Module 15

   

Updated: 02/18/2014

Module 15    

 

 

 

Principles of

Television Color 

 

Knowledge of the physics of color will add to the effectiveness of your work and help eliminate production problems. In fact, it will help you with everything from white-balancing a video camera to color-coordinating your wardrobe.

First, note from the illustration below that visible light represents only a small portion of the electronic spectrum.

This is a band of energy that starts with low frequency radio waves, moves through VHF-TV, FM radio, UHF-TV (which now includes the new digital TV band of frequencies), all the way through x-rays.

Electromagnetic Spectrum

The visible light portion of the electromagnetic spectrum consists of all the colors of the rainbow (as shown in the enlarged segment above), which combine to produce white light.

The fact that white light consists of all colors of light added together can be demonstrated with the help of a prism.

If you project white light through a prism, as illustrated below, the light will be expanded to show the individual color components within the light.

Conversely, the opposite is also true: if you add all of the basic colors of light together, you can create white light.

light prism

Infrared and ultraviolet in the illustration above are not in the visible light range. But within these boundaries, red, orange, yellow, green, blue, indigo and violet are. A simple way to remember this sequence is with the acronym, Roy G. Biv.

By keeping these concepts in mind, you will have the key to the additive color television process.

Before we get further into the additive color process -- a process that's basic to color television -- we need to take a look at a process that's probably better understood: the subtractive color process.    


color wheel Subtractive Color  

>>The color of an object is determined by the colors of light red light absorptionit absorbs and the colors of light it reflects.

When white light falls on a red object, the object appears red because its surface subtracts (absorbs) all colors of light except red.

The light that is absorbed (subtracted) is transformed into heat. This explains why a black object, which absorbs all of the colors of light hitting it, gets much hotter in sunlight than a white object that reflects all colors. subtractive color

When the primary subtractive colors of cyan, yellow and magenta pigments are mixed together, the result is black -- or, because of impurities in the pigments, a dark shade of something resembling mud.

To solve the "mud" problem, sophisticated color printing processes use CYMK, with "K" standing for black.

With either process all color is essentially absorbed where all of the inks or pigments overlap, as seen in the center of the above illustration.

Also note what happens when you mix the three primary subtractive color pigments (yellow, cyan and magenta).  

You can also see that yellow and cyan produce green, magenta and cyan produce blue, etc.

If you take a few minutes to drag around the colored squares in this additive and subtractive color demonstration you can see in a very clear way how the primary colors interact.

Note: This may take a minute or so to load the first time and you may have to okay a notice about an earlier Java version.

 


red filter

>>When a colored filter or gel is placed over a camera lens or light, the same type of color subtraction takes place.

For example, a pure red filter placed over a camera lens will absorb all colors of light except red.  

Many people erroneously assume that the red filter simply "turns all of the light red," which, as you can see, is not the case.


Additive Color

>>Thus far we have been talking about the subtractive color process -- the effect of mixing paints or pigments that in various ways absorb or subtract colors of light.

When colored lights are mixed (added) together, the result is additive rather than subtractive.

Thus, when the additive primaries (red, green and blue light) are mixed together in the right proportions, the result is white.

additive color This can easily be demonstrated with three slide projectors.

 Let's assume that a colored filter is placed over each of the three projector lenses -- one red, one green, and one blue.

When all three primary colors overlap (are added together) on a white screen, the result is white light. Note in this illustration that the overlap of two primary colors (for example, red and green) creates a secondary color (in this case, yellow).

Again, you can clearly see this in the additive color part of the interactive color demonstration.

>>The standard color wheel is the key to understanding many issues color wheelin color television.

Red, green and blue are TV's primary colors, and yellow, magenta, and cyan are considered secondary colors.

If you take the time to memorize the color wheel on the left, you will find it useful in many areas -- not just TV.

If any two colors exactly opposite each other on the color wheel are mixed, the result is white.

Note that instead of canceling each other as they did with subtractive colors, these complementary colors combine for an additive effect. (One definition of complementary  is "to make whole.")

" Objects with colors that are opposite each other on the color wheel tend to exaggerate each other when seen together. For example, blue objects will look 'bluer' when placed next to yellow objects and reds will look 'redder' when placed next to cyan."

 

>>It may be obvious at this point that by combining the proper mixture of red, green and blue light, any color of the rainbow can be produced.

Therefore, in color television only three colors (red, green and blue) are needed to produce a full range of colors in a color TV picture.

In essence, the color TV process is based on the process of separating (in the camera) and then combining (in a TV set) different proportions of red, green and blue.

Although this explanation has long sufficed for a basic understanding of the process, technically, things go beyond this. For a far more in-depth explanation, red indicator click here.


Simultaneous Contrastsimultaneous contrast example

>>Question:  Which of the small rectangles in the center of these illustrations is the lighter shade of blue?

Answer: they are exactly the same. It's the level of brightness (saturation) of the surrounding color that can make the square on the left appear lighter.

According to the concept of simultaneous contrast the way we perceive the brightness of an object depends on its background.

In television production this concept can be especially important in commercials for wearing apparel where certain colors and shades are critical to harmonizing accessories, or where an advertiser wants to promote subtle colors that are "in vogue."

Not understanding simultaneous contrast can lead to some unpleasant surprises. One TV director was doing a food commercial for tuna fish and, unfortunately, saw fit to put the tuna on a magenta-colored plate. This made the tuna fish look green--not exactly an appetizing appearance for this kind of product. 

