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 (1 trillion = 1,000,000,000,000) 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 0,00078 mm or 780 nm (nanometer) to a wavelength of 0,00038 mm (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.
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.
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.
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 = magenta (purple) red + green = yellow blue + green = cyan (blue/www.magnavox.com/electreference/videohandbook green) 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 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 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 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.