SINGLE CAMERA SYSTEM FOR CLOSE RANGE
INDUSTRIAL PHOTOGRAMMETRY
Knut Ämdal, Metronor AS
(P.O.Box 238, N-1360 Nesbru, Norway)
Commission V
Abstract
This paper gives a description of an on-line photogrammetry system designed for close
range applications (SCS Single Camera System). The system has the ability to perform
3-D point measurements, employing one
measurement tool. The measurement
single CCD camera in combination with a
tool is equipped with LEDs serving as
photogrammetric targets, which are precalibrated in the local tool coordinate system.
Accuracy results obtained from simulation studies and measurement tests are reported.
The possibilities of accuracy improvements by post processing the SCS measurements
in a free network bundle adjustment is also discussed.
Key Words: Accuracy, Close Range, Industrial, Photogrammetry
1. Introduction
The Metrology Norway System (MNS) developed by
Metronor AS, is an on-line photogrammetry system
designed for close range applications (see [1] and [2]). By
employing high resolution CCD cameras in combination
with the Light Pen, the MNS becomes a portable
Coordinate Measuring Machine (CMM). The system allows
for rapid and accurate data collection for a variety of
metrology tasks.
The metrology concept is based on measuring the spatial
(XYZ) coordinates of reflected laser beams or Light
Emitting Diodes (LEDs). The laser spots and LEDs consti-
tute the photogrammetric targets. To achieve a favorable
signal-to-noise ratio of the sensor observations, the
amount of light emitted from each LED is regulated
individually for each exposure.
To obtain a favorable accuracy and a user friendly system,
the CCD cameras in the system are laboratory calibrated
by Metronor AS. This one time calibration process, which
involves measuring more than 10 million calibration points
for each camera, turns the high resolution CCD camera
into an "ideal" photogrammetric camera. Due to the
laboratory calibration the system does not need to rely on
a full self calibration in the setup. The calibration method
is a unique and proprietary technique developed and
patented by Metronor AS, and will not be further
described in this paper.
2. Background
The usual way of operating an on-line digital
photogrammetric station is by employing dual- or multi-
camera setups. Operation of the system is usually divided
into an initialization phase (setup), followed by a
measurement phase. System initialization usually involves
a relative orientation of the cameras and a scale transfor-
mation, eventually also a camera calibration and calcu-
lation of exterior orientation. In the measurement phase,
3D coordinates of target points are calculated by
intersecting rays of the projected target in each of the
cameras, employing the exterior and interior orientation
parameters estimated in the setup. To obtain on-line 3D
measurements, this technique requires two or more
cameras operating simultaneously.
By employing the Light Pen as a measurement tool, it is
possible to think of new strategies for on-line
photogrammetry. The Light Pen is equipped with a
number of LEDs (5 or 6), all precalibrated in the local
coordinate system of the Pen (XP, YP, Z;). The
interchangeable Pen tip is also calibrated in the same
coordinate system. Figure 1 shows the Light Pen.
When the LEDs are imaged on the sensor of a camera,
the sensor observations (x?, y?) together with the given
3D local coordinates of the LEDs (XP, YP, ZP) gives
enough information to estimate the position of the Pen as
referenced to the camera (X^, Yo“, Z,^). The coordinates
of the Pen Tip, which is the touch point when doing a
measurement, can then easily be estimated in the camera
coordinate
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