The unit was defined as that illumination emanating from a lamp burning pure colza oil (obtained from the seed of the plant Brassica campestris) at a defined rate.
4) Two one million candlepower lamps pointing downwards at an angle to estimate height above the forest canopy, using a similar principle to that adopted for night flying during the 617 Squadron Dams raids in World War 2 and developed by Barnes Wallis (for whom the machine was named),