Читать книгу Optical Engineering Science - Stephen Rolt - Страница 77

A microscope cover slip 0.17 mm thick is to be used with a microscope objective with a numerical aperture of 0.8. The refractive index of the cover slip is 1.5. What is the root mean square (rms) spherical aberration produced by the cover slip? The aberration is illustrated in Figure 4.7.

From Eq. (4.24):

Figure 4.7 Spherical aberration in cover slip.

Substituting the above values we get: K_sa = 0.003 22 mm or 3.2 μm.

The wavefront error (in microns) is thus given by:

where p is the normalised pupil function.

For reasons that will become apparent later, in practice, wavefront errors are usually expressed as a fraction of some standard wavelength, for example 589 nm. The above wavefront error represents about 0.4 × λ when expressed in this way. An rms wavefront error of about λ/14 is considered consistent with good image quality. This level of aberration is, therefore, significant and measures must be taken (within the objective) to correct for it.