In addition, fixture lenses get dirty and discolor with age, and the timing and choice of a replacement can alter light output by more than 20 percent. Failure to keep lenses clean can cut output by another 10 percent. A new clear acrylic lens transmits over 92 percent of a fixture's light, as shown in Figure 2, while an opalescent (also called "milky white") diffuser may allow less than 50 percent of light to pass through it.
Figure 2: Light loss through fixture lenses
Old and dirty lenses can absorb as much as one-third of the light that tries to pass through them. In this case, a brief cleaning reduced losses from 29 percent to 19 percent (relative to a new K-12 lens).