There are three different kinds of lenses available for Desire fixtures: linear, oval, and round. For each kind there is a small range of available beam sizes.
Linear lenses are useful for things like wall grazing, where you might want to spread the light horizontally across the width of the wall but keep the vertical spread very tight for good control and focus. Linear lenses can be double-stacked, and depending on how you rotate them, you can create different custom shapes. For example, with the lenses rotated 90 degrees from each other, you can make a square or rectangular beam. In general, beam edges stay fairly tight with linear lenses (similar to a soft, barn-door-type cut).
Oval lenses are similar to the linear ones, but whereas linear lenses spread the beam in only one plane, oval lenses both lengthen and widen the beam, albeit in one direction more than the other. Oval lenses produce a much softer beam edge compared to the linear lenses, which is useful when blending light from multiple fixtures.
Round lenses produce a round beam. The widening is uniform in all directions, which is why these lenses are cut in a square shape (since rotating them in a gel frame would have no effect). Like oval lenses, these produce a softly tapered beam edge.
Each one of these lenses is far more optically efficient than standard diffusion media. We have measured approximately 8-10% light loss on each lens. This is all related to standard Fresnel light loss, and for that reason, they are more efficient with the rough or patterned side facing inward toward the light source.
Any combination of multiple lenses may be used. Expect a roughly 10% reduction in output with each lens. When stacking linear lenses, if their rotation is significantly less than 90 degrees apart from each other, you may sometimes see a kind of moire effect of light and dark stripes. This never happens with oval and round lenses, due to the random microstructures that make them work.
You can find an Excel spreadsheet here that shows very detailed beam distribution and efficiency information for each lens individually:
The worksheet tabs are labeled "__R" for round lenses, "__L" for linear, and "__O" for oval.
We initially labeled these lenses with a specific beam-angle designation (e.g., 30deg round), but we found that this was creating confusion because our different LED fixtures sometimes have slightly different native beam angles, which changes the effect of these secondary lenses. The softly tapered edge of the oval and round lenses also made it difficult for users to consistently agree at what angle the beam edge lies. Generic beam designations are simply more appropriate for these accessory products.