Various high-intensity discharge (HID) lamps can increase PPF levels in controlled environments. Barrier materials such as glass, Plexiglas (acrylic) and water are usually placed between the lamps and the growing area to provide ventilation of the lamp space for removal of heat. Bubenheim et al. (1988) observed that spectral compositions (in the 400 to 800 nm range) produced by any of several lamp types tested were not significantly changed by filtering through any of these barrier materials. The dry-tempered, 4 mm glass and the 5 mm Plexiglass single sheet filters reduced PPF 7%.
PPF==Photosynthetic Photon Flux (PPF)

http://ncr101.montana.edu/Light1994C...ung%20text.htm

.