, all possible colors are still within the horse shoe inside of the triangle.

Relative amounts of [x],[y],and [z] needed to match A give spectral color

Ways of mixing color by addition Simple addition  Happens with different light sources illuminates the same region.  Stage lighting: a number of different colored lights aimed at stage with different intensities.  The first color photographs were projected using three projectors in register, one for each of the additive primary.

Large screen color television projection (spots arenas and home projection systems)  The TV projector consists of three different mirrors or lenses that project enlarged images of three small very bright picture tubes, each of which contains the TV images in one of three additive primary colors.

Partitive Mixing Placing small separate source close to each other. Your eyes do not see the separate sources, but the mixed color.  Color TV: one picture tube but three electron guns. Electrons from different guns are directed to different points on the screen. The screen consists of dots of three different phosphors, each of which will produce one of the additive colors. (less light from each color)

Pointillist painters: put small dabs of different color paint near each other, and look at the picture from a distance  Mosaic  Stained glass  Trees in the fall  Textiles

Positive afteriamge Put the colors “near each other” in time. The colors change so rapidly that the positive afterimage of one mixes additively in your eye with the image of the next.  Rapidly rotating wheel with different color segment (Color wheel)  Two eyes are exposed to pictures of different colors!

Why do we see color? Most of the light sources contain all possible frequencies in the visible light window (the sun, light-bulbs,…) We see colored surfaces because they absorb the light of certain frequencies very strongly. The reflected light then have colors complementary to the color spectrum of the absorption.  A red surface absorb cyan very strongly.

Subtractive Mixing When you mix red and green paint, you don’t get yellow. You get black! This is because the red paint absorbs cyan, and green absorbs magenta. When mixed together, it absorbs all frequency. Therefore three primary colors for subtractive mixing are not red, blue and green, they are cyan, yellow, and magenta.

Subtractive Mixing Rules: Mixing Cyan + Magenta, one gets Blue Mixing Cyan + Yellow, one gets Green Mixing Magenta + Yellow, one gets Red Mixing any two of the Blue, Red, Green one gets Black. The above results assume the light-sources contain all possible frequencies.

Dependence of subtractive color on the light source The color of the light reflected from an object also depends on the light source.  Under the “golden white” sodium lamps on highways, some objects lose their color because there is little green and red light in it. To find the color of an object under a non- white light source, we need to know the intensity distribution of the light source and the absorption spectrum of the surface.

Even if two illuminating lights look the same, an object may still appear different colors in them.  Under a white light consisting of two narrow bands of cyan and red, the yellow object can only appears as red or black.  However, the same yellow object appears yellow under sun light.

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