The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
writers of this paper measured approximately 500 tooth replicas 
using a mechanical stylus method, (Mitchell et al., 2003). Such 
a procedure is clearly very protracted and inefficient. Many 
other dental surface researchers carry out similar measurements, 
(see, e.g., Azzopardi et al., 2000; Bartlett et al., 1997; Mehl et 
al., 1997; Pesun et al., 2000; Pintado et al., 1997 ). 
1.4 Goal 
It is seen as highly advantageous for dental research to be based 
on direct intra-oral measurement. Firstly, direct intra-oral 
measurement would overcome the tedium of the casting 
procedure for patients as well as dental workers. Secondly, the 
use of photogrammetry could improve the efficiency of the 
measurement 'procedure, as has been done for replica 
measurement (Grenness et al., 2008). 
1.5 Alternatives 
Various techniques for dental replica measurement are 
imaginable. However, for intra-oral measurement, any 
instrumentation must meet severe access criteria, especially if 
any side of the tooth - not simply the most easily accessed - is 
to be measured. Electron microscope methods fail in this 
regard, as indeed do other methods involving microscopes. 
Among the optical methods, the obvious alternative is laser 
scanning but it faces a far high equipment expense than does 
photogrammetry. Other optical options include shape-from- 
shading, moiré fringing and structured light techniques. The 
feasibility and value of optical intra-oral measurement is 
signified by the CEREC (Sirona GmbH, 2008) and D4D (D4D 
Technologies, 2008) intra-oral measurement systems, even 
though they measure using optical but not photogrammetric 
principles. The cost of optical dental measurement techniques 
based on structured light patterns is hard to estimate, because 
the commercial units are sophisticated and moreover are 
available only when linked to crown machining software and 
hardware. 
1.6 Photogrammetry’s advantages 
Intra-oral measurement by optical methods seems to be a 
feasible option. Photogrammetric measurement can be 
executed at low cost, because the only significant hardware 
requirement is a camera. Intra-oral cameras are commercially 
available. Although there is a likely requirement for ancillary 
items such as lighting, lenses and mirrors, these are relatively 
cheap. The other significant photogrammetric components are 
software. Given the need for automation, and given the current 
sophistication of automated image matching, photogrammetry 
was seen by the writers to be deserving of some 
experimentation. 
1.7 Requirements 
The above description suggests that there is a demand for 
development of a tool which could be used to measure any 
surface of any tooth in the mouth to aid any dental research 
which involves studies of the loss of the external enamel 
surface of the tooth. However, to replace alternative and 
existing procedures which involve taking castings, and perhaps 
replicas, of the tooth for subsequent measurement by 
mechanical or photogrammetric means, as is intended for such 
purposes as studying tooth surface loss caused by decay and/or 
erosion, the technique must be superior in other aspects as well. 
The requirements are therefore seen to be: 
Able to measure any surface of the tooth, this defining 
access requirements. 
Quick, comfortable and safe for the patient, i.e. easier 
than taking castings which can take a few minutes. 
Simple and easy for the dentist and/or researcher, and 
accordingly the photogrammetric measurement must be 
entirely automatic. 
Providing an absolute accuracy of about 0.01 mm in 
absolute terms. This accuracy represents a low relative 
accuracy of 1:1000 if the tooth is about 10 mm across. 
2. THE PHOTOGRAMMETRIC DESIGN 
2.1 Imaging 
The problem of access to all teeth can be overcome by using 
commercial intra-oral dental cameras. This work is based on a 
Flexiscope Piccolo camera, providing colour imagery at 768 x 
576 pixels size. Collecting suitable multiple images is a serious 
difficulty. If a single camera is used, movement of the camera 
in close proximity to the human patient while controlling 
translations and rotations of the camera relative to the patient 
can be difficult. The use of two cameras is therefore attractive; 
Grenness et al. (2008) decided that “close range applications, 
by their compact nature, are well suited to fixed base stereo 
camera equipment. If the stereo camera is sufficiently rigid then, 
once fully calibrated, object space control is not required". 
However, the use of two cameras can severely limit access to 
certain regions in the mouth, and, moreover, the use of two 
separate cameras complicates image file handling, and this has 
also been seen as undesirable for this project. Good estimates 
of parallax values are also needed to get reliability and 
matching success, and this demands careful control of the 
camera position. 
2.2 Calibration 
The Flexiscope camera lens has a short but unknown focal 
length with a 105° of field of view (Inline, 2008) and, not 
surprisingly, high distortion levels, and it demanded calibration. 
Figure 1. Intra-oral camera image of calibration object, only 
approximately 10 mm in size. Distortion is apparent. 
Calibration is problematic with small objects. A determination 
of both principal distance and distortion parameters has been 
undertaken using a small in-house test object, approximately 10