TECHNIQUES OF SLIT-LAMP ILLUMINATION 167 . . : Figure 5. Camera set-up for slit-image stereophotography. (Courtesy of Carl Zeiss, Inc.) may be employed for a second observer (Fig. 4). Camera attachments are available for both anterior segment photography and for slit image photogra- phy (7). Stereophotography (8) can be accomplished with the aid of two cameras (Fig. 5). TYPE OF SLIT-LA5IP ILLUI•{INATION There are txvo basic types of slit images used in biomicroscopy: a parallele- piped and optical section. When employing the parallelepiped, a rectangular beam (1-2 mm wide and 5-10 mm high) is projected through the cornea •vith the slit illuminator at a 45 ø angle. The shape of the illuminated area is similar to a parallelepiped prism whose outer and inner surfaces are curved due to the curvature of the cornea. When using the optical section slit image, the width of the beam is narrowed to almost its minimum and is projected from an angle of about 45 ø through the cornea. The result is a sagittal view or optical section similar to a thin histological section. The width of the anterior and posterior faces of the optical section should be about 20 ./z.

168 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The types of illumination used with the described slit images are: direct illumination, diffuse illumination, direct retro-illumination, indirect retro-illu- mination, sclerotic scatter, indirect illumination, and specular reflection (9). With direct illumination, the beam and the microscope are sharply focused at the same area in the same plane (Figs. 6 and 7). The microscope is directly in front of the eye and the angle between the slit illuminator and the corneal microscope is 45 ø. A rectangular beam, with no greater thickness than 2 mm, is projected from an angle of 45 ø to the optical medium. When the parallele- piped is formed on the cornea, three general areas may be observed: the epi- Figure 6. Direct illumination. Slit illuminator beam and cornea] microscope focused on same area and plane. CM--• corneal microscope SI----slit illuminator C--cornea I---- Figure 7. Direct illumination of normal rabbit cornea ... ..