HydroFlex, Inc. - Lighting - Changing Beam Patterns

To change the beam pattern (i.e. spot vs. flood) of any of the HydroPar or SeaPar lamps, the head must be opened out of the water and the lens or globe exchanged. Admittedly, it would be easier to change the diffusion lens if it was simply clipped on the outside of the lamp. However, the diffusing power of the lens would be seriously diminished if both sides of the lens were immersed in water. We feel it is important to retain the normal properties of the diffusion glass that technicians expect. Below is a more technical explanation of this as graciously provided by Mark Olsson, president of Deepsea Power & Light

"The ability of a lens (or Par cover/diffuser) to bend or spread light refractively is based upon the DIFFERENCE (actually the RATIO) in index of refraction between the glass and the medium (usually air) that it is immersed in. Glass has an index of 1.474 (Nd), air 1.000 and water about 1.33. When a par cover is placed underwater so that both surfaces (especially the inner shaped surface) are wet, the ability of the diffuser to spread light is very significantly reduced. This is a 2 to3 fold reduction in diffusing power. It is possible to stack 2 or 3 of these diffusers and achieve an equivalent 'in-air' effect. However, stacking can result in a bit more scattering, loss of light and has the problem of trapping bubbles between all the additional surfaces.

Underwater, the glass in the par lens has an effective index of refraction of 1.474/1.33 - 1.108, and hence, not a lot of bending power."