A submersible light meter that measures and records irradiance versus wavelength over the range 400–750 nm is described. A data acquisition system coupled to the radiometer permits collection and processing of large numbers of field observations. The incident spectrum is determined by an optical system incorporating a wedge interference filter monochromator and photodiode. Recorded data are converted to spectral irradiance (μw cm−2 nm−1) at 5-nm intervals by applying standard lamp calibration and immersion factor corrections. Minimum detectable radiation varies from 0.05 to 0.016 μw cm−2 nm−1 depending on wavelength. Response of the meter is not affected by temperature in the range of 3.5–23.0 C and angular dependence of the light collector follows Lambert’s Cosine Law for incident angles less than 55° from the vertical.Changes in the spectral composition of incident light with depth are demonstrated for two lakes.