In Lufwaffe raids the bombers had formidable protection from night fighters and there was an urgent need to find a way of giving our defending fighters a better chance against them. Britain was ahead in the radar race and the idea was conceived to miniaturise radar sets to be small and robust enough to be carried on our own night fighters. A downward-looking radar system called “H2S” was already in use in bombers, which gave some rough view of the ground through cloud cover. The need was for forward-looking radar with the detailed resolution to pick out a fighter aircraft and distinguish it from a bomber, with a display that the pilot could use for guidance towards his target and aim. The equipment hardware was made by EMI to use special valves developed by STC: small, tough, with suitable power and able to operate reliably at the necessary frequency. The design fell to Chris and the engineers in the Development Lab.
It is not an exaggeration to say that carrots can be described as nothing less than ideal for night vision.
When the time came for testing, a prototype set was fitted into a Beaufighter and flight tested by John Cunningham. Already an established fighter ace, he had also been a part-time test pilot for DeHavilland before the war. After initial testing, when the DeHavilland Mosquito became available it was found to be a natural test-bed for all manner of purposes and Cunningham was the natural choice to fly it for further radar development.
His mounting successes against night fighters were announced in BBC news broadcasts and celebrated as a booster for public morale. It was important to avoid Germany guessing that Britain had developed viable airborne radar. A cover story was put about, that Cunningham’s success came because he ate a lot of carrots thus improving his night vision. The nickname "Cat's-eye" was created for him, suggesting that he could see in the dark, like a cat. Testing continued until the system was well enough developed and was put into major production for supply to all fighters engaged in night flying.
credit: http://www.bbc.co.uk/ww2peopleswar/stories/14/a9033914.shtml