Page 84 - Fundamentals of Light Microscopy and Electronic Imaging

THE DIFFRACTION IMAGE OF A POINT SOURCE OF LIGHT 67

Point source
Pin hole

(a)

I

(b)

Figure 5-5
The image of a point source of light viewed through a pinhole is a perceptible disk. (a)
Viewing a point source of light through a pinhole. (b) Relative sizes of the diffraction
disk on the retina when a point source is viewed with normal vision (dotted line) and
through a pinhole (solid line). With a pinhole, the diffraction disk expands about 10-fold
to 30–40 m, covering a patch of hundreds of cone cells on the retina, thereby
allowing perception of a visible diffraction disk and surrounding rings.

In cameras and telescopes, the image of a star is likewise always a finite diffraction
disk (not a point) with linear radius r 1.22λ (f/D), where f/D is called the focal ratio
or f-number. Roughly, the diameter of the diffraction spot in m is the focal ratio in mil-
lionths of a meter. Thus, the diameter of the diffraction spot in the primary focal plane
of the 250 cm diameter, f/5 mirror of the Hubble space telescope is 5 m. In the case of
the human eye or photographic camera, the image of a point source on the retina or film
has a diameter of about 3 m. In all of these optical systems, the image of a point source
corresponds to a diffraction disk, and the terms NA and f/D are measures of the effective