The basic elements of an imaging system are shown in Figure 5.1. The light from a source, either coherent (e.g., a laser) or incoherent (e.g., an incandescent lamp or an arc lamp), is collected by the illumination optics (e.g., a condenser lens) and projected onto the object. An image is then formed by an objective lens upon a screen, a photographic plate, a CCD camera, the retina of an eye, etc. Assuming that the objective lens is free from aberrations, the resolution and the contrast of the image are determined not only by the numerical aperture of the objective lens but also by the properties of the light source and the illumination optics.
The source and the illumination optics
Three types of illumination will be considered. For collimated and coherent illumination we assume a monochromatic laser beam brought to focus at the plane of the object with a condenser lens having a very small numerical aperture (NA). Figure 5.2(a) is the logarithmic intensity distribution at the object plane, produced by a 0.03NA condenser. This distribution has the shape of an Airy pattern, with a central lobe diameter of 1.22λ/NA ≈ 41λ, where λ is the wavelength of the light source. Since the objects of interest will be small compared to the Airy disk diameter, and since they will be placed near the center of the Airy disk, this illumination qualifies as coherent, fairly uniform, and nearly collimated.