pixel is attained at latter diameter. This is only slightly
improved for even larger targets to 0.004 pixel.
a) Targets
Internal precision [Pixel]
0.020 4
0.015 —
0.010 —
0.005 4
0.000 I I I T T
4 9 14 17 Size [Pixel]
b) Internal precision of target location.
Figure 2 Analysis of internal precision as a function of
target size.
The circle appears to be the best form for point position-
ing tasks as it is isotropic. If targets are allowed to cover
large areas and the imaging scale is similar throughout
the measurement range, targets with radially varying in-
tensity can be used to increase the amount of gradients.
2.3 Optical Elements of Camera
Figure 3 shows an exploded view of the optical system of
a typical CCD-camera. The radiometric and geometric
Lens
IR-filter
Diffusor
Cm
~~ Cover glass
Solid-state sensor
I
Ceramic substrate
Figure3 Functional elements of a solid-state camera.
characteristics of lens and optical system are well docu-
mented in literature. The geometric calibration of zoom
lenses has also been addressed in recent years (e.g. Wiley
and Wong, 1990). The fall-off of light to the corners in-
duces radial displacements which are absorbed via pa-
rameters for radial symmetric distortion. The purpose of
the IR cut filter was already explained. The diffusor is of-
ten incorporated into cameras with Interline Transfer
sensors as well as color-cameras. It reduces aliasing by
generating a double image displaced by 1/2 the sensor el-
ement pitch. The cover-glass over the sensor serves as
chemical and mechanical protection.
The modulation transfer function (MTF) of the optics
must be adapted to that of the sensor. This is relatively
unproblematic for typical sensors having spacings of 10
to 20 uum. Cameras employing the micro-positioning
technique with pixel spacings of a few micrometers
(spacing of the pixels of the generated image) require ap-
propriately designed lenses (MTF and aberrations).
The IR-filter, diffusor and cover glass should have plane
parallel surfaces which are normal to the surface of the
sensor. This is not provided in most CCD-cameras, i.e.
specifications are only rarely provided. The diffusor is
usually akin to a thin sheet of plastic of rather instable
nature. The stability of the assembly of optical elements
and sensor is for most CCD-cameras questionable. A sta-
ble fixture of the sensor to the camera body as well as
specifications of the location of the optical axis in the im-
aging plane, its orthogonality with respect to the sensor
surface, and the planeness and parallelity of the surfaces
of other optical elements are some of the items to be re-
quired in the specifications of off-the-shelf CCD-camer-
as. Furthermore the MTF, aberrations, and spectral
characteristics should be given.
2.4 Sensor
Solid-state sensor exhibit excellent radiometric and geo-
metric characteristics. The Photo Response Non-Unifor-
mity (PRNU), the surface characteristics, and the
regularity of the sensor element aperture and spacing are
of special interest.
The dark signal non-uniformity (DSNU) and photo re-
sponse non-uniformity (PRNU) can be used to assess the
basic radiometric performance. From an analysis of the
DSNU electronic and other disturbances, independent of
any sensor illumination, can be detected. The PRNU can
be used to gain an overall impression of the quality of the
imaging system. Effects of light-fall-off, dirt, and other
degrading factors do often have larger impact than the
PRNU of the sensor itself. Basically a radiometric cor-
rection could be performed on a pixel by pixel basis to
correct for DSNU and PRNU. It is at this point in time
questionable wether or not any improvement can be at-
tained as the PRNU of typical off-the-shelf CCD-camer-
as is already in the range of 1%. The dependence of the
Figure 4 Photo response non-uniformity of a typical
lens and camera.
PRNU on the spectral characteristics and the difficulty to
generate a uniform illumination which is significantly
better than 1% (Ulbricht spheres specify a uniformity of
