The underwater irradiance meter or submarine photometer described here is designed to operate in the upper 100 to 150 m. It permits direct measurement of the downward blue-green irradiance (480 mµ) to a depth of 100-120 m on a clear day in water with an attenuation coefficient in the blue-green region of the spectrum of approximately 0.04/m. The meter can accommodate as many as five individual detector units as well as an associated depth-sensing element....