Cooke triplet

The Cooke triplet was the first lens which was quite well color corrected for two colors (achromatic lens) and also corrected against astigmatism (anastigmat), patented in 1893 by Dennis Cooke. The only later basic efforts made in lens history were the correction for three colors (apochromatic lens), the coating against flares, and the mass-production of aspherical lenses.

The combination of two convex lens elements made of crown glass with a concave one which is made of flint glass is known since 1765 as a means against chromatic aberration. Achromatic lenses enable well focused projection of blue and green light. The draft exaggerates the different optical paths of the blue and the green light shares of the light coming from a blue/green (cyan) light mixture. The Cooke triplet was derived from this simple triplet lens construction.

Cooke Triplet lens from a 1916 advt. scanned by Geoff Harrisson  (Image rights)