Optical Systems lecture 6

Lecture 6
Chapter Two
Optical Instrumentation

1- STOPS, PUPILS, AND WINDOWS
Generally, not every light ray from an object point, directed toward or into an optical system, reaches the final image. Depending on the location of the object point and the ray angle, many of these rays are blocked by the limiting apertures of lenses and mirrors or by physical apertures intentionally inserted into the optical system. An aperture, in its broadest sense, is an opening defined by a geometrical boundary.
The apertures are often purposely inserted into an optical system to achieve various practical purposes:
1. Apertures can be used to modify the effects of spherical aberration, astigmatism, and distortion.
2. In other applications, apertures may be introduced to produce a sharp border to the image.
3. Apertures may also be used to shield the image from undesirable light scattered from optical components.

In any case, apertures are inevitably present because every lens or mirror has a finite diameter that effectively introduces an aperture into the system.
The presence of apertures in an optical system influences its image-forming properties in two important ways:
1- By limiting the field of view.
2- By controlling the image brightness.

2- THE CAMERA
The simplest type of camera is the pinhole camera, illustrated in Figure (2-1) a. Light rays from an object are admitted into a light-tight box and onto a photographic film through a tiny pinhole, which may be provided with any simple means of shuttering. An image of the object is projected on the back wall of the box, which is lined with a piece of film. As stated earlier, an image point is determined ideally when every ray from a given object point, each processed by the optical system, intersects at the corresponding image point. A pinhole does no focusing and actually blocks out most of the rays from each object point. Because of the smallness of the pinhole, however, every point in the image is reached only by rays that originate at approximately the same point of the object, as in Figure (2-1) b.
Alternatively, every object point sends a bundle of rays to the screen, which are limited by the small pinhole and so form a small circle of light on the screen, as in Figure (2-1)a. The overlapping of these circles of light due to each object point maps out an image whose sharpness depends on the diameter of the individual circles. If they are too large, the image is blurred. Thus, as the pinhole is reduced in size, the image improves in clarity, until a certain pinhole size is reached. As the pinhole is reduced further, the images of each object point actually grow larger again due to diffraction, with consequent degradation of the image. Experimentally, one finds that the optimum pinhole size is around 0.5 mm when the pinhole-to-film distance is around 25 cm.