Friday, 2 May 2014

TV Camera Tubes

TV Camera Tubes: Camera is the first and basic equipment in a TV. The input to a camera  is the light from the picture or scene to be televised and output obtained from camera is the electrical pulses corresponding to the information contained in picture.

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 transducing action convertsit 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.

The image-orthicon, vidicon and plumbicon are some important electronic scan camera tubes which find wide applications these days.

1. Image Orthicon:

It is a sensitive tube and is capable of handling a wide range  of light values and contrast. In a single envelope, it includes three sections:

(a). Image Section: This section includes:
1. a photo sensitive surface, called photo cathode, operated at a very large negative potential.
2. a target plate which is a thin plate of glass of low resistivity. Thickness is less than 0.0002 in.
3. a screen located very close to target plate and has about 500,000 openings per square inch.

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.

Merits and Demerits:

1.It has high Sensitivity
2. The S/N ratio is better and its typical value is 30 dB.
3. It’s spectral response is close to eyes.
4. The ratio of signal current to illumination os gamma and it varies from unity at low light to 0.5 at high light levels.
5. It produces no lag.
6. Size of image orthicon is bulky in nature.
7. It’s operation is elaborate.
8. It is very costly camera tube and life time of this camera tube is nearly 1500 to 6000 hrs.

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.

Advantages:

1. Low cost.
2. Simple Adjustment.
3. Sensitivity is large.
4. Resolution of the order of 350 lines can be achieved under practical conditions.

Disadvantages:
1. Owing to the fact that the resistance of the photoconductive film does not change instantaneously with change of light intensity, different levels of light intensity are adjusted with slight time slag.
2. The response characteristic is non-linear.

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.

Merits and Demerits:

1. In plumbicons, the uniluminated or the dark current is negligible and also it is temperature independent.
2. It has got high sensitivity and a high signal to noise ratio.
3. Resolution is good but not as good as that of a vidicon.
4. Operational gamma is unity.
5. It is compact and exhibits simplicity of operation.
6. It is free of spurious signals.
7. Susceptibility to damage by over loads is not as severe as it is in vidicons.
8. There are some forms of PbO which have spectral limitations.

Comparison of various camera tubes:


Parameter
Image Orthicon
Vidicon
Plumbicon
01
Photo electric Conversion
Photo emission
Photo conduction
Photo conduction
02
Sensitivity
High
Good
Better
03
Illumination (lumen)
750 - 1000
1500 - 2000
750 - 1000
04
S/N Ratio
30 dB
50 dB
50 dB
05
Speed
No lag
Severe lag
Reduced lag
06
Resolution at 400 lines (5MHz)
30 – 50 %
55 %
40 – 50 %
07
Special response
Close to eye
Good
Poor in Red
08
Gamma
1.0 near black.0.5 near high lights
0.4 to 0.9
0.9 to 1.0
09
Operation
Elaborate
Simple and Quick
Simple and Quick
10
Sizes
Length- 15 to 20 inches. Diameter 3 to 4 inches
Length- 5 to 8 inches. Diameter 0.6 to 1.6 inches
Length- 8 inches. Diameter 1.2 inches
11
Cost
Costliest
Cheapest
Costlier
12
Life (hours)
1500 – 6000 hours
5000 – 20000 hours
2000 – 20000 hours


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