4-2-1
CALIBRATING A ZOOM LENS CCD CAMERA FOR A TERRESTRIAL IMAGE BASED SURVEY SYSTEM
Y.D. Huang and D. Chen
School of Surveying
University of East London
United Kingdom
Y.Huang@uel.ac.uk D.Chen@uel.ac.uk
KEY WORDS: Camera Calibration, Zoom Lens, Videotheodolite, Terrestrial Image Based Survey.
ABSTRACT
The paper presents the technical detail of calibrating a zoom lens CCD imaging unit in a motorized videotheodolite system. The camera-
on-theodolite calibration method was used. Both the camera interior parameters including lens distortion parameters and the exterior
parameters with respect to the theodolite were determined at various focal l.ength settings. The accuracy of the calibration was assessed
objectively. The repeatability of the calibrated parameters against zooming action was evaluated. The paper gives conclusions and
recommendations for future work.
1 INTRODUCTION
A terrestrial image based survey system is being developed for
3D city modelling at University of East London. The core part of
the system is a zoom lens CCD camera mounted on the telescope
.of a motorised total station. Although such a CCD-theodolite
combined unit has many working modes and applications (Huang
1992), in the current project the unit is to work mainly in the
manner that the camera captures images for photogrammetric
measurement and photo rendering with the theodolite providing
precise orientation parameter values for all the images (Huang
1997). The main reason for choosing a zoom lens is to enable the
unit to achieve relatively uniform object pixel size for objects at
varying distances and to consequently gain more efficiency and
flexibility for the system.
One of the fundamental tasks for the project is the calibration for
the interior camera parameters and the camera-to-telescope
parameters at various focal settings within the range of zoom.
Another is assessing the repeatability of those parameters against
zooming action and time.
Although zoom lenses have many advantages, previous research
in photogrammetric community has not offered extensive
experience in calibrating these lenses and the existing
applications of zoom lens is in fact rather limited in
photogrammetry. Nevertheless, Wong and Wiley reported very
encouraging experiment results on their chosen cameras and
lenses (Wong & Wiley, 1990, 1995). That has proved the
existence of quite calibratable zoom lenses for photogrammetric
purposes. As photogrammetry meets machine vision and the use
of CCD/video cameras increases for metrology, the desire for
exploiting the on-site flexibility zoom lenses offer is increasing
and more experience in using them for photogrammetry is quite
foreseeable.
This paper reports on the calibration of an off-the-shelf zoom lens
CCD camera. The terrestrial image based survey system in which
the camera is used is briefly described. The relatively new
camera-on-theodolite calibration method (Huang and Harley
1989) used for this calibration is explained. The accuracy for
image location and the camera warm-up effect are assessed. The
calibration results and repeatability test results are presented.
Conclusions are drown and future work recommended.
2 THE SYSTEM AND CALIBRATION REQUIREMENTS
The videotheodolite is as shown in Fig. 1. During a survey, this
videotheodolite is set at a number of stations. At each station, the
theodolite rotates at angular steps for the camera to capture
images to cover the panorama. The theodolite angular readings
are recorded on capture of each image for calculating the
orientation parameters of the image. The zoom is adjusted
according to the average object distance within each frame so that
the average image scale is uniform among all the frames of
image.
The zoom CCD camera used is the Sony EVI-371, which is
designed for use in camcorders. It is a self-contained block which
encloses the CCD chip, the circuitiy and an internal zoom lens all
built in as shown in Fig.l. The CCD chip is a 1/3 inch interline
transfer chip with 752 x 582 imaging cells and the cell size is
about 6.50(H) x6.25(V) microns. The lens can zoom from 5.4 to
64.2 mm focal length in 12 electronic settings. Focusing is
switchable between auto and manual modes. For this calibration,
the focus was fixed at a certain distance. The focus tracking
feature enables image to remain in focus during zooming. The
