From a large body of data of quanta irradiance (350–700 nm) in different water masses the depth has been derived at which the percentage of surface irradiance of quanta is 30%, 10%, 3%, and 1% respectively. It has been shown that the relationship between the 10% level and the other levels can be expressed by simple equations with an accuracy sufficient for most practical purposes. Thus three of the levels can be computed if one is known. This implies that a classification of sea water in terms of transmittance of quanta irradiance is established for the whole euphotic zone.