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47
OPTICAL TERMINOLOGY
9.4 Strehl ratio
When a point light source is observed with an aberration-free
optical system and an aberrated optical system , the former
concentrates the focal point to a point at the image formation
position. In contrast, the latter fails to produce a focal point,
instead causing a spread in the intensity distribution of the point
image (this is known as “point spread”). The specific
appearance of such a point image (i.e. point spread) is shown in
Fig. 9-9.
With the proportion of light concentrated in the image plane
(intensity of light concentrated in the Airy disk) by an aberration-
free optical system serving as 100%, the proportion of light
concentrated by an aberrated optical system is known as the
Strehl ratio. When graphed, the Strehl ratio reveals peaks in
intensity as shown in Fig. 9-10. The higher the SR, the closer an
optical system is to being aberration-free.
A Strehl ratio of 80% is typically called the diffraction limit, and
lenses with a lower ratio lack the performance required to serve
as an objective lens. A ratio of over 95% means that the lens’
performance in general observations is comparable to that of an
aplanatic lens (which is corrected for spherical aberrations and
coma).
Note) A laser interferometer is used for actual assessment of
optical performance, so assessment is done at a single
wavelength. Unless otherwise noted, Strehl ratio measurements
are at the e-line (544nm).
Figure 9-9 Appearance of condensed light in
the image plane (point spread)
Aberration-free optical system Aberrated optical system
Figure 9-10 Strehl ratio
Intensity
Aberration-free
optical system
A
berrated
optical system
Image plane
100%
SR