Efficiency is usually a ratio of energy (or work) output by a device compared to the energy required to run that device. The ratio is always less than one unless you believe in perpetual motion machines. Focusing on optical efficiency, we typically ratio the total number lumens output by a luminaire to the total number of lumens produced by the light source inside that luminaire. Since there are losses in all optical systems, the ratio is again, less than one. We usually multiply the fraction by 100 and express efficiency in terms of percent.

Generally, the more complex an optical system is the more losses occur, so the efficiency is usually lower for spot luminaires than for wash fixtures. For typical, state of the art automated spot fixtures, efficiencies run from 15% to around 25%. A popular automated spot uses a 1200 watt arc lamp that emits around 110,000 lumens. This fixture can put about 18,000 lumens on the wall. Every aperture inside the fixture, every surface of each lens, each filter eats away at the total amount of light that passes. These losses can be minimized by taking extreme care in the optical system design and by employing highly efficient optical coatings on all optical surfaces to minimize the losses there. With careful attention to detail, optical efficiencies of very complex automated spotlights can be as high as 40%.