Anomaloscope

An anomaloscope (1) comprises two light sources that can be viewed side-by-side. The first light source is a referente white light source (6) which is made up of a blue LED which excites a yellow phosphor on its cover and a green LED, which when viewed together present a white light. The second light source is a comparison light source (7) which comprises a blue LED and a yellow LED. A subject-adjustable mixture control (8) is provided for adjusting the proportions of yellow and blue light in the comparison mixed light source (7) to allow a subject to try to match the comparison mixed light source (7) to the referente white light source (6).