Kerr cell.

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Kerr cell.

Postby Stephen » Thu Apr 26, 2007 12:00 am

Has anyone utilised a Kerr cell as a light source modulator in recent years? It would seem to me to be ideal for modulating a high power light source such as an incandescent bulb for NBTV display use, but for perhaps the relatively high drive potential needed. If so, I would be interested in the design details.
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Postby Panrock » Mon Apr 30, 2007 3:01 am

I made two of these many years ago using a short wave variable condenser (fully closed) as the 'plates' with the light shining through the sides and cross-polarised films either end. The nitrobenzene had been specially obtained, with some difficulty. Polythene containers (I think) were used.

I never got them to work properly, even with 1000 volts across the plates, at which point I gave up for fear of an explosion ejecting poisonous nitrobenzene in all directions!

I then stored them in a secret never-visited corner of my house for years, almost as carefully as nuclear waste. Then about two years ago I disposed of them (along with the bottle of nitrobenzene) carefully with the assistance of the local council.

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Postby Viewmaster » Mon Apr 30, 2007 3:21 am

Although Nitrobenzine is the best liquid, although dangerous as you say, one can use safer liquids such as Carbon Disulphide or Acetate.
The plates should only be a few thou apart so a variable condenser may have been the reason it didn't work as their plate spacing is probably in the order of 10 thou plus.
Also volts can be up to 30k....stand well back :)

There is also a Pockel Cell using a crystal of some kind.
Not worth all the bother.....or danger. :shock:
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Postby M3DVQ » Mon Apr 30, 2007 7:59 am

maybe I've misunderstood the kerr cell... but as I understand it you are attempting to modulate a high intensity beam of light. could a liquid crystal cell not be used for this? it has the advantage of not requiring thousands of volts or being full of extremely hazardous chemicals :wink:
has anyone experimented with liquid crystal cells for NBTV modulation I wonder? this could be used to modulate a headlamp or suchlike for a flying spot projector perhaps
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Postby DrZarkov » Wed May 02, 2007 4:26 am

Liquid crystal cells should be easy and cheap to get, just look for shutter glasses for use with a PC to watch 3D. Those glasses are available since nearly 20 years by now (it started on Atari in that time), I've got some two years ago for 50 Cent. Unfortunally I gave them away after somebody asked me what are that for funny glasses...
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Postby Klaas Robers » Wed May 02, 2007 10:36 pm

I wonder if the speed of LCD as a light modulator would be fast enough to modulate the light up to a frequency of 10 kHz....
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Postby Stephen » Wed May 02, 2007 10:55 pm

Klaas Robers wrote:I wonder if the speed of LCD as a light modulator would be fast enough to modulate the light up to a frequency of 10 kHz....
My recollection is that LCD panels have sufficient bandwidth to display one line of information at a time with standard television formats, thus meaning that they can at least switch in the 15 kHz range. Therefore they should be fine for NBTV applications. What I do not know is the linearity of response of such panels to analogue input signals. The common way to modulate them is by way of pulse duration modulation. Such an LCD modulator might need an analogue to digital converter to drive the LCD panel.
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Postby DrZarkov » Thu May 03, 2007 12:29 am

Sometimes you find very cheap LCDs for use with an overhead-projector. For us the elder ones in black-and white are more interesting, I think. Those displays have a common VGA-connector (which is analogue), some of them have a composite connector, which would make things much easier. Another possibility wouldbe a pocket-TV (for example the old Casio black and white models.). Exspecially the Casio has no backlite, but a diffusor, it uses sunlight to make the picture visible. This TV is a curiosity by itself, because you view the picture in a mirror, not on the TFT. (Like in a prewar Telefunken TV). You can use that display with very little modification I guess. Now you can get such a TV for about 10 EUR, but I bet that it will become a collector's item in a few years...

Update:

Here are the photos:

Image

The white array is not the display, but the diffusor!

Image
On the upside is the black and white TFT, you see the TV-picture in the mirror. Here is this area are no longer any programmes analogue available, so I left the TV-set switched off.
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Postby Klaas Robers » Sun May 06, 2007 5:46 am

LCD's for TV display are not faster than 25 Hz. One pixel is addressed once per frame and keeps that brightness for at least 1/25 of a second. In LCDs for PC display the speed (slowness) sometimes is already a problem when displaying fast movements. Then a "tail" or "blurr"follows the moving object on the screen, especially when the screen is cold. This is an advantage for TV, because it is not needed to store the brigntness from one frame to the next.

If you want to display NBTV on a LCD that could be done. The usual flicker will be much less than with a Nipkow disc. But for using it as a light modulator they are far too slow. Only Kerr cells or Pockel cells can be used. They are fast enough.
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Liquid Crystal Kerr Cell.

Postby Stephen » Tue May 08, 2007 7:02 am

Ordinary LCDs may be unsuited for light valve use, although liquid crystal light valves of other configurations or that use other liquid crystalline substances may be fine. An early liquid crystal light valve patent, British Patent 441,274 filed 13 July 1934 by Barnett Levin and Nyman Levin of Marconi, describes Kerr cells that use liquid crystalline substances as the birefringent material instead of nitrobenzine. They claim that ethyl-alisalamino-cinnamate has a Kerr constant of 7 when used in the nematic phase of its liquid crystalline state. Nitrobenzine only has a Kerr constant of 4 × 10<sup>-5</sup>! They also claim that with the construction of cell that they describe that this reduces drive potential from 4000 volts for nitrobenzine to about 200 volts. This is a usable level. I have posted this patent in the Patents and Articles section for reference.

Also see http://www-optique.enst-bretagne.fr/18_LCOASLM.htm for the description of an optically activated liquid crystal spacial light modulator. Possibly the scanning area of a Nipkow disc could "write" an image on the photosensor surface and then a high power light source would reflect the reflective "read" image off of the liquid crystal surface. I have no idea how much these little modulator devices cost, but they are interesting.
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