The Lichtspricht 80

In these days of fibre optics and interest in signals transmitted by light, it. is well to remember the above sophisticated device which was the only light speech transmitter which was other than a bad joke on either side during the 1939/45 war. Made by Zeiss it gave a cast-iron link over 5 miles using infra-red light from a car headlamp bulb consuming no more than 5 watts.

From the, earliest days in the Desert, forward troops occasionally picked up what looked like a large pair of binoculars, always greatly prized loot. However, ones labeled as per the title above had plug sockets and one small eyepiece which, on further investigation peeped coyly out of the front between the two 80 mm lenses. In 1942 a number of discarded units were available for investigation.

This proved that one lens was the transmitter and the other the receiver of a. most efficient modulated light transmitter. The source was a very ordinary 5 watt bulb which passed through a mechanical modulator and a series, of prisms which covered monitoring and target location of which more later. The light circle at one mile was about 20 feet across, for ease of lining up this could be widened by putting in an extra lens to spoil the focus. For night use an infra-red filter could be introduced which had a few db loss and made the beam invisible after 50 feet or so. The receiver focused the light on to a small spot of thallium sulphide working in the range of 50 K ohms or so and this was followed by a not very clever head amplifier with a fair amount of bass cut which helped the signal if it were flickering in the hot rising air over the sands of the Desert. Battery case. and main amplifier completed the kit together with magneto caller and hybrid, It had a tripod mount.

The most ingenious part was the modulator. Driven by something like the old fashioned cone motor, a tiny prism about 2 x 5 mm was pressed against another glass surface in the light path at an angle such that the light was totally reflected if no contact was obtained between the glass surfaces. The small prism had been deeply etched with 4 or 5 lines in one direction and 10 or 12 lines in the other giving 50/60 little squares. These had been polished with an excess of polishing material such that each of the facts had become a little convex lens. When pressed against the plane surface Newton’s Rings were obtained ranging from 0 to nearly 100% either passing the light on or reflecting it as before. A very perfect linear modulator was so made which on overload left the surface for relatively long intervals giving gaps in one direction and full light in the other.

The use of the whole device was as follows:—

1.    You set up roughly using a rifle sight on the case

2.    You looked through the telescope and saw a red image of the filament (produced over one of the prism paths with a supplementary lens) floating about. You adjusted the head to bring this on target: (Another LS80).

3.    You switched the spreader lens in and spoke, adjusting to centre.

4.    You switched the spreader lens out and readjusted.

5.    You turned up the wick until you could see flickering in a little green square in the telescope due to the effect previously noted of the prisms separating at the modulator surface. (Half the light went for transmission and the other half for this monitoring).

If you were out of speech range you could use a Morse key and you had an Aldis lamp. Its use?  It was originally meant to bridge gaps in overhead or surface speech lines. It is doubtful if it was used much for this, as later in the Greek Islands it was used by patrols and strong points to keep contact with the next Garrison. Its design changed little – by 1945 a few extra light baffles. What a device: Still, it did have a rotten head amplifier!    So nothing is perfect!

Eric Vast – December 1980.

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Popular pages

Get your amateur radio licence - Find out more about amateur radio licence training.
Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
G3BDQ - John Hey's Rare QSL Cards.
Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
Vital Spark - A selection of articles re-published from the Vital Spark.
RSGB News - Find out how to get RSGB news on your mobile or PC.
Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
BBADL - Bath Based Distance Learning Course.
Conquest Hospital Radio - Presented by HERC member Antony (G4CUS).
Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
Club QSL Cards - A selection of QSL cards the club has received over the years.
Other Newsletters - Excellent newsletters and magazines from other clubs.
TX Factor episodes - Take a look at the TX Factors YouTube videos.
John Taplin - A bio of the late John Taplin.

Amateur Radio Resources

Other Radio Clubs & RAYNET

BSARS - Brede Steam Amateur Radio Society

RAYNET - The Hastings and Rother RAYNET Group.

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Sigord - Gordon Sweet
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