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1.1 Introduction This first part begins with identifying the basic elements that make up an optical system: source or target, aperture stop, field stop, image plane, entrance and exit pupils, entrance and exit windows. These elements and necessary radiometric definitions are discussed and applied in analyzing the extended simplified radiometric performance equation [Eq. (1.7)]. This equation is structured to include the major contributions to the detected signal in an optical system: the thermal radiation laws that describe the transmission through the atmosphere, the optics of the system, and finally the response of the detector. These concepts are discussed in sequence, except the third one, which will be addressed last and in more detail, because it embraces the major subject of this tutorial, optics. 1.2 Basic Optical Relations An axial ray travels from the axial object point through the lens and on to the image plane. The axial ray that passes through the edge of the aperture stop is called the marginal axial ray. The principal ray, also referred to as the chief ray, is an oblique ray from an off-axis object point through the center of the aperture stop. The marginal principal ray begins at the edge of the covered object and travels through the center of the aperture stop and the edge of the field stop. As will be discussed in Chapter3, these two marginal rays are the only ones needed to calculate the primary, third-order aberrations, u and up are the angles formed by the axial ray and the principal ray relative to the optical axis. This is indicated in Fig. 1.1. The aperture stop is the physical opening in the optical system that limits the size of the axial energy cone from the object. The image of the aperture stop in the object space is the entrance pupil, and the image of the aperture stop in the image space is the exit pupil. In lens systems, the object space is to the left of the first lens surface. The image space is to the right of the last lens surface. The opening that limits angle up of the principal ray is the field stop. Its image in object space is the entrance window. In the image space, the field stop image is the exit window. In Fig. 1.1, D is the size of the aperture stop and d′ is the size of the field stop. Figure 1.1 also indicates that an object located a distance s to the left of the lens is imaged at distance s′ to the right of the lens.
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