Electronic Image Reproduction
At the center of all video field production systems is the portable video camera. A marvel of modern electronics, the video camera is charged with the responsibility of producing the electronic signal. All video cameras have a number of standard components, including an image sensor, viewfinder, camera control unit, and lens assembly. This post will focus on portable video cameras and on the function of the first three components listed above. We will look at lenses in our upcoming posts.
Function and Types of Camera Image Sensor
The most important component of the video field camera is the image sensor. Its function is to change light into electrical energy. Technically speaking, the camera image sensor is an optical-video transducer, which simply means that it changes incoming light (physical input) into an electrical video signal (electric output). By way of comparison, microphones are also transducers-they change incoming sound waves (physical input) into an electrical audio signal (electrical output). Two types of camera image sensors are now in use: CCDs (charge coupled device) and vacuum tubes (usually simply referred to as pickup tubes)
CCD (Charge-Coupled Device) Principles
A charge-coupled device (CCD) image sensor is a solid-state semiconductor that converts incoming light into a video signal. In the CCD, incoming light strikes a layer of metal oxide or silicon semiconductors where it is converted into an electric charge. Each one of these conducting points is referred to as a pixel. The number of pixel in the chip determines the amount of detail or resolution; the camera will be capable of producing. These pixels, or picture elements, are arranged in precise horizontal and vertical rows on the chip. High-quality CCDs often contain more than 300,000 individual pixels, or picture elements. The camera lens focuses the scene before it on this array of pixels, each of which is of reproducing one tiny part of the picture. After the incoming light is converted into an electronic charge, it is transferred and stored in another layer of the chip, and then the information is read out one frame at a time in a line-byline sequence in conformity with normal television scanning rates. CCDs are very small. Typically, the image sensing area of the chip is either 1/3”, ½” or 2/3”, measured diagonally. CCDs are much smaller, lighter, and more reliable, and they consume less power than traditional pickup tube type systems. Because they are much more rugged than conventional pickup tubes, CCDs have largely replaced pickup tubes in cameras and camcorders designed for home video market. They are also used in most professional cameras and camcorders designed for remote use, where the need is for a durable camera that will stand up to more physical abuse than is typically received by a camera used in a controlled studio environment. Indeed, CCDs may soon replace pickup tubes entirely as the imaging device in all consumer and professional video cameras.
Electronic Shutter
All CCDs cameras are equipped with an electronic shutter. The electronic shutter controls the amount of time that the incoming light hits the photosensitive layer of the chip. In recording situations where the recorded image tends to flicker or become blurred because of high speed movement of the subject in front of the camera, shutter speed can be increased to reduce these picture artifacts and improve the sharpness of the image. Although shutter speed position of OFF, 1/60, 1/100, 1/250, 1/500, 1/1000, and 1/2000 are fairly common on professional cameras and camcorders, the number and range of shutter speed positions available will vary depending on the quality (and cost) of the camera or camcorder.
The Camera Pickup Tube
Vacuum tubes have been in use since television cameras were first invented, and although they have been largely replaced in new camera designs by CCDs, pickup tubes continue to be used today in few cameras particularly large studio-type cameras and some cameras designed for high-definition television (HDTV). The process through which light is changed into the video signal by a pickup tube is one of the most fascinating things about television. Although it may seem complicated at first, it is really quite a simple process. The pickup tube in a video field production camera is a small vacuum tube usually 3” or 4” long and ½”, 2/3”, or 1” in a diameter. The tube has several main components. The tube itself is made of glass. Attached to the inside of the glass face of the tube is an extremely thin, transparent photosensitive coating. Next to this is a layer of photoconductive material. Immediately behind the photoconductive layer is a target, which has a slight positive electrical charge when the camera is turned on. Light from the scene that is being recorded is focused by the lens of the camera onto the face of the pickup tube. It passes through the glass and the photosensitive coating, onto the photoconductive layer. As light hits the photoconductive layer, it causes the charge on the target to change in proportion to the relative intensity of the light. The video signal is produced as the target is scanned by the beam of electrons. The beam of electron is emitted by the electron gun at the back of the pickup tube. The electron beam is focused on the target and scanned it in a series of horizontal lines. Each line of the picture is composed of about 500 pixels, or bits of information. Bright spots on the photoconductive layer change the charge on the target greatly, and when the particular spot is scanned by the electron beam, a great number of electrons pass through the target than pass through the places where the image on the photoconductive layer is darker. These changes in the charge on the target plate produce the video signal.
read more:
The Production Company
Working of Production Company
Types of Production
Initial Components of Video Field Production
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