Television:Basic Principles
The Sound and Light Spectrum
Video is a combination of light and sound, both of which are made up of vibrations or
frequencies. We are surrounded by various forms of vibrations: visible, tangible,
audible, and many other kinds that our senses are unable to perceive. We are in the
midst of a wide spectrum which extends from zero to many millions of vibrations per
second. The unit we use to measure vibrations per second is Hertz (Hz).
Sound vibrations occur in the lower regions of the spectrum, whereas light vibrations
can be found in the higher frequency areas. The sound spectrum ranges from 20 to
20,000 Hertz (Hz). Light vibrations range from 370 trillion to 750 trillion Hz. When
referring to light, we speak of wavelengths rather than vibrations.
As a result of the very high frequencies and the speed at which light travels (300,000
km per second), the wavelength is extremely short, less than one thousandth of a
millimeter. The higher the vibration, the shorter the wavelength.
Not all light beams have the same wavelength. The spectrum of visible light ranges
from wavelength of 780 nm to a wavelength of 380 nm. We perceive the various
wavelengths as different colors. The longest wavelength (which corresponds to the
lowest frequency) is seen by us as the color red followed by the known colors of the
rainbow: orange, yellow, green, blue, indigo, and violet which is the shortest
wavelength (and highest frequency). White is not a color but the combination of the
other colors. Wavelengths which we are unable to perceive (occurring just below the
red and just above the violet area), are the infrared and ultraviolet rays, respectively.
Nowadays, infrared is used for such applications as remote control devices.
Visible light as part of the electromagnetic spectrum.
Note: visible light is only visible because we can see the source and the objects
being illuminated. The light beam itself cannot be seen. The beams of headlights in
the mist for instance, can only be seen because the small water drops making up the
mist reflect the light.
Luminosity
Besides differing in color (frequency), light can also differ in luminosity, or brightness.
A table lamp emits less light than a halogen lamp, but even a halogen source cannot
be compared with bright sunlight, as far as luminosity is concerned. Luminosity
depends on the amount of available light. It can be measured and recorded in a
numeric value. In the past, it was expressed in Hefner Candlepower, but nowadays
Lux is used to express the amount of luminosity.
Brightness Values:
Candle light at 20 cm
10-15 Lux
Street light
10-20 Lux
Normal living room lighting
100 Lux
Office fluorescent light
300-500 Lux
Halogen lamp
750 Lux
Sunlight, 1 hour before sunset 1000 Lux
Daylight, cloudy sky
5000 Lux
Daylight, clear sky
10,000 Lux
Bright sunlight
> 20,000 Lux
Luminosity is the basic principle of the black-and-white television. All shades between
black and white can be created by adjusting the luminosity to specific values.
Color Mixing
There are two kinds of color mixing: additive and subtractive color mixing. The mixing
of colorants, like paint, is called subtractive mixing. The mixing of colored light is
called additive mixing. Color TV is based on the principle of additive color mixing.
Primary colors are used to create all the colors that can be found in the color
spectrum.
Additive Color Mixing
In video, the color spectrum contains three primary colors, namely red, green and
blue. By combining these three, all the other colors of the spectrum (including white)
can be produced.
red + blue
red + green
blue + green
= magenta (cylamen)
= yellow
= cyan (turquoise)
green + magenta
= white
red + cyan
= white
blue + yellow
= white
red + blue + green = white
Making colors in this way is based on blending, or adding up colored light, which is
why it is called additive color mixing. Combining the three primary colors in specific
ratios and known amounts enables us to produce all possible colors.
By combining the three primary colors red, green and blue, other colors
can be mixed, including white.
White light is derived from a ratio of 30% red, 59% green, and 11% blue. This is also
the ratio to which a color TV is set for black-and-white broadcasts. Shades of grey
can be created by maintaining the ratio percentages and by varying the luminosity to
specific values.
30% red + 59% green + 11% blue = white
Light Refraction
Light refraction is the reverse process of color mixing. It shows that white light is a
combination of all the colors of the visible light spectrum. To demonstrate refraction a
prism is used, which is a piece of glass that is polished in a triangular shape. A light
beam travelling through a prism is broken twice in the same direction, causing the
light beam to change its original course.
Beams with a long wavelength (the red beams) are refracted less strongly than
beams with a short wavelength (the violet beams), causing the colors to fan out. The
first fan out is enlarged by the second fan out, resulting in a color band coming out,
consisting of the spectrum colors red, orange, yellow, green, blue, indigo, and violet.
There are no clear boundaries between the various colors, but thousands of
transitional areas. A rainbow is a perfect example of the principle of light refraction in
nature.
When white light, such as sunlight passes through a prism, it is
refracted in the colors of the rainbow.
Color Temperature
Color temperature relates to the fact that when an object is heated, it will emit a color
that is directly related to the temperature of that object. The higher the color
temperature, the more 'blue' the light, and the lower the color temperature the more
'red' the light. Color temperature of light can be measured in degrees Kelvin (K).
Daylight has a color temperature between 6000 and 7000 K. The color temperature
of artificial light is much lower: approximately 3000 K. In reality, color temperatures
range from 1900 K (candlelight) up to 25,000 K (clear blue sky). Television is set to
6500 K, simulating 'standard daylight'.
Various light sources with different color temperatures. Color
temperature is expressed in degrees Kelvin.
The Human Eye
The eye tends to retain an image for about 80 milliseconds after it has disappeared.
Advantage is taken of this in television and cinematography, where a series of still
pictures (25 per second) create the illusion of a continuously moving picture. Other
characteristics of the human eye are that it is less sensitive to color detail than to
black-and-white detail, and that the human eye does not respond equally to all
colors. The eye is most sensitive to the yellow/green region, and less in the areas of
red and (particularly) blue.