Biomedical Engineering Reference
In-Depth Information
A
F
1
F
2
O
I
B
H
H
′
F
1
F
2
A
1
A
2
C
H
H
′
O
I
Figure 6-1.
A.
In a surface-by-surface approach to locating an image formed by a thick
lens, the image formed by the first surface serves as an object for the second surface.
In this example, the object for the second surface is virtual.
B.
A thick lens showing
the location of its primary (
H
) and secondary principal planes (
H
′
). The front and back
surface apices are labeled
A
1
and
A
2
, respectively.
C.
For an equivalent lens, all refrac-
tion is assumed to occur at the principal planes.
focal length can be measured from a principal plane, the anterior surface of the
lens, or the posterior surface of the lens. When the focal length is measured from
the primary equivalent plane to the primary focal point, it is called the
primary
equivalent focal length
(
f
e
)
,
and when it is measured from the secondary equiva-
lent plane to the secondary focal point, it is called the
secondary equivalent focal
length
(
f
e
′
)
.
[Since this lens is surrounded by a single medium (air), the primary
and secondary equivalent focal lengths are equal to each other.] The
back vertex
