HOW DOES TV WORK?
As you already know, the central principle of all video equipment is that a focused image must be converted into an electrical signal, which can be transmitted either by cable or by a radio frequency. When the signal is received, it can be reassembled on a screen as a visible image. All visible images may be thought of as variations of light and shade. In order that they can be converted into electrical signals, they must be broken down into a very large number of dots, ranging from black to grey to pure white, which are in turn arranged as a series of slightly slanted horizontal lines on the screen – 625 in Europe, 525 in the US and Japan. In order to convey the effect of motion, each of these lined pictures must last no more than 1/25 of a sec (1/30 sec in US), but in practice even this very brief duration would appear to flicker when viewed, so each picture of action is scanned twice by the tube by a series of interlaced lines which when combined produce a complete picture. Thus the field frequency of each system is double the picture frequency – that is 50 Hz in Europe and 60 Hz in US and Japan. In short, one frame in video is made up of two fields.
Color television systems work with the additive primary colors of light: red, green and blue. These colors combine to form white light, as well as any other color of the spectrum. Color tv systems take the light that enters the lens and break it into its red, green and blue components (RGB) with the help of a small prism block, through a series of dichroic mirrors that reflect certain colors and let others pass or through a stripe filter. The two components of the color signal are luminance, which refers to the black and white brightness information, and chrominance, which refers to the color information and includes the components of hue (color) and saturation (intensity of color)
The TV Camera and its Components
The most important component of the video camera is the image sensor. It is essentially an optical video transducer, which means that it changes light into electrical energy. A CCD or a charge coupled device image sensor is one of the most popular sensors. It is a solid-state semi conductor that converts incoming light into a video signal. In the CCD, incoming light strikes a layer of metaloxide or silicon semi conductors where it is converted into an electric charge. Each of these conducting points is referred to as a pixel. The number of pixels in the “chip” determines the amount of detail, or resolution, the camera will be capable of producing.
- Aperture: the variable opening by which light enters a camera.
- Electronic shutter: it controls the amount of time that the incoming light hits the photosensitive layer of the chip.
- Focal length: it is the distance from the optical centre of the lens to the point where the image is in focus.
- Depth of field: it refers to the portion of the scene that is in focus in front of the camera. It is directly proportional to the amount of light entering the camera and the distance from the camera to the subject and in inversely proportional to the focal length of the lens.
- Lens: A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.
- Normal: A normal lens is a lens that reproduces perspective that generally looks “natural” to a human observer under normal viewing conditions.
- Wide: a wide-angle lens refers to a lens whose focal length is substantially shorter than the focal length of a normal lens for the image size produced by the camera.
- Zoom: Zoom lenses have a focal length that varies as internal elements are moved, typically by rotating the barrel or pressing a button which activates an electric motor. Commonly, the lens may zoom from moderate wide-angle, through normal, to moderate telephoto; or from normal to extreme telephoto.
- Telephoto: a telephoto lens is a specific construction of a long focal length photographic lens in which the physical length of the lens is shorter than the focal length. In these lenses the optical centre lies outside of its physical construction, such that the entire lens assembly is between the optical centre and the focal plane.
- Fisheye: A fisheye lens is a wide-angle lens that takes in an extremely wide, hemispherical image.