Image Processing Reference
In-Depth Information
Fig. 2.5
Before introducing a barrier, the rays of light from different points on the tree hit multiple
points on the sensor and in some cases even the same points. Introducing a barrier with a small
hole significantly reduces these problems
to an incorrect design. Building optical systems is indeed a complex science! We
shall not dwell on all the fine details and the following is therefore not accurate to
the last micro-meter, but the description will suffice and be correct for most usages.
2.2.1 The Lens
One of the main ingredients in the optical system is the lens. A lens is basically
a piece of glass which focuses the incoming light onto the sensor, as illustrated in
Fig.
2.6
. A high number of light rays with slightly different incident angles collide
with each point on the object's surface and some of these are reflected toward the
optics. In the figure, three light rays are illustrated for two different points. All three
rays for a particular point intersect in a point to the right of the lens. Focusing such
rays is exactly the purpose of the lens. This means that an image of the object is
formed to the right of the lens and it is this image the camera captures by placing a
sensor at exactly this position. Note that parallel rays intersect in a point,
F
, denoted
the
Focal Point
. The distance from the center of the lens, the
optical center O
,to
the plane where all parallel rays intersect is denoted the
Focal Length f
. The line on
which
O
and
F
lie is
the optical axis
.
Let us define the distance from the object to the lens as,
g
, and the distance from
the lens to where the rays intersect as,
b
. It can then be shown via similar triangles,
see Appendix B, that
1
g
+
1
b
=
1
f
(2.2)
f
and
b
are typically in the range
. This means that when the object
is a few meters away from the camera (lens), then
[
1mm
,
100 mm
]
1
g
has virtually no effect on the
equation, i.e.,
b
=
f
. What this tells us is that the image inside the camera is formed
