An apparatus comprises a light source which produces a first radiation within a first wavelength range and which produces a second radiation within a second wavelength range when a property of the light source is changed. The apparatus further comprises an optical device which receives the first radiation and attenuates the first radiation to emit a first attenuated radiation having a first perceived brightness. The optical device also receives the second radiation and attenuates the second radiation to emit a second attenuated radiation having a second perceived brightness. The first and second perceived brightnesses are approximately equal.