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In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B 
ment probe (see Fig. 4) is a scaled replica of the profile height 
h{r). 
Figure 3: Scheme for the laser light section technique with the 
geometric triangle drawn in produced by the line shaped light 
source. 
z 
d(r) 
Camera 
SM =tan <^ (1) 
h(r) 
d{r) 
tan(<£>) 
(2) 
Figure 4: Top view from the laser light section technique scheme. 
The smallest clearly recognizable profile height depends on the 
triangulation angle, p, which is measured between the incidence 
direction of the light source and the surveying direction of the 
camera and the pixel pitch. For optical reasons (reflection prop 
erties of the coil) and resolution considerations the triangulation 
angle p should be between 30° and 60° (Schaffer and Kirchhoff, 
2004). 
To minimize the influence of the varying lighting conditions a 
powerful diode laser with 100 mW at 660 nm is used as light 
source and additionally an optical bandpass filter is used to reduce 
the ambient lights influence. The optical bandpass (see Fig. 5) 
consists of two separate filters, a near infrared blocking filter and 
a red filter. The combination of both fiters results in an opti 
cal bandpass with a maximum transmission of 94 % at a central 
wavelength (CWL) of 685 nm and a full width at half maximum 
(FWHM) bandwidth of 100 nm. Compared to a standard inter 
ference filter the assembled optical bandpass has a larger FWHM 
bandwidth but shows no dependency on the angle of incidence, 
which is known to cause spectral side effects for interference fil 
ters. 
h 0.8 
1 0.6 
0 
1 0-4 
I 
H 0.2 
0 
200 
400 600 800 
Wavelength in nm 
1000 
Figure 5: Diagramm of the filter transmittance, the near infrared 
blocking filter (green), the red filter (red) and the resulting optical 
bandpass (blue). 
2 EXPERIMENTAL SETUP 
Before a recognition system based on the laser light sectioning 
technique is realized, the camera characteristics and furthermore 
the depth resolution limits have to be determined. First of all an 
approximation of the minimum detectable object size is neces 
sary, therefore the specifications of the provided camera and the 
expected viewing geometry must be known: 
• Pixel pitch p: 4.65 pm 
• Focal length /: 30 mm (camera constant c « /) 
• Distance between camera and coil Z: 5 m 
By using Eqn. 3, which describes the central projection theo 
rem (Luhmann, 2003) (with the object dimensions in real world x 
and on the CCD chip x'), we obtain a minimum detectable object 
size a: m in of 0.7486 mm. 
The minimum detectable object size rr m i n can now be used to 
designate the triangulation angle p (in Eqn. 2). In Fig. 6 the re 
lationship between p and the minimum detectable winding dis 
placement Amin is plotted. 
Angle tp in ° 
Figure 6: Relationship between p and the minimum detectable 
winding displacement h m in- The required and realized operating 
points for the minimum detectable winding displacement h m i n 
are marked with red dashed lines. The gray colored area indicates 
the interval of technically relevant triangulation angles p. The 
green hatched area labels the interval of valid triangulation angles 
p for the realization. 
The minimum detectable winding displacement h m ¡ n was spec 
ified to be equal to or less than 1mm. Therefore the triangula 
tion angle p is choosen to be 58°, and the resulting minimum 
detectable winding displacement h m ¡ n is 0.47 mm. Due to the 
measurement error which will be calculated in Sec. 5 a value less 
than 1mm is advantageous. A major requirement for using the 
laser light section technique is the presence of a diffusely reflect 
ing object. This can be proved with the Rayleigh criterion (Hor- 
bach, 2008) shown in Eqn. 4. 
Ra < 
A 
16 cos (p) 
(4) 
The Rayleigh criterion is an inequation including the wavelength 
of light A, the triangulation angle <p and the mean roughness in 
dex R a . If the mean roughness index R a of the illuminated ob 
ject area is less than A/16cos(</?) no diffuse reflection appears 
The second possibility to minimize the influence of the lighting 
conditions is to reduce the shutter time. This is also necessary 
with regard to the movement of the coil during the measurement.