How is the intensity of lights measured?

To test the shape of a reflector, two brass gauges are used, one for the back and one for the concave face^[1]. Its reflecting power is then proved by trying a burner in the focus, and measuring the intensity of the light at various points of the reflected conical beam^[1].

From experiments made by Sir James Douglass, it appears that the intensity of the light of each candle must have been about 2*8 candle units each, so that the aggregate intensity of radiant light from the twenty-four candles did not exceed sixty-seven candle units^[1].