Clearly, a little knowledge can save you problems and embarrassment.


Three-Chip Video Cameras

>>Let's use our knowledge of color to understand how a three-chip video camera works. (You will recall that we covered chips and CCDs in Module 8 .)

The full-color image "seen" by a professional video camera camera prism blockgoes through a beam-splitter (on the right half of the drawing) that separates the full-color picture into its red, green and blue components.

Note, for example, that all red light in a color scene is split off by a color-selecting mirror and directed to one of the three ▲CCDs.

In the same way, all of the blue light in the original picture is directed to the blue receptor. The green light is allowed to pass through to the CCD at the back of the prism block.

Thus, what was a full-color picture has now been separated into the percentages of red, green and blue light contained in the original scene.

>>The CCDs in multiple-chip cameras are basically "color blind"; they just respond to the amount of light focused on their surface.

The red, green and blue information from a full-color picture is shown below.

When the appropriate color is added to each of the three "black and white" images (the first three illustrations), and combined, you get the full-color result shown in the final picture.  

red chanel

Red Channel

blue channel

Blue Channel

green channel

Green Channel

combined colors

The Three Colors Combined


Note that the red laser light is detected primarily by the red channel and that the blue-green laser housing (bottom-right of each picture) is detected primarily by the blue and green color channels.

Few colors are "pure"; most contain some white light. Thus, they are normally detected to some degree by more than one color channel.  Note that the white shirt is detected equally by all three color channels.

>>This takes care of color; but how does a color camera detect pure black or white?

You can probably guess. 

Since white is the presence of all colors, the camera's chips or imaging devices respond to pure white as the simultaneous presence of all three colors. Black is simply the absence of all three colors.

One-Chip Color Cameras

>>Interestingly, one-chip cameras are split between low-end and very high end cameras, with not much in between.

Although Color Chip Filter many professional cameras use three chips, it's possible to use one chip with an overlay of millions of tiny colored filters. A greatly enlarged section of one type is shown on the left.

At the low end the chip size is quite small (with the image quality compromises that represents) and at the high end the chip size is roughly equal to the image size on 35mm motion picture film, meaning the image quality can be very high.

In this Relative chip sizes.illustration here the blue area is the approximate size of the one-chip, high-end professional camera and the light colored bottom rectangle the size of a simple, one-chip camera.

The difference in size can make a major difference in image quality -- and also price. The professional cameras with the larger chips used in major productions go for tens-of-thousands of dollars.

Some of the professional advantages of the large, one-chip cameras are discussed in Module 17.

In one-chip cameras the image is not separated by color and sent to separate chips, as discussed previously, but a mosaic filter sits on the chip itself and the colors are separated by electronic circuitry.

A greatly enlarged section of ▲another type of mosaic color filter is shown in this pop-up illustration.

After the color image from the lens activates points on these mosaic filters that respond to the red, blue, and green light, an electronic circuit is able to separate each of the three colors and send them on their way as three separate electronic signals.

It is also possible to make a mosaic camera filter that responds to only two colors of light, with the third color added through extrapolation. (See "A Little Simple Algebra" below.) 

Although mosaic filters make possible smaller and less expensive camcorders, with small image areas this approach sacrifices resolution (image sharpness) and light sensitivity.


How the Eye Sees Color  

>>You might assume from the above that in color television "white" would simply result from an equal mix of the primary colors.

Unfortunately, it's not that simple. For one thing, the human eye does not see all colors with equal brightness.

The eye is much more sensitive to yellowish-green light than to either blue or red light.

Due to the greater sensitivity of the eye to the green-to-orange portion of the color spectrum, an equal percentage mix of red, green and blue colored light will not appear white.

Because of this, and because of the limitations and variations of the color phosphors used in different TV screens, the actual color mix used in color television to produce white ends up being an unequal mix of red, blue and green.


A Little Simple Algebra

>>In the equation: A + B + C=100, if the values of "A" and "B" are known, it's easy rca color to figure out "C."

In the same way, it's not always necessary to know the values of all three primary colors, only two. Thus, color cameras can be made that have only two chips.

Assuming that for a particular TV system 59 percent green and 30 percent red and 11 percent blue equals white (a common mix), then the camera would only need to "know" two of the three colors when it was white-balanced.


Component and Composite Video

>>Although using all three colors throughout the TV process may be the most consistently accurate way of reproducing color, the requirement of three separate color signals at every stage of the process can be technically demanding.

Using even more complex math, it's possible to reduce the three color signals into a single signal.  When the three color signals are combined into one, we refer to it as composite video. (Definitions for composite include "merged" or "combined.")

Unfortunately, in the process of combining the three signals subtle interactions take place between the colors (color bleeding); plus, there is a general loss of video quality.

Although these problems may not be noticeable to the untrained eye, they become progressively worse (and noticeable) when the video is copied.

Since this is what happens in the editing process, editing composite analog video can be a problem.

One solution it to keep the color signals separate and to use high-quality (and expensive) digital video equipment.

Unfortunately, this can put camera and recording equipment out of the price range of most consumers. So, we are looking at a trade-off between price and quality.

We'll revisit this issue when we discuss the various video recording processes in Module 46


Green, Yellow, Blue, and Red Square Reminder


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