We use Cathode Ray Tubes (CRT) in both televisions and computer monitors. CRTs are made up of three overlapping layers of phosphors with each layer corresponding to one of the primary colors RGB. Electrons are shot at the phosphorous layers and as they pass through each layer, it changes the wavelength to correspond with the correct color. One layer passes electrons on at 400-500 nanometers (nm) causing us to see blue on the screen. Another layer passes electrons on at 500-600 nm to create green light and the third layer moves electrons at 600-700 nm causing us to see red. Because the three layers are overlapping, any color in the spectrum can be created by combining different amounts of the three colors.
Because CRTs are so commonly used, color scanners, digital cameras, etc. also use RGB. This saves having to convert from another type of color description to RGB to view the image on a CRT monitor.
CRT monitors do have problems however when it comes to color imaging. It is impossible to select and match color effectively by eye on the monitor for a variety of reasons. Because the monitor uses three phosphors to emit light, we see more intense and saturated light through the monitor than we do on reflective substances such as printer paper. It is also difficult to see color on the monitor as it will print on paper because monitors have adjustable brightness and contrast settings. To make matters worse the brightness and contrast of a monitor can change depending on the manufactor, the size of the CRT, even the age of the monitor.
Keep going Amos, you're doing great
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