BRUEL & KJAER 4912 Instruction Manual page 12

8
Fig.3. 1. Polar diagram showing typical beam geometry for the
491 2 light source
llluminance
Amount
Distance from lamp (m)
off axis
0, 1 0,25 0,5 1,0
00
3400 3000 1000 330
10
2950
980 300
20
3300 2800
900
220
30
2300 700 150
40
2000 450 100
50
3000 1500
300
70
60
1000 200 45
70
2400 500 100 31
100
1500 170
30
8
15° 800
20° 250
25° 150
30° 100
800158
Table 3. 1. Typical beam geometry characteristics for the 4912 light
source
Sometimes it can be difficult to distinguish multiple and single images of a rotating ob-
ject which is rotationally symmetrical, such as a fan or gear wheel. In such cases it is
helpful to stop the machine and make a distinctive mark, with chalk or ink, on the rotat-
ing part. Start the machine again and obtain a stationary imagine. Check that the mark
occurs at only one point in its orbit, and if it occurs at several, reduce the Stroboscope
frequency. If exclusion of ambient light proves to be difficult, the contrast of such a mark
can be improved by the use of a patch of retroreflective adhesive tape. This material has
the property of reflecting light back along its incident direction. A patch of this tape stuck
to the rotor should be viewed from a position immediately adjacent to the Stroboscope in