Vacuum Optical Array Transducers
Vacuum Optical Array Transducers
Vacuum optical array transducers are based on a 2-D photosite array, that is there is no vacuum 1-D equivalent of the solid state linescan camera, and have a typical construction as shown in Figure 1. These devices include
the television cameras and have traditionally been very popular as optical transducers even though, they are increasingly being challenged by solid state cameras in the robotics field. Vacuum television cameras include the recently introduced high sensitivity Newvicon and Chalnicon as well as the longer established Vidicon and Plumbicon cameras. These latter two types are by far the most popular and will therefore be described in more detail in this Section. The main difference between the various vacuum television cameras lies in the photosensitive area (usually known as the ‘target’) which is coated with lead oxide (PbO) for the Plumbicon, selenium or antimony compounds (e.g. Sb2O3) for the Vidicon and zinc and cadmium tellurides for the Newvicon and Chalnicon.
The scanning of the photosites is achieved via an electron beam in the same way as in cathode ray tubes, namely by the use of magnetic deflection coils. The principle of operation, illustrated by Figure 2, is as follows :
as the electron beam scans across the target area it ‘closes’ the switch S on each photosite (i.e. it connects it to the cathode which is at ground potential) thereby charging its equivalent capacitor Ceq to a specific voltage V1. While the beam is scanning another area of the target (i.e. switch S is open), the capacitor discharges through its equivalent parallel component and attains a new, lower voltage V2.
As the electron beam scans across the target area again, it ‘recharges’ the photosites’ capacitors back to V1, at a rate Ic proportional to the voltage difference (both the capacitor value Ceq and the time interval Δt are in fact constants), as shown in eqn (3.8) :The value of the parallel component, however, is light dependent which means that the capacitor ‘recharge’ current Ic is proportional to the incident irradiation Ein : by monitoring the value of Ic during the scanning period we therefore have a waveform Vout proportional to the photo sites illumination levels (i.e. to the scene focused on to the target area) , as required.
Figure 3.13 shows that in a Vidicon the parallel component is equivalent to a Light Dependent Resistor (LDR) and its output characteristic is therefore non-linear, the non-linearity being created by the gamma factor γ (sometimes referred to as gamma distortion) shown in eqn (3.11):The Plumbicon parallel component, on the other hand, is equivalent to a reverse biased photodiode which has a linear characteristic, as illustrated by eqns (3.13) and (3.14)A comparison between the vacuum television camera types (J. Loebl, 1985) shows that the Vidicon can handle the largest input dynamic range and is therefore less susceptible to image ‘blooming’ (an apparent defocusing of the bright regions of the television image), it also has the lowest unit cost, the lowest sensitivity (that is it requires a higher minimum input light level) and a less linear input-output relationship due to the gamma distorsion.
The Plumbicon, by contrast, has the lowest image lag (it only takes 2 or 3 scans to ‘forget’ the last image compared to 4 or 5 for the Vidicon), a good sensitivity and a linear input-output response but does exhibit a high unit cost, caused by the necessity to maintain the lead oxide air-free during the entire manufacturing process. Both the Vidicon and the Plumbicon have a wavelength response similar to that of the human eye, unlike those of the Newvicon and Chalnicon which are far more red and infra-red sensitive.
Newvicons and Chalnicons, on the other hand, feature the highest sensitivity (they will give a useful image in a semi-darkened room) at a unit cost similar to that of the Vidicon.