Color-corrected optical systems

An optical system comprising a plurality of refractive elements, the refractive elements of said system being made of only two different optical materials, said optical materials being exclusive of calcium fluoride, each of said optical materials having a characteristic dispersive power, the dispersive powers of said two different optical materials being related to each other so that color correction of said system at more than two wavelengths is possible, the refractive elements comprising said system having specified geometrical configurations and positions with respect to each other along an optic axis so that said system is color-corrected at more than two wavelengths and so that said system is well-corrected for monochromatic aberrations, said system being made by a process that includes selection of said optical materials, said optical materials being selected by:

• a) representing the dispersive power of each optical material by a polynomial expression in a variable that is a function of wavelength, the polynomial expression for each optical material being of a degree that is numerically one less than the number of wavelengths for which color correction is required;
• b) comparing a ratio of coefficients in the polynomial expression for one of said optical materials with corresponding ratios of coefficients in the polynomial expressions for others of said optical materials;
• c) identifying particular optical materials for which said corresponding ratios of coefficients in said polynomial expressions are substantially equal;
• and
• d) choosing optical materials for designing said system from among those particular optical materials for which said corresponding ratios of coefficients are substantially equal.