The Science of Color

[Dermot]

The Science of Color
In grade school we learn that there are three primary colors - red, yellow and blue - that when combined produce secondary colors such as green, purple and orange, and all the various shades and variations on the primary and secondary colors. However, these are the primary colors of pigment and different systems apply to the colors that we see on computer screens and the colors that we see in printed media and photographs.
Colors would not be possible without light because color is, in essence, light either reflected off of objects we see or viewed directly from the light source. When light is passed through a prism, the light beam bends and the wavelengths are separated, causing the prism to produce the colors of the electromagnetic spectrum. Rainbows are light bent through water particles in the air. Humans can only perceive a small portion of the electromagnetic spectrum - from 4.3-7.5 x 1014 Hz in frequency - and it is the frequency of the radiation that determines the color of light that we are seeing. Each color has its own wavelength. Infrared, microwave, radio, television and electrical power have lower frequencies than visible light, and ultraviolet, X-rays and gamma rays have higher frequencies.

Unlike the primary colors of pigment, the primary colors of light are red, green (not yellow) and blue, commonly referred to as RGB. The pigment color system does not apply to computer monitors because colors are created on monitors by adding light. RGB is an additive color system, which means that color is added to a black background. Black is the absence of light and therefore the absence of color. Secondary colors, such as cyan, magenta and yellow, are created by combining the primary colors. The color white is achieved by adding the three primary colors together in equal amounts.

There is, however, a third color system called subtractive, commonly referred to as CMYK. The primary colors of the subtractive system are cyan (C), magenta (M) and yellow (Y), the same colors that are the secondary colors of the RGB system. The letter "K" in CMYK stands for black. The subtractive color process is based on light reflected from an object and passed through pigments or dyes that absorb certain wavelengths, allowing others to be reflected. Unlike the additive system, which begins with black and adds color, the subtractive system begins with white and subtracts color. CMYK is commonly used in printing as most print begins with a white page that reflects white (RGB) light. To reproduce color on the paper, transparent pigments (cyan, magenta and yellow) are used to filter out the RGB wavelengths in various combinations. In theory, the combination of these three colors should produce black, but the fourth color black in CMYK is needed to produce true black. The secondary colors of the subtractive system are red, green and blue, the primary colors of the additive system.

 

[Less]

Wooooow, thanks Dr. Kearns! That sheds a lot of light! So that explains why Printers' always want our files to be CMYK.

I am involved with designing my own ads, gallery invitations, Giclee printing, and I often work with a printer/publisher. I work with Photo Shop and find this explanation very useful. So many modes, <strike>so little time.</strike> too lazy to read.

Now, can you help me figure out what I need RGB for?

 

[Jason Maranto]

RGB is the monitors natural color space since we are dealing with additive color. Anytime you see color on an electronic device you are seeing a variation of RGB.

Interesting fact is most RGB gamuts are larger than the monitors visual range.

CYMK is a compromise at best when it comes to reproducing color... most good printing presses nowadays can add several suplimentary colors to expand the gamut.

The reason why colors shift when you convert to CYMK from RGB is because you are trying to cram all the color data a shape made of RED GREEN and BLUE extremes in to shape which shares none of those extremes... tho' usually the worst shifters are greens.

Jason.

 

[Dermot]

Converting RGB to CMYK

The most common conversion of color spaces is between RGB (red~~green-blue additive primaries) and CMYK (cyan-magenta-yellow-black subtractive primaries) color spaces. The reason for the popularity of this conversion is that all scanners scan in RGB, and once an image is scanned, the colors must be converted to the four-color space used on the printing press.

The Kodak PCD Professional Imaging Workstation's scanner scans in RGB, then converts into a generous color space called YCC. The YCC space can ultimately be converted back into RGB, CIELAB, or CMYK spaces (or others) for display, print, or storage purposes.

The RGB display color space is a somewhat truncated color space compared to the RGB scan color space, accounting for a slight gamut reduction in the display. In other words, the monitor cannot display all the values of red, green and blue that a scanner sees when it digitizes the film, so some information is lost in the translation to the RGB display. YCC color space records the scanned image, and can later convert the image to the RGB space that a monitor can display.

RGB display space is described by a triangle on the CIE chromaticity chart with its strong points at the red, green and blue corners of the trangle. There is no allocation within this triangle for strong cyan, yellow or red all of which fall outside the perimeter of the RGB triangle. In order to convert from one to the other, a color coordinate system rotation must occur. Mathematically, this is a simple task, but it runs headlong into a photomechanical barrier which prevents it from being completely successful when producing color separations for printing.

This barrier is created by the fact that we cannot make perfect block absorbers (pigments) which transmit perfectly outside the range of colors we want to absorb. No pigments can be made as pure as we would like, and as a result we get a certain amount of contamination in the printing of CMY images.

The theory of mixing three pure primary pigments to get black does not work in the real world. Because of contamination in the minerals and synthetics used to make printing inks, the result of a three-color primary CMY mix is deep purple-brown, a color which is far from clean black. So the printing industry adds actual black ink to overcome the failure of the chemical process of printing ink pigments.

Pigment impurity, and the compensation that must take place to correct for it, requires the mathematics of color conversion to take a turn here and there to make the colors come out in the right places. The 120-degree rotation is made, but then a series of rule-based distortions of colors must occur to build the black separation from the known (or predicted) failures of the purity of the pigments, and to get increased density in dark areas.

Another necessary operation in the color separation process is the calculation of gray component replacement (GCR)and under color removal (UCR). These calculations are based on the colors of sets of pixels in the same position in the matrix The calculations are needed to prevent excessive ink layers from being created and for the simplified balance of neutral gray in printing.

Rich, saturated reds, greens and blues become less brilliant than the same colors viewed on an emissive device (an RGB monitored) while yellows magenta and cyan colors become more dominant The separation process requires understanding of the effect of color conversion Color separations are made worldwide that look beautiful and have brilliant colors that take advantage of the four pigment primaries used on the printing press Strong yellows magentas and cyans can rule the scene

 

[Ruth]

Are you working for the Great Yellow God, Dermot? (Kodak)