Videre Design STH-DCSG, Руководство пользователя

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STH-DCSG U
SER
’
S
M
ANUAL
©
2005 V
IDERE
D
ESIGN
16
6.7 Focal Length
The focal length is the distance from the lens virtual viewpoint to the
imager. It defines how large an angle the imager views through the lens.
The focal length is a primary determinant of the performance of a stereo
system. It affects two important aspects of the stereo system: how wide a
field of view the system can see, and how good the range resolution of the
stereo is. Unfortunately there’s a tradeoff here. A wide-angle lens (short
focal length) gives a great field of view, but causes a drop in range
resolution. A telephoto lens (long focal length) can only see a small field of
view, but gives better range resolution. So the choice of lens focal length
usually involves a compromise. In typical situations, one usually chooses
the focal length based on the narrowest field of view acceptable for an
application, and then takes whatever range resolution comes with it.
6.8 Range Resolution
Range resolution is the minimum distance the stereo system can distinguish.
Since stereo is a triangulation operation, the range resolution gets worse
with increasing distance from the stereo head. The relationship is:
d
bf
r
r
∆ = ∆
2
,
where b is the baseline between the imagers, f is the focal length of the lens,
and
d
∆
is the smallest disparity the stereo system can detect. For the STH-
DCSG/-C, b is 90 mm, and
d
∆
is 0.325 um (pixel size of 5.2 um, divided
by the interpolation factor of 16).
Table 3 plots this relationship for several focal lengths. At any distance, the
range resolution is inversely proportional to the focal length.
6.9 Field of View
The field of view is completely determined by the focal length. The
formulas for the FOV in horizontal and vertical directions are:
) / 92 . 1 arctan( 2 f HFOV
=
) / 44 . 1 arctan( 2 f VFOV
=
where f is in millimeters. For example, a 2.8 mm lens yields a horizontal
FOV of 87 degrees.
The following table shows the FOV for some standard focal lengths.
Figure 6-1 Range resolution in mm as a function of distance, for
several different lens focal lengths.