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mits images in an analogue form. This will have a corresponding adverse 
effect on the tolerance that must be placed on each measurement taken. 
A number of "digital" cameras were evaluated prior to making the decision to 
develop a digital camera to meet the precise requirements of the Non-Contact 
Measurement System. Most of these cameras were discovered not to be true 
digital cameras as they involved one or more analogue stages within the image 
capture and transmission path. Any analogue stage looses the integrity of the 
image data and so precludes the identification of the individual pixel from the 
CCD which was responsible for forming each individual part of the image, as 
well as reducing image quality in general. 
In addition to normal photogrammetric calibration parameters, it is important 
to be able to calibrate the performance of the CCD in terms of the dark current 
(the residual image pattern in the CCD prior to exposing it to the subject) and 
the relative sensitivity of the individual pixels. Both calibration factors 
become crucial when automating the relative orientation and measurement 
process, which rely on image matching techniques. 
Some digital camcras were of the slow scan form, from which the image cannot 
be extracted from the imaging device quickly. To preserve image integrity, 
which could be damaged by a continued dark current build-up over a short 
period of time, these cameras are normally cooled in liquid nitrogen and so are 
not suitable for operational use. 
Other digital cameras did not allow a buffer memory adjacent to the camera 
head, so not allowing an image to be captured and then transmitted over a slow 
transmission system. 
The digital cameras developed for the Non-Contact Measurement System have 
the following benefits: 
* The high resolution CCD (1242 x 1152 pixels) gives at least four times. 
the area resolution of video cameras, so allowing measurements of higher 
accuracy or larger areas to be covered in one image to achieve the same 
accuracy. 
*  À high grey scale resolution of 256 levels, which allows better interpre- 
tation of the subject. 
*  Thedata form of a digital image means that image quality is not degraded 
in the transmission system. 
* Full digital control of the formation of the image which makes the 
calibration of the camera and the automation of measurement tasks more 
reliable. 
* High quality lenses are readily available, so increasing the quality of the 
images, and hence the accuracy of the measurements, correspondingly. 
The main applications for digital input are when high spatial resolution is 
required. The performance of the digital camera can be adequately compared 
to a 35mm monochrome photographic film image. 
However, they also have disadvantages: 
» They are monochrome devices. 
* They require digital transmission systems which, due to the larger 
amounts of data produced by the larger and higher quality image, tend to 
be slower. 
* They are more expensive than video cameras. 
2.2 Transmission Systems 
When the cameras are being used remotely from the analysis system the image 
data must be transmitted from the cameras to the system. In some cases within 
the offshore environment, the camera is separated from the analysis system by 
1200 metres or more of electrical umbilical which is used to power and control 
the ROV, as well as to bring back the images and other data. 
Conventional ROV transmission systems are suitable for the use of video input 
into the Non-Contact Measurement System, but slow down digital input to the 
extent that it can take up to one minute to transmit a single image. Modern 
fibre optic systems are necessary to allow full digital images to be transmitted 
in real time. 
2.3 Image Processing System 
The processing of the images captured from the video or digital cameras is 
carried out within the PC mounted image processing system. This is a modular 
system based on the Texas Instruments TMS 34020 graphics chip. The image 
processing system is mounted in the bus of the host computer, and is controlled 
from the host computer. However, the image capture, processing and display 
functions are controlled over a high bandwidth local bus, hence by passing the 
PC bus. The PC based image processing system, together with the sterco 
viewing system, is illustrated in figure 5. 
The use of the modular system brings the benefit of being able to tailor the 
system to the precise requirements of the user. A different capture board is 
used for video and for the digital camera. Dedicated image processing 
functions are carried out on other boards. Stereo viewing of real time video 
requires an add- in board. However, the base system is identical. Video and 
digital boards may bc installed concurrently, allowing versatility in the use of 
the system. 
The system brings the following additional benefits: 
The use of a PC means that the software has been written as a Microsoft 
Windows application and hence provides the benefits of working with this 
popular graphics user interface. 
* The system can be based on an office or industrial computer chassis and 
hence can bc tailored to withstand the rigours of offshore working without 
penalising desktop users. 
= The system has a lower capital cost than dedicated workstation based 
image processing systems that are often no faster for the tasks that are 
required to be carried out. 
* The system is easily transportable. 
* The PC environment allows images to be stored on magnetic or optical 
disks, or to be sent over standard telephone or radio transmission systems, 
with no image quality reduction. Images can be transferred to be viewed 
on a standard VGA compatible computer screen with a slight reduction 
in quality. 
* The PC environment allows easy access to other software packages, such 
as word processing, graphics and CAD such that the measurement data 
extracted is immediately usable for reports, drawings and for further 
processing. 
2.4 Stereo Viewing System 
A design requirement was that the Non-Contact Measurement System should 
be able to be used by engineers, without specialised knowledge and training. 
This placed a special emphasis on the design and use of the stereo viewing 
system. 
Test marketing indicated that the stereo viewing system should allow multi- 
access to allow the extraction of data to be discussed, and to facilitate 
introductory training. 
Conventional optical stereo viewing systems were rejected. 
  
Figure 2. A typical Offshore Remotely Operated Vehicle (ROV) which will 
have to carry out the work of divers in deep water.