Flying spot camera usable with ambient light.

[Stephen]

John Logie Baird invented the "spotlight" or flying-spot method of scanning, as described in his British Patent 269,658, filed 20 January 1926. In it, he scans an "image spot" across an object by means of his lens disc, which he also invented as described in British Patent 230,576, filed 29 December 1923.

Curiously, the patent, like his earliest spotlight cameras, uses the lens disc to not only scan the image spot on the object, but also to simultaneously scan the spot-illuminated object onto a photocell behind the disc. This is unlike the flying spot arrangement of later cameras, with banks of photocells arranged in front of the spot-illuminated object to pick up light reflected off of the object. I thought it curious that the preferred embodiment in the patent, as well as Mr Baird's early spotlight cameras, placed a photocell behind the disc rather than the later arrangement using banks of photocells outside of the camera or spotlight projector.

The reason for scanning the photocell as well as the image spot just occured to me over the weekend after re-watching the video "JLB-The Man who Saw the Future". In the video there is a computer recreation of the spotlight camera as shown in the '658 patent. I realised after watching the beam pattern return back into the camera to the photocell that this arrangement allowed this version of the spotlight camera to work in ambient light and even outdoors!

Since the lens disc scans the object image simultaneously over an aperture in front of the photocell behind the lens disc, the photocell simply adds the ambient light to the image spot light for each image pixel to produce a slightly brighter image. In contrast, having banks of photocells arranged around a spotlight projector exposes the photocells to ambient light over the whole image simultaneously, not just the area of the scanned spot as shown in the patent. Thus, ambient light will "swamp out" the image spot from the spotlight camera so that it must operate in near darkness.

The disadvantage to the spotlight camera as shown in the '658 patent is that pickup is limited to a single photocell mounted in the camera itself. Perhaps the limited sensitivity of the photocells in the 1920s and 1930s required banks of photocells for sufficient signal and this is the reason that the embodiment shown in the patent was not later used.

However, it seems to me that with photocells now available that using this arrangement would allow a spotlight or flying-spot camera to function indoors and out with ambient lighting. I suspect that outdoors the ambient lighting would provide most or all of the illumination whilst indoors it might be just the spotlight or some combination of ambient light and the spotlight. The spotlight would serve as the equivalent of "fill-in flash" for photography.

Both the 269,658 and 230,576 patents are in the Patents and Articles section of the forum.

[DrZarkov]

I think the idea of a single photocell behind the scanning disc was the orginal idea by Paul Nipkow? I think this arrangement would be the first idea of everybody who experimented with television, and this was the reason, why other big companies in Germany and the USA had less success in the 20th than Baird, because it didn't work properly with the selen-cells in that times?

[Stephen]

I think the idea of a single photocell behind the scanning disc was the orginal idea by Paul Nipkow?

Of course you are right, Volker, with respect to a natural reflected-light camera. Mr Baird's earliest cameras used the same reflected light principle as set forth in Herr Nipkow's patent, except that they used Mr Baird's lens disc instead of the apertured Nipkow disc.

Mr Baird introduced the flying-spot camera to overcome the problem of providing sufficient natural reflected-light to activate the photocell in the camera. However, most all flying-spot cameras that used a scanning image spot, rather than natural reflected light, to analyse an object used separate photocell banks not behind a scanning disc. The camera described in the '658 patent is a flying-spot camera that uses a scanning image spot in combination with a photocell that analyses a scanned image of the object, which is not an obvious combination. This would allow operation both in low natural indoor lighting as well as outdoor lighting conditions.