Fabrication and Assembly of a High Production Injection Molded Optical System

The Polaroid Spectra Camera contains an unusual telescopic viewfinder comprised of injection molded components and glass mirrors. The reverse telephoto design includes seven lenses, five aspheric surfaces, and two prisms. Figure 1 depicts the Spectra Camera and the location of the viewfinder 24 and the orientation of the optical axis OAv of the viewfinder. Figure 2 depicts the folded optical path. Note that the image is erected using a 4 mirror system. Figure 3 is a layout of the unfolded optical path. The surfaces marked by asterisks are aspheric. All of the lenses are fabricated by injection molding acrylic plastic. Camera geometry dictated a folded optical path and imposed severe packaging requirements. Figure 4 is an exploded view of the entire viewfinder assembly. All of these components (with the exception of parts 184 and 198) are machine assembled, aligned and tested using an automated assembly machine. Parts are placed into the viewfinder housing using pick-and-place units or robots and are retained by snaps, clips, or retainer housings. Cycle time for each assembly step is less than 5 seconds. The viewfinder is bore sighted by adjusting the vertical tilt of 40 (mirror 1) and the lateral position of the field mask 48. The system focus is verified using a modulation measurement technique which images a rotating radial grating through the viewfinder and onto a slit. There is no adjustment for focus. Poorly focused assemblies are discarded; hence, it is imperative that high piece part quality be maintained and that the design allow for reasonable tolerance stack up. Figures 5 and 6 depict details of a snap mount and the one mirror alignment screw.