Opening the aperture of the slit to its widest setting creates a full, broad illuminating beam. The intensity of this beam can be dampened by the introduction of the neutral density filter to reduce the discomfort experienced by the patient. The wide beam provides diffuse illumination such as would be obtained from the illumination provided by a penlight or muscle light.

This technique is used to perform a general inspection of the anterior segment and adnexal structures under low magnification. It offers the observer the “big picture,” which might otherwise be missed if an area of interest is immediately isolated with high magnification. This technique is useful in assessing the overall degree of ocular erythema, identifying areas of corneal opacity or surface irregularity, localizing corneal and conjunctival foreign bodies, and determining patterns of normal and abnormal pigmentation (Fig. 6-1).

Examination with broad-beam illumination should also be performed while the light source is actively rotated from side to side. As the beam is applied tangentially at different angles, variations in elevation and depressions are emphasized and contrast is increased. This permits the observer to determine whether the lesion of interest is flat or if it has any degree of elevation (Fig. 6-2).

By narrowing the beam and shortening its height, the examiner can create a “spotlight” to permit viewing an object of interest in isolation without illuminating surrounding tissues. An essential derivative of this technique is the optical section. The slit-lamp biomicroscope provides the unique ability to view an optical cross section of the semitransparent (i.e., translucent) tissues of the eye (cornea, aqueous, lens) in vivo. Although the vast majority of light rays pass through the cornea unimpeded, slight irregularities in its substance result in reflection of light back to the examiner. This fact can be used to localize abnormalities to one or more layers of the cornea. Rotating the illumination arm allows the narrow slit beam to strike obliquely the lesion of regard. When angled at 45 degrees from the observation axis, maximal information about the internal structure of the cornea is revealed. At a lesser angle, the optical section collapses, whereas more than 45 degrees decreases brightness and gives a false impression of angular distance between the corneal layers.

The slit lamp is designed so that the slit beam and biomicroscope are focused at the same plane, thus allowing for movement of the illumination and viewing arms by the examiner to appreciate best the nature and depth of the lesion while remaining in focus. With this slit-beam technique, each layer of the cornea and tear film may be examined. Thus, the location of a lesion in the cornea may be determined by noting where it falls in the optical section. (Fig. 6-3).

The thin slit beam also permits the examiner to note the thickness and curvature of the cornea. Localized depressions result in the beam of light appearing to bend posteriorly. Similarly, elevated lesions within the cornea cause the light beam to appear to bend toward the observer (Fig. 6-4). This ability to detect changes in thickness may also be applied to the conjunctiva, where deviation in the appearance of the slit beam can detect elevations and depressions.

Focal illumination with a small, narrow slit beam also permits examination of the aqueous. The normal aqueous is transparent: Light rays pass through it unimpeded. White blood cells or increased protein content (flare) present during
inflammation result in reflection of light, or Tyndall’s phenomenon. A 0.2 × 0.2 light beam is applied at an oblique angle. On Haag-Streit and similar instruments, the light tower should be tilted forward. This technique is enhanced by observing the findings in the light beam against the black background provided by the pupil. The degree of inflammation is graded using a standard system (Tables 6-1 and 6-2).

Total internal reflection of light rays in the anterior segment requires the use of a gonioscopy lens to view the angle structures. Similarly, a portion of the light rays directed at the cornea undergoes total internal reflection between the anterior and posterior corneal surfaces. This can be maximized by directing the light beam at the corneoscleral junction. In the normal cornea, this technique results only in a faint glow at the limbus; the remainder of the cornea is not affected and remains clear. However, irregularities in the cornea, if present, cause the light being reflected in the stroma to be scattered and, in part, reflected toward the examiner (Fig. 6-6).

Sclerotic scatter is performed by directing the light beam tangentially toward the limbus until the entire limbal circumference appears to glow. Observation of the cornea is then performed. This technique is particularly useful for detecting subtle irregularities that might be missed with other examination techniques.