CHAPTER 7

Optical Instruments and Low Vision Aids

In this chapter, we discuss magnification, telescopes, and other concepts of optical engineering and then explain the design of various instruments used to examine the eye. After that, the chapter describes various optical and nonoptical aids available to assist patients with impaired vision.

Magnification

The magnification an optical system provides may be defined several different ways. Three of these are as follows:

1.transverse magnification perpendicular to an optical axis

2.axial magnification along an optical axis

3.angular magnification, which is a measure of an instrument’s ability to provide the viewer’s retina with an enlarged image

Transverse (linear, lateral) magnification is the height of the image with respect to the optical axis divided by the height of the object with respect to the optical axis. Axial magnification, the change of depth along the axis, is the square of the transverse magnification. Angular magnification differs from transverse and axial magnification in that it is not a comparison of the size of an image to the size of an object. Angular magnification is a comparison of the size of the retinal image of an object viewed with and without an optical instrument; it is defined as the angle subtended at the nodal point of the viewing eye by an object viewed through an instrument, divided by the angle subtended by the same object when viewed by the naked eye. Increasing this angle proportionally increases the size of the image on the viewer’s retina so that finer details can be discriminated.

The simple magnifier is a special case of angular magnification. An object is placed at the primary focal plane of a plus lens, so that a pencil of rays emanating from any point on the object emerges from the lens with its rays parallel, ready for viewing by the emmetropic eye (Fig 7-1). Magnification of the simple magnifier is defined to be the quotient of 2 angles. The numerator is the angle subtended at the eye by the object as seen through the magnifying glass, while the object is positioned at the focal length of the magnifier lens and the viewing eye is somewhere on the other side of the lens. The denominator is the angle subtended at the eye by the object when it is viewed 25