Ambient light intensities in the ocean at depths between 1500 m and 4700 m near Hawaii Island were measured around the one photoelectron level with 5″ diameter hemispherical photomultipliers. Measurements of count rates above variable thresholds were carried out in ship-suspended and bottom-tethered configurations. The ship-suspended rates show considerable fluctuation and their mean value decreases with depth approximately as exp [−x(m)/877]. The bottomtethered rates are about an order of magnitude lower than the ship-suspended rates and show little fluctuation. The calibration of our instrument indicates an absolute flux at 4700 m depth based on the bottom-tethered measurement of 218 −60 +20 photons/cm2·s, which is consistent with calculated intensities due to β-decay electrons from40K. The difference in the two cases is attributed to bioluminescence due to environmental stimulation.
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