Page 110 - Modern Optical Engineering The Design of Optical Systems

The Primary Aberrations 93

Figure 5.20 The secondary spec-
trum of a typical doublet achro-
mat, corrected so that C and F
light are joined at a common focus.
The distance from the common
focus of C and F to the minimum
of the curve (in the yellow green
at about 0.55 ) is called the
secondary spectrum.

The second major chromatic residual may be regarded as a variation
of chromatic aberration with ray height, or as a variation of spherical
aberration with wavelength, and is called spherochromatism. In ordi-
nary spherochromatism, the spherical aberration in blue light is over-
corrected and the spherical in red light is undercorrected (when the
spherical aberration for the yellow light is corrected). Figure 5.21 is a
longitudinal spherical aberration plot in three wavelengths for a typical
achromatic doublet of large aperture. The correction has been adjusted

Figure 5.21 Spherochromatism. The longitudinal aber-
ration of a “corrected” lens is shown for three wave-
lengths. The marginal spherical for yellow light is
corrected but is overcorrected for blue light and under-
corrected for red. The chromatic aberration is corrected
at the 0.707 zone but is overcorrected above it and
undercorrected below. A transverse plot of these aberra-
tions is shown in Fig. 5.24k.