Human vision color

In human vision color is reduced to 3 co-ordinates. Still, in most color printing, 4 colors are used. In high quality color print 7 colors are used. Explain the advantages of using 4 or 7 colors, when we can only see 3 colors.

From the electromagnetic energy spectrum, our eyes can detect the small portion of the wavelengths which is 380 nm to 770 nm. This range of wavelength is the visible light spectrum. The short wavelength of visible spectrum is blue and longer one is red. All the colors in nature we can see is in between blue and red. Red, Green and Blue are the primary colors. Something creates color by absorbing certain wavelengths of color while reflecting other wavelengths back to the viewer. Other way to produce color is when a particular source emits only certain wavelengths.

Retina is the light sensitive tissue in human eyes. It is consists of rods and cones. Rods functions in less intense light and cones in bright light. There are three kinds of cones-red absorbing cones that absorb long wavelength light, green absorbing cones that absorb middle wavelength light and blue absorbing cones that absorb short wavelength light. Human eyes define a color by perceptual attributes of brightness, hue and colorfulness.

Cyan, magenta and yellow are the secondary colors in between three primary colors. The additive combination of two primary colors in proportion gives the perception of a secondary color. Red and green yields yellow, blue and red yields magenta, blue and green yields cyan and other colors can form this way. In printing, color is defined in terms of reflectance and absorbance of cyan, magenta, yellow and black inks on the paper. In 4 color printing- cyan, magenta, yellow and black inks are used which act as filters and subtract portions of the white light strike the image on paper to produce other colors. Paper reflects unabsorbed light back to the viewer.

Color printing gives a reproduction of an image or text in color. In 4 or 7 color printing, the color used is the different proportion of RGB and give rise to the visual sensations of all other colors.