**11. Related imaging techniques**

### **11.1. Fundus photography**

rect the diverging pencils of rays toward the observer. Because these elements are so close to the eye, the field of view can be very wide. Indeed, without moving the lens, the view reach‐ es 200 degrees, that is, from equator to equator, 4 to 5 times the diameter (16 times the area)

**Figure 9.** Contact lens arrangement for wide-angle indirect biomicroscopy. A high-power contact lens forms an in‐

**Figure 10.** Indirect ophthalmoscopy of a high myope. The myopic eye forms its own aerial image (dotted lines) with‐ out the help of the ophthalmoscopy lens. Without the lens, only the central part of this image would be visible (dash‐

The size of the image inside the front lens is 70% of the retinal size; for detailed examination, therefore, 50% more microscope magnification is required than with the other slit-lamp methods. However, the principal use of this lens is not for its magnification but for its over‐ view, an overview previously achievable only in fundus drawings or photocompositions. Similar contact lens arrangements are used in specially designed fundus cameras that allow fundus photography of areas 100 degrees or more in diameter. With lenses such as these, the spectrum of our examining methods can be extended not only toward higher magnification than with direct ophthalmoscopy but also, at the other end, toward an overview of the fun‐

As the technology to calculate, design, and manufacture lenses with aspheric surfaces has improved, it has been possible to make lenses with higher powers and better light gathering abilities. The number and variety of lenses for indirect ophthalmoscopy and of contact lens‐

ed lines, limited by the patient's pupil). With lens (solid lines) the image is limited by the lens rim.

dus considerably beyond that obtainable with regular indirect ophthalmoscopy.

es for slit-lamp microscopy has grown accordingly.

verted image (F') inside a spherical element, which redirects the light toward the observer.

of regular indirect ophthalmoscopy or of the El Bayadi lens.

282 Glaucoma - Basic and Clinical Aspects

Fundus cameras have greatly improved the ability to document and follow fundus lesions. Eduard von Jaeger often spent countless hours drawing a single fundus, but today a photo‐ graphic image is available in a fraction of a second. For reasons mentioned earlier, fundus cameras are built on the principle of indirect ophthalmoscopy. The observer's lens and reti‐ na are replaced by a camera lens and film. Because all components are enclosed in a rigid housing, more accessories can be built in. This includes a dual illumination system, which includes a constant light source for focusing and a flash for photography, and filters such as for fluorescein angiography. Rather than placing the viewing and illumination beams side by side, the illumination beam generally uses the periphery of the pupil and leaves the cen‐ ter for the observation beam.[38]

An angled glass plate that can be flipped to the right or to the left can be used to slightly deviate the observation beam to the right part or the left part of the patient's pupil to pro‐ duce photo pairs that can be viewed stereoscopically.

Because newer lens designs have allowed the construction of wide-angle cameras, a special challenge has been to construct the optical system in such a way that the curved retina is imaged in a plane that can be captured on a flat film.

#### **11.2. Adaptive optics**

The optics of the eye are not perfect. Even if major errors are corrected with spherical and cylindrical lenses, small irregularities across the pupillary opening persist. The technique of adaptive optics was developed for astronomical telescopes to counteract image degradation by atmospheric irregularities. An adaptive optics system uses a grid to divide the pupillary opening into many small areas and determines a separate small correction for each area. The information is fed to a slightly deformable mirror with microactuators. Thus the image qual‐ ity can be enhanced to the point at which the cone mosaic can be clearly visible. The setup is too laborious for use in routine photography. Because the corrective system has to be fixed in relation to the pupil, it cannot be implemented in glasses or contact lenses. However, the technique, also known as wavefront analysis, has found a place in the refractive sculpting of the cornea.[39]
