Some manufacturers specify the illuminance in lux (lx) of their underwater lamps, but tell nothing about the boundary conditions, for which the specified light intensity is reached. A high illuminance is reached even with a small light flux of the light source in combination with small opening angles of the optics which focus the emitted light on a small area and short distances betweem light source and illuminated area.
The calculator below establishes the relationship between light flux and expected illuminance at a given geometry and optics. It should be added that in the calculation, according to "keep it simple", some simplifications have been made. In particular, a non uniform distribution of light on the illuminated area was not considered. Furthermore it is assumed that the entire area is completely flat and the illuminated area is a circle with a sharp border between illuminated and not illuminated areas, which does not occur for various reasons in practice. Therefore, the Note: the results of calculation may differ from the actual situation in practice. They are "useful approximations" with an expected error of less than 10% usually.
One question is still unanswered: Which illumination is needed? Some typicall values (more here):
- cloudy summerday: 20.000lx
- cloudy winterday: 3.500lx
- illumination TV-studio: 1.000lx
- office: 500lx
- good Illumination for video- and digitalcam: 100lx
- night of a full moon: 0,25lx