Method of ophthalmoscopy using a blue-green filter in the illumination system. This gives a better contrast between the retinal vessels and the background and helps to differentiate more easily between retinal and choroidal lesions: retinal lesions appear black while choroidal ones appear grey. However, in most ophthalmoscopes that use tungsten filament bulbs, the amount of light of short wavelength is so small that the observation is difficult and a filter that lets more long wavelengths pass is used, such as a yellow-green one. As a result of this compromise there is only a slight increase in contrast between the retinal vessels and the background.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann