Lesson 11 – Imaging
Among the most important applications of optics is the creation of images. This is accomplished by bending the path of light as it travels using curved surfaces of lenses and mirrors. These imaging optics can cause light to converge or diverge by various amounts depending on their effective focal length (EFL). When locating the image of an object, the distance from the object to the imaging optics is important. The object distance to the imaging optics and the focal length of the imaging optics may be used to calculate the image distance between the image and the imaging optics.
Two important rays may be traced from an object through a lens or mirror to find an image, the marginal ray and the chief ray. Both rays start at the top edge (the “height”) of the object. The marginal ray travels parallel to the optical axis, coming from optical infinity, so it is refracted through to the focal length of the imaging optics. The chief ray is the only ray that is not deviated by the imaging optics—the ray’s angle going in is the same as it is going out. When these two rays intersect again in image space, both the size and location of the image may be determined.
Other parameters are common to describe imaging system performance. The field of view (FOV) gives the angle to the object height as seen from the imaging optics. The clear aperture (CA) or effective diameter (Øe) is the area over which light interacts with the imaging optics. The f-number is the ratio of the EFL to the CA, and the numerical aperture may be determined from the f-number; both represent the imaging system’s speed, or ability to collect light.