Biomedical Engineering Reference
In-Depth Information
6
Thick Lenses
In the previous chapter, we learned how to use a surface-by-surface approach to
locate images formed by thick lenses. In this chapter, we'll learn how to construct
what is referred to as an
equivalent lens
to solve thick lens problems. This approach
is particularly helpful when dealing with complex optical systems. We'll also discuss
front and back vertex power, two commonly encountered clinical measurements.
DEFINITIONS
To refresh your memory, Figure 6-1A reviews how we can locate an image by taking
into account the refraction that occurs at each lens surface. Light rays that emerge
from the object, O, are refracted at the front surface and subsequently refracted again
by the back lens surface to result in the image, I. An alternative strategy is to construct
an equivalent lens. This lens, which doesn't exist physically, has two imaginary prin-
cipal planes; these are drawn as dotted vertical lines in Figure 6-1B, and labeled
H
(the primary principal plane) and
H
(the secondary principal plane). The principal
planes are redrawn in Figure 6-1C without the lens, but with the object and image.
′
When working with an equivalent lens, all refraction is assumed to
occur at the principal planes.
Object distance is measured from the primary
principal plane to the object, and image distance is measured from the secondary
principal plane to the image. Object and image vergences are calculated using the
refractive index of the medium in which the lens exists (generally air), not the lens's
index of refraction. Later in this chapter we'll solve a problem with this approach,
but before doing so, we'll learn how to specify the power of a thick lens.
Figure 6-2 shows a thick glass lens situated in air. Labeled are the primary (
H
)
and secondary (
H
) focal
points.
This thick lens does not have a single focal length—it has several focal lengths.
The
′
) principal planes and the primary (
F
) and secondary (
F
′
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