TV Camera

Camera is the production equipment of pictures. Camera fetches light from the picture or scene to and converts it in to electrical signals with the help of camera mechanism (like lenses & reels) camera tubes and some chemical process.  But digital era  solid state devices replaces the role of chemical reactions and magnetic tapes using electronics components like CCD & CMOS and start storing picture using compactable ssd storage devices

The TV Camera is just analogous to human eye. The basic principle of all TV cameras is based on the fact the each picture of all TV Cameras is based on the fact the each picture may be assumed to be composed of small elements with different light intensity. The camera picks up each element and by transuding action converts it into “electrical signal” proportional to its brightness there is a photosensitive layer called target or image plate in each camera which performs this job. At the same time simultaneous, pick up of this information is also necessary for this purpose. There is an electron gun (which produces an electron beam) which scans the image plate at a fast speed. Thus opto-electric conversion as well as pick-up of the signal takes place simultaneously and at a fast speed.

Analog Camera Tubes 

1. Image Orthicon

When the optical image is focused on the photo cathode, photoelectrons, in proportion to the amount of light impinging, are emitted. Most of the photoelectrons pass through the screen and hit the target plate.

As the photoelectrons are accelerated to several hundred electron volts, they liberate several secondary electrons from the target plate surface, and are then collected by the nearby-screen which is at a small positive potential. The emission of secondary electrons from target plate leaves a distribution of positive charge on its surface. The low resistivity of target plate resists the lateral charge flow on its surface and thus the image charge pattern, formed on the plate, is truly restored as such. Since the plate is thin, this charge pattern also appears on the other side (away from screen) of the plate.

(b). Scanning Section: The otherside of the pattern is now scanned by a beam of low velocity electrons generated by an electron gun. The beam is deflected on the plate in vertical and horizontal directions and enables the electron beam to scan the whole plate. This beam gives up the number of electrons required to neutralize the positive charge at that point and thus the returning electron beam varies in magnitude in accordance with the brightness variation of the image. It should be noted here that since the target portion affected by the white portion of the image will be positively charged and hence the electron beam has to give up large number of electrons to neutralize the positive charge at that point, i.e., the intensity of returning electron beam is much reduced and the video signal developed across the output resistor for this part will be small. It, therefore, concludes that the brightest part of image are transmitted as the signals of low amplitude which is very advantageous in avoiding the effect of strong noise at the receiver.

(c). Electron Multiplier Section: An electron multiplier is located within the pick-up tube for amplifying the electron density variation in the returning beam.

2. Vidicon:

This camera tube based on the photo conductive properties of semiconductors i.e., decrease in resistance with the amount of incident light. The tube is shown in figure. It consists of

(a). Signal Plate: Which is a conducting metallic film very thin so as to be transparent. The side of this film facing cathode is coated with a very thin layer of photoconductive material (amorphous selenium). This side is scanned by electron beam. The optical image is focused on the other side of this film.

(b).Scanning System: The electron beam for scanning is formed by the combination of cathode, control grid-1, accelerating grid-2 and anode grid-3. The focusing coil produces an axial field which focuses the beam on the film. Vertical and horizontal deflection of the beam, so as to the scan the whole film, is accomplished by passing saw-tooth current waves through deflecting coils which thus produce transverse horizontal and vertical magnetic fields respectively. The alignment coils are for initial adjustment of the direction of electron beam.

Operation: When the scanning beam passes over the photo conductive material of the signal plate, it deposits electrons so that the potential of this side of plate is reduced to that of the cathode. But the otherside of the film (plate) is still at its original potential. Consequently a potential difference across a given pointon the photoconductive material is created. It is approximately 30 V. Before the next scanning (which may be done after an interval of 1/50 or 1/25 sec.) the charge leaks through photoconductive material at a rate determined by the conductivity of the material which, in turn depends upon the amount of incident light.

White portions of the object will project more light on the film and make it more conductive. This charge leaked to photoconductive side of the film will vary according to illumination of the object. As a result, potential at every point on the photoconductive side will vary. Now the electron beam again starts scanning the photoconductive side of the film but this time the charge deposited by the beam in order to reduce its potential towards zero (cathode potential) will vary with time. Therefore current through RL (and hence the output voltage) will follow the changes in potential difference between two surfaces of the film and hence follows the variations of light intensity of successive points in the optical image.

3. Plumbicon:

The construction of a plumbicon camera tube is similar to that of a standard vidicon except for the target material. The plumbicon has a new type of photo-conductive target, i.e., lead oxide of the form PbO. The figure below shows the constructional features of a plumbicon camera.

Operation: The operation of a plumbicon camera tube can be best explained from the diagram. Initially, when there is no light input, the PIN diode is reverse biased due to a positive potential appearing on SnO₂ coating (n-type) and p-type stabilized at a potential slightly below the cathode due to negatively charged scanning beam. This results in a very small output current which is almost negligible. This is the greatest advantage of a plumbicon camera tube especially when used with color systems. The photo electronic conversion is almost similar to that of a standard vidicon except for the method of discharging each storage element. In standard vidicon each element acted as a leaky capacitor with leakage resistance decreasing with more light. Here when light falls on the target, the diode becomes forward biased upon the extent depending upon light intensity. The forward bias on each diode results from the photo excitation of the pure PbO and doped PbO junction. Thus the target behaves as a capacitor in series with PIN diode.

Digital Cameras

The digital camera is one of ­the most remarkable instances of this shift because it is so truly different from its predecessor. cameras depend entirely on chemical and mechanical processes -- you don't even need electricity to operate them. On the other h­and, all digital cameras have a built-in computer, and all of them record images electronically

The new approach has been enormously successful. Since film still provides better picture quality, digital cameras have not completely replaced conventional cameras. But, as digital imaging technology has improved, digital cameras have rapidly become more popular.

Just like a conventional camera, it has a series of lenses that focus light to create an image of a scene. But instead of focusing this light onto a piece of film, it focuses it onto a semiconductor device that records light electronically. A computer then breaks this electronic information down into digital data. All the fun and interesting features of digital cameras come as a direct result of this process.

Instead of magnetic tape, a digital camera has a sensor that converts light into electrical charges.

The image sensor employed by most digital cameras is a charge coupled device (CCD). Some cameras use complementary metal oxide semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors convert light into electrons.

CCD Sensor

Once the sensor converts the light into electrons, it reads the value (accumulated charge) of each cell in the image. This is where the differences between the two main sensor types kick in:

·         A CCD transports the charge across the chip and reads it at one corner of the array. An analog-to-digital converter (ADC) then turns each pixel's value into a digital value by measuring the amount of charge at each photosite and converting that measurement to binary form.

·         CMOS devices use several transistors at each pixel to amplify and move the charge using more traditional wires.

Differences between the two types of sensors

·         CCD sensors create high-quality, low-noise images. CMOS sensors are generally more susceptible to noise.

·         Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip is lower. Many of the photons hit the transistors instead of the photodiode.

·         CMOS sensors traditionally consume little power. CCDs, on the other hand, use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.

·         CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality pixels, and more of them.

Although numerous differences exist between the two sensors, they both play the same role in the camera -- they turn light into electricity. For the purpose of understanding how a digital camera works, you can think of them as nearly identical devices.