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RGB and CMYK




 

On a microscopic level, the retina of a human eye is made up of two types of light sensitive cells, rods and cones. Rodes are best at scotopic or low-light-level night vision, while the cones are best at photopic or high-light-level, high resolution colour vision. Each retina has about 120 million rods, and 6 to 7 million cones, each is about 1 to 3 micrometer in diameter. The human eye has three types of cones which receive short (S) - blue, medium (M) - green and long (L) – red wavelengths. They are also known as the blue, green and red receptors. We see colours because these cones are stimulated.

Visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has a wavelength in the range of about 380 nm (nanometres) to about 740 nm – between the invisible infrared, with longer wavelengths and the invisible ultraviolet, with shorter wavelengths.

The additive primary colours are red, green and blue (RGB). Coloured lights are mixed using additive colour properties. Light colours are combining two or more additive colours together, that creates a lighter colour that is closer to white. Combining all three additive primary colours in equal amounts will produce the white colour. So, adding different colours creates white, and the absence of all light equaling black.

 

Additive colours combined in equal parts:

Blue + Green = Cyan

Red + Blue = Magenta

Green + Red = Yellow

Red + Green + Blue = White

 

By changing the brightness of each of the three primary colours, by varying degrees, you can make a wide range of colours.

 

Additive colours combined in unequal parts:

1 Green + 2 Red = Orange

1 Red + 2 Green = Lime

1 Green + 1 Blue + 4 Red = Brown

 

Computer monitors and televisions are an application of additive colours. These devices use a mosaic of red, green and blue dots. Our eyes do not distinguish the dots.

Look closely at a white spot on your computer monitor using a strong magnifying glass or eye loop. You should be able to see that the white dot isn’t really white, but rather is a combination of red, green, and blue dots all located very close to one another.

The distinction between additive and subtractive colours is based on a fact that the image is derived from a light source, like a TV set that uses glowing phosphorus, or reflected natural light, as from a book, photograph, wall or any other object.

It is possible to print colour pictures using just three colours of ink, but you have to work in reverse of the process of mixing light colors. We see light colours by the process of emission from the source. We see pigment colour by the process of reflection (light reflected off an object). The colours which are not reflected are absorbed (subtracted). The subtractive primary colours are cyan, magenta and yellow. These are three colours used in printer’ ink cartridges.

 

combine absorbs creates
Blue + Green Red Cyan
Red + Blue Green Magenta
Green + Red Blue Yellow

 

Subtractive colour mixing

combine absorbs leaves
Cyan + Magenta Red + Green Blue
Cyan + Yellow Red + Blue Green
Magenta + Yellow Green + Blue Red
Cyan + Magenta + Yellow Red + Green + Blue Black

 

 

According to the table above, combining all three subtractive pigments yield black. In practice it doesn’t yield really as true as black colour as printing with black directly. So, most colour printing is done with four ink colours: cyan, magenta, yellow and black, or CMYK for short, where “K” is used instead of “B” to avoid confusion with blue.

 


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