Iconoscope Design and Function

[old_tv_nut]

Donald Fink explains the operation of the iconoscope in Television Engineering, 2nd Ed, 1952, pp.102-104.

1) [Paraphrising Fink] The simple and wrong explanation is that each element of the mosaic takes charge from the scanning beam depending on the photoelectric charge of the element. If this were true, it would produce a sensitivity gain of about 150,000 times compared to a non-storage device (image dissector). However, experiments show that the electron beam in the iconoscope has practically infinite resistance, that is, it always delivers a constant current to the mosaic. What is varying is the amount of secondary emission depending on the voltage on each mosaic element, which in turn depends on how much photoemission has taken place. Each element is also being discharged by a shower of secondary electrons from all the other elements. The result is that the sensitivity of the iconoscope is only about 100 times that of an image dissector, since the variation between a light and dark "pixel" is only the variation in secondary emission.

I think the improvement from using a collector ring is in having fewer secondary electrons fall back on the target, thus increasing the net charge on the mosaic elements, although Fink simply assumes that a collector is used.

By the way, the beam does not "overcome the resistance" of the insulator. The mosaic elements are miniature capacitors and current flows by charge/discharge with a change in electric field, but no electrons flow through the support plate.

2) All photo-emitters are delicate - if a substance gives up electrons easily, it usually means it is chemically somewhat unstable as well; dependent on the exact formulation and treatment in manufacturing, etc. In fact, when I worked on a flying-spot video player using film and photomultipliers, we found that taking a photomultiplier out of its box under bright fluorescent lighting would damage the photocathode and significantly increase the noisiness of the tube. So, I believe the sensitivity of the tubes could easily be different from unit to unit.

I don't know about differences in spectral peak. I think tubes of similar formulation would be similar, but perhaps differ somewhat. According to Fink, the usual iconoscope mosaic had high sensitivity in the red and near infrared, which would be modified to reduce infrared response by a final step of evaporating a small amount of silver onto the mosaic after the cesium deposition.

3) I don't know what the construction of the two sided iconoscope target consisted of. If it had similar globules on an insulator, that would mean the insulator had to be transparent. Perhaps it was too difficult to make in the large size for the studio iconoscope.