How the lamps work
The altimeter in the Lancaster was useless at low levels and could not give the height of the plane to the precise height of 150 feet (later 60 feet) that was required for a successful Upkeep launch.
This problem was solved by the Royal Aircraft Establishment. They advised attaching two spot lamps, one in the nose and one behind the bomb bay aligned to meet when the plane was at exactly 150 (or 60) feet.
Too low and the lamps will not meet. Too high and they will pass each other and shine appart again. At the correct height though, the two lamps meet on the surface of the lake water. At that time, the plane will be at exactly the correct height to release Upkeep.
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