e 
D 3 
AS 
ts 
on-line digital 
g dual- or multi- 
is usually divided 
followed by a 
n usually involves 
a scale transfor- 
ation and calcu- 
isurement phase, 
e calculated by 
ot in each of the 
nterior orientation 
obtain on-line 3D 
os two or more 
urement tool, it is 
ies for on-line 
equipped with a 
ated in the local 
D 2°) The 
ated in the same 
Light Pen. 
1sor of a camera, 
1er with the given 
p, YO Z^) gives 
tion of the Pen as 
). The coordinates 
int when doing a 
ated in the camera 
coordinate system. By employing this technique, on-line 
3D measurements can be done with only one camera. 
The idea of a 3D measurement system employing only 
one camera (SCS Single Camera System) was initiated at 
SAAB Scania Aircraft Division in Linkóping, Sweden, and 
developed by Metronor AS. Some of the reasons for 
developing such a system are: 
- To have a system that is designed for 
measurements on large constructions in the 
aircraft industry. Typical applications are 
straightness measurements of airplane 
fuselages and measurement of wing 
contour. The SCS is ideal for these 
applications due to its high angular accuracy 
(see Section 4). 
= To have a system designed for fast 
operation. The SCS does not need any 
system calibration (relative orientation) and 
is therefore favorable for time critical 
measurements. 
= To reach difficult places where a dual- or 
multi-camera setup is not possible to 
employ due to physical restrictions on the 
survey site. 
= To benefit from the redundancy when having 
several sensor observations for each mea- 
sured object point (several LEDs on the 
Light Pen), leading to a lower stochastic 
variance of object point coordinates. 
= To have the possibility of measuring hidden 
points (points that can not be seen directly 
from the cameras), both reference points 
and ordinary measuring points. 
= To have the possibility of establishing a 
photogrammetric network with a strong 
geometry. 
3. Single Camera System 
The Light Pen forms an integral part of the SCS, and the 
measurement accuracy and functionality depend much on 
the geometry of the Light Pen. As a tool in the design 
process of the Light Pen, simulation studies were utilized 
. to arrive at a favorable solution. Various shapes with 
various numbesr of LEDs were simulated before ending 
up with the Light Pen showed in Figure 1. 
Each Light Pen contains five or six (depends on Pen size) 
embedded LEDs. There are several interchangeable tips 
for the Light Pen, each specially designed for different 
measurement applications. The Light Pen is compatible 
with commercially available CMM probes. Tips can also be 
manufactured on site by the user, and calibrated by em- 
ploying a special calibration routine that follows the 
System. 
The most important features to consider when designing 
the Light Pen were: 
= User friendliness. The Light Pen has a slim 
construction and is fabricated of light weight 
composite material. This makes the Light 
Pen easy to use, also in narrow places. 
- Stability. Durable composite construction 
minimizes the effect of temperature 
variations and other physical constraints. 
= Fast and reliable point identification. 
When operating the system, the target 
points on the Pen are automatically identified 
by the system based on the given Pen 
geometry. Due to the simple geometry, the 
points are easily identified by the point 
identification software for all Pen orientations. 
= Unique mathematical solution. A minimum 
of 3 observed target points are required to 
obtain a non-singular mathematical solution 
for estimating the relation (transformation 
matrix) between the local Light Pen 
coordinate system and the camera 
coordinate system. However, only 3 points 
may lead to ambiguous mathematical solu- 
tions. Employing 5 or 6 points with the 
geometry showed in Figure 1 gives a unique 
mathematical solution, and enough 
redundancy to eventually detect gross 
observation errors. 
= Fast convergence. The SCS software is 
based on linearized equations (Equation 1 
is linearized) which is solved in iterations. 
The depth information (Light Pen pointing 
towards the camera) together with good 
approximate values for the unknown 
parameters, gives a fast convergence 
towards the correct solution. 
The SCS is based on bundle adjustment software. 
Equation 1 express the relation between the sensor 
observations and the 3D coordinates of the corresponding 
target points (perspective transformation): 
x sh S QC^Xg a Y^- Y) aZ - A) dx 
831 (X^ -X9) +83 ( Y?- Ys) * ag (Z^ -Zy) (1) 
yf-fs a (X^ - X9) a (Y - Yy) *aj(Z/ -Z5) «dy 
ay (X, X9) ag (Y? - Yo) * a (Zi - Zo) 
  
  
Equation 1: Perspective Transformation