301 
In the seventies laser-interferometry established a new standard in distance measurement, the wavelength of light. 
Resolution improved to ca. 10nm and relative accuracy to 1ppm or better. The limiting factor was and still is the state of 
the atmosphere, influencing the speed of light and hence the wavelength. Fig. 1 shows the principal set-up of a 
comparator of this kind. 
The laser emits a coherent beam of light. A beamsplitter reflects part of it directly to the interferometer. The remaining 
beam is reflected by a cornercube mounted to the object under test" and then guided to the interferometer where it 
superimposes the direct beam. The HP-laser-interferometer uses different frequencies for the directly reflected and the 
measuring beam and determines the movement of the cornercube reflector by the frequency shift of the measuring 
beam (HP Product Note 5527A-2). 
The microscope is used to position the rod at certain marks (it is not feasible to measure every mark for economic 
reasons), the interferometer determines the distance between them. Considering the atmosphere to 1°C in 
temperature and 1 pa (mbar) in pressure, accuracy is some ppm relative- or some um absolute for the distance 
between any two marks of the rod. 
One snag remains: manual calibration described above is a tedious job and certificates specifying rods and scalebars 
are almost as expensive as the tool itself. The wish to automate the system arose. A photoelectric microscope using a 
light sensitive double diode was developed and successfully implemented in several comparator systems (Schlemmer, 
H., 1975). 
So the idea to use a photoelectric device for automatic detection of rodmarks is not entirely new, but we have decided 
to use a linear CCD-sensor instead of the photodiode. We were induced to this Step by a new development in levelling, 
the "Digital-Level", using a coded rod with scale marks of different width, varying from 1 to 20 mm. The comperator 
systems using a double-diode were designed to detect the centre of marks with constant width. The choice of a CCD- 
sensor gave us access to a very useful photogrammetric tool: edge detection. Instead of the centre of the scale mark 
we aim for its edges and need not worry about different width. The automated calibration System at the IGP meets an 
urgent need of the industry and supplied Mr. Ch. Schmid, who developed the control- dataacquisition- and processing 
software with an interesting and demanding topic for his diploma thesis. 
2. HARDWARE-COMPONENTS OF THE AUTOMATED ROD CALIBRATION SYSTEM AT THE IGP 
The automated calibration system equals the manual one described above in many respects. 
The following changes have been made: 
- The microscope has been substituted with a linear CCD-camera 
- Anew interferometer with plug in data acquisition board for a PC has been installed 
- The manual drive of the trolley has been modified to a motor-driven one 
- A 486 PC is now part of the system to perform data acquisition, processing and control - 
Technical data: 
Laser-interferometer: 
Type: HP 5519A 
Wavelength: À = 633 nm 
Resolution: better than 0.1 um 
max. velocity: 0.7 m/s 
CCD-camera: 
Type: LINCAM 03 
Focus: modified to 41mm 
CCD-sensor: TOSHIBA TC 104 
Pixels: 3648 linear, pixel size 83x8um 
grey-value resolution: 8 bit 
Motor: closed loop DC-motor with attached encoder as used in LEICA theodolite TM3000 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995 
+