1. Investigations of Digital Levels for High Precision Measurements
Georg L Gassner
Robert E Ruland
Brendan Dix
Metrology Department
Stanford Linear Accelerator Center
Work supported by the U.S. Department of Energy under contract number DE-AC03-76SF00515
Thanks chairmen / person
SLAC Stanford DOE
TITLE
Encountered Problems not explainable
No test stand in US
Started in 2002

2. Factors influencing the accuracy:
Introduction
Requirements at SLAC:
150 mm / 300 m
50 mm / single components
Factors influencing the accuracy:
Scale factor
Critical distances and focusing
End section of the staff
Illumination
Tested Equipment:
Leica DNA03
Trimble (formerly Zeiss) DiNi12
(Leica NA3000)
Digital level + fast and normally accuratly measurements
Known sources - scale factor regular basis

3. Design and Hardware
Principle idea is to move the levelling rod controlled by an interferometer and measure the height differences with the level at the same time
Only two other in the world Graz and Finland
Sounds pretty easy but it is not z. B. the staff is moveable over a range of 3m with an accuracy of about 1micrometer
The digital level is mounted in a moveable cage on the ceiling which allows us to measure at sighting distances from 1.8m up to 30m
Fully automated
Mention tour to SLAC

4. Determination of the scale factor
Scale Determination
Determination of the scale factor
Repeated regularly – ensures equipment works correctly
Explain X axis y axis
Two purposes
1.) determination of the scale factor
2.) test of equipment

5. Critical Distances – Leica
1 code element (2.025mm) is projected onto exactly 1 pixel, or integer multiples
Leica NA3000 critical distance at m (causes up to 0.4 mm misreading)
Leica DNA03 critical distance at m
At certain distances it appears that digital levels don’t give the same accuracy as usual.
Investigation this effects peaple found that this happens when one code element projected on the ccd array has the same size as one pixel. An explanation therefore is that if one code element has a dark image projected on it and the next a light image an effect called crosstalk occures meaning that the not illuminated pixel gets activate by the neighboring pixl
A real problem occurs with a instrument no longer in use at SLAC the NA3000 There one code element is projected onto the ccd array with the size of 1 pixel at m.

6. Critical Distances – DiNi12
There are 251 critical distances between 1.5 m and 15 m
Size of 1 code element is 20 mm
Example: At m 1 code element is projected onto 38 pixels

7. Defocused Measurements
Leica DNA03 and Trimble DiNi12: critical distances do not cause deviations > 30 mm
Defocused measurements increase these values
focused set up
defocused set up
Easy to take measurements with not accuratly set up instrument

8. End Section of the Staff (1)
Measurements at the end section of the rod
Only parts of the staff are visible
Inaccurate measurements are the consequence
Can also occur if parts of the rod are covered
Good practice to avoid lower end of the rod because of atmospheric effects

9. End Section of the Staff (2)
DiNi12
300 mm code section
H [mm] = start of visible code on the staff mm
Leica DNA03
DNA03 uses 1.1° code section (moveable)
H [mm] = start of visible code + 20 mm + 6.9·sighting distance [m]
invisible code section

10. Illumination
Only illumination at a steep angle
causes biased measurements of up to 100 mm (only correct for the instruments tested)
Measurements in dimly lit environments require artificial illumination

11. Conclusion
Determination of the scale factor on a regular basis gives the confidence of quality checked measurements.
The level has to be horizontal and properly focused.
When the highest accuracy is desired, the rod end sections and every obstruction have to be avoided.
Illumination at a steep angle must be avoided.