Vogt introduced Koehler illumination, and the reddish Nernst glower was replaced with the brighter and whiter incandescent lamp.[1] Special mention should be paid to the experiments that followed Henker's improvements in 1919. On his improvements the Nitra lamp was replaced with a carbon arc lamp with a liquid filter. At this time the great importance of color temperature and the luminance of the light source for slit lamp examinations were recognized and the basis created for examinations in red-free light.[4]