This is the Optoma BX-DL080 Short Throw Lens. It is compatible with HD8600, TX785 and TW775 projectors.
The "throw distance" of video projectors describes the maximum distance the projector can "throw" or project the image to the screen. Most lenses provide one foot of screen per every two feet of space. When there's a need for larger images within a smaller space, an ideal solution is the use of a short throw lens.
The BX-DL080 yields approximately 20% larger that standard lenses at the same distance from the screen.