Figure 7-31 Optical principle of the Optos Optomap laser ophthalmoscope. V = vertical spinning polygonal mirror; H = horizontal spinning polygonal mirror. (Courtesy of Optos, Inc.)

Scheimpflug Camera

In a Scheimpflug camera, the image and object planes are tilted with respect to the instrument’s optics. Although this tilting yields a distorted image, it allows for a greatly increased depth of focus. Thus, such a camera can image, in sharp focus, a slit lamp’s optical section of the eye, which would not be feasible with an ordinary camera’s more shallow depth of focus.

Autorefractors

Various optical principles have been used in the design of automated objective refractors, some of which will be discussed briefly here. Retinoscopy, also called dynamic skiascopy, is the analysis of the direction and speed of motion of the edges of shadows. The optometer principle is illustrated in Figures 7-23 and 7-32. Autorefraction may also be accomplished by illuminating a small patch of the retina, which reflects light to emerge from the eye, passing through the eye’s optics. That emerging light, passing through the pupil, can then be imaged, and the image analyzed, in order to infer the optical power of the eye’s optics. The final result is calculated based on repeating this measurement along numerous paths through the subject’s pupil. The Scheiner principle gives another method of measuring the power of the eye’s optics.

Figure 7-32 The optometer principle. Instead of using interchangeable trial lenses, a single converging lens is used,

placed at its focal length (f) from the spectacle plane. Light from a target (T) on the far side of the lens enters the eye with vergence of different amounts (0, minus, or plus), depending on the position of the target. Vergence in the spectacle plane may thus be changed smoothly and is directly proportional to the axial displacement of the target. This arrangement exactly simulates a spherical trial lens having smoothly variable power. (Modified from Duane TD, ed. Clinical Ophthalmology. Vol 1.

Hagerstown, MD: Harper & Row; 1983:2.)

The Scheiner principle

Suppose that someone uses 1 eye to look at a small, distant light source. An occluder with 2 pinholes, one above the other, isolates 2 pencils of parallel rays coming from a distant small source. If the eye is emmetropic, the 2 pencils converge at the retina and the viewer sees 1 spot. If the eye is too short (ie, hyperopic) or too long (ie, myopic) for its optics, then 2 separate spots are illuminated on the retina. If we momentarily cover one of the holes, say the top one, which spot disappears? The myopic eye sees crossed images, which are projected mentally as uncrossed diplopia, so the patient says that the top spot disappears; conversely, the hyperopic eye sees the bottom spot disappear (Fig 7-33).

Figure 7-33 The Scheiner principle. Double pinhole apertures placed before the pupil isolate 2 small bundles of light. An object not conjugate to the retina appears doubled instead of blurred. (Modified from Duane TD, ed. Clinical Ophthalmology. Vol 1.