Some interesting experiments have been made on the visibility of the electric light at sea by the government of the United States, Germany and the Netherlands. The word “visible” in the report on the tests means visible on a dark night with a clear atmosphere. The result of the experience of the German committee was that a white light of one candle power was visible 1.4 miles on a dark, clear night, and one mile on a rainy night. The American tests resulted as follows: In very clear weather a light of one candle power was plainly visible at one nautical mile; one of three candle power at two miles; one of ten candle power was seen by the aid of a binocular at four miles; one of twenty-nine candle power faintly at five miles, and one of thirty-three candle power plainly at five miles. On an exceptionally clear night a white light of 3.2 candle power was readily distinguished at three miles; one of 5.6 candle power at four miles and one of 17.2 candle power at five miles. In the Dutch experiments the results were almost similar, but a 16 candle power light was plainly visible at five miles. For a green light the power required was two for one mile, fifteen for two miles, fifty-one for three miles and 106 for four miles. The result of tests with a red light were almost identical with those with green, but it was conclusively proved that a white light was by far the most easily seen.
Candle power is one way of measuring how much light is produced by a lamp. Its value depends on whether the beam is focused or not, also. It is not the same as a foot-candle which is a measure of how much light hits an object a foot away from the source. Ain’t physics fun?
It is left as an exercise to the reader to plot the results. I did, ignoring all questions of reflectors, beam focusers, etc. Nyah.
I’m lucky if I can see a light across the back yard, let alone a mile away! That is the unfortunate consequence of our love of lit sidewalks and streets. I seldom see the stars here in AA.