Originally discussed by Boys in 1893, the rotating mirror streak camera is designed to determine the time differences of a sequence of events. Previous work describes various geometries that define the operation of streak or smear cameras. A simplified streak geometry, as shown in Fig. 1, describes the locus of any image point produced by placing a rotating mirror in a bundle of converging light cones. Absent from Fig. 1 is the objective lens that views the subject or primary object under study and presents an image of the subject on the slit plate that defines the linear element of the object to be recorded. The linear image defined by the field stop, i.e., camera slit, becomes the object for the relay lens that in turn makes a final image of the slit via the rotating mirror on the focal surface. The final image of the slit is swept normal to its length to produce a one-dimensional space-time recording usually called a smear or streak record.
Purpose and Use of Field Lenses
In any optical system in which multiple image planes are formed, a potential problem always is that a cone of light from the primary objective lens will not be captured by succeeding relay lenses. Thus, the use of field lenses is important because a field lens, as shown in Fig. 1, ensures that each cone of light emanting from any point on the slit will be redirected to strike the entrance pupil of the relay lens without loss of illumination. The cone of light of an image point located on the otpical axis will, of course, always continue to be symmetrically intercepted by the following relay lens. Nevertheless, one can always find a sufficiently off-axis image point such that its cone of light will, after forming the image point at the slit, completely miss the succeeding relay lens. This condition is called complete image vignetting and is llustrated in Fig. (2). Intermediate image points will have various degrees of vignetting where only some part of the cone of light will be intercepted by the relay lens. The purpose of a field lens located near the image plane at the camera slit, as shown in Fig. 2(b) is to bend or refract the cone of light back onto the succeeding relay lens. The field lens behaves as a variably angled prism. Thus, as an image point moves from the center to the edge of its image field, the field lens contributes to forming increasing angles of incidence and consequently exhibits increasing refractive power. This goal of refracing and redirecting image-forming cones of light is accomplished when the field lens' focal length is chosen to image the exit pupil of the objective lens onto the entrance pupil of the camera relay lens.