Earlier field examinations of the primary productivity of an eupsammic algal community in a sandy beach of Lake Piaseczno have shown vertical stratification of the primary production of algae and chlorophyll a concentration in the ground. The chlorophyll concentrations decreased almost uniformly or from the ground surface, or their highest values were reported in the layer of 10-20 mm, depending on the time of measurements. On the other hand, the highest quantities of primary production were reported at a depth of 30-40 mm under the sand surface, regardless of the date of investigation. In the same station, on eight days of July (1994) with different solar conditions, light intensity (in the range of 400-700 nm – PhAR i.e. Photosynthetically Active Radiation) was measured, using the differential photometer "Aquatic PAR-Meter" (of Sonopan). One of its two sensing elements measured the light intensity on the sand surface as 100%, while the other, covered by the required thickness of sand, simultaneously registered light intensity as a percentage of the above figures. The measurable figures of light intensity reached a depth up to 6 mm only, in spite of the fact that primary production of algae in the sand reached a depth of up to 60 mm at least and its maximal figures were recorded at a depth of 30-40 mm.