Glossary: APOCHROMATIC

Apochromatic means "free from chromatic and spherical aberration", and is said especially of a lens in which rays of three or more colours (typically R, G and B) are brought to the same focus. As normal lenses focus different wavelengths of light (ie. the different colours in an image) differently, apochromatic lenses to ensure all colours in the image are focused at the same plane. This again ensures perfect display of details, and perfectly focused images down to sub-pixel level - it is for instance easy to show single pixel mirrors in a focused image, with the hinge clearly showing in the middle. Apochromatic lens designs thus improve contrast and sharpness by not letting the light scatter around on the projection screen, and do not discolour projected images.