Stray light is defined as unwanted light that reaches the focal plane of an optical system. Figure 1.1 illustrates an example of noticeable stray light. This photograph of clear, featureless sky was taken with a digital camera and zoom lens, with the sun just outside the field of view (FOV). The bright spots (called artifacts) are caused by light from the sun that strikes the camera lens and reaches the focal plane by physical mechanisms such as scattering from surface roughness, scattering from particulate contamination, and ghost reflections. These mechanisms are discussed in Chapters 4, 5, and 7, respectively.
Another example of stray light is shown in Fig. 1.2. This picture, also of clear, featureless sky, was taken with a Maksutov–Cassegrain telescope whose baffles were shortened to admit stray light. Light from the sun, which is located just outside the FOV, bypasses the primary and secondary mirror and passes directly through the hole in the primary to shine directly on the image plane. This type of stray light path (called a zeroth-order path) is discussed in Chapters 3 and 9.
The effect of aperture diffraction is shown in Fig. 1.3. Light from the street lamp inside the FOV diffracts from the iris of the camera and results in the radial streaks seen in this image. Aperture diffraction is discussed in Chapter 7.
In Figs. 1.1–1.3, stray light in the optical system resulted in unwanted light in the final image. The artifacts were not intended by the designer of the optical system; they occur because it is not possible to perfectly control the path of light between the scene and the focal plane of the system. The best that the designer can do is to use stray light control techniques to reduce it to a level that ensures proper functioning of the system. Stray light control is important in all optical systems.
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