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frame is shot during the non-saturated excitation of the fluorescent
light, see Fig. 139.
It can be said, that the light at this point in time is not white and the
light output is less.If the phase of the camera shutter remains constant
with respect to the mains phase, the automatic light control and the
white balance circuits in the camera will compensate for these
effects and stable pictures are produced.This situation is obtained by
locking the camera frame synchronisation to the mains (mains lock).
When there is no fixed phase relation between the scanning
frequency of the camera (free running) and the mains frequency,the
camera will take a snap-shot of the scene at varying phases of the
fluorescent lamp light output.This causes a colour fading to become
visible.The extent of colour fading is depending on the lighting
design of the area.
In applications where the scene is illuminated with just one fluoresecent
lamp or other gas-discharge lamp, stabilised by conventional gear,
the colour fading risk is at its maximum. It is recommended that
cameras be locked to mains frequency and the phase of the camera
synchronisation be adjusted such that the camera signal output is
maximum. If mains lock is not possible in such an application, the lens
iris should be closed to the point where the colour fading just
disappears. Now the shutter speed is less (full frame integration)
and the additional benefit is that the sensor smear effect is less.This
solution cannot be used in applications that need short shutter speeds
to suppress movement blurr.
In all other cases (combination of inductive and capacitive circuits,
three-phase installation of high-frequency stabilised) this phenomenon
will not occur.
3.The movement of the arc on the electrode(s) (flicker) has no fixed
frequency and will only become noticeable in exceptional cases. It
5
144
Fig. 138.The 20 msec frame integration
time of a CCD colour camera with the
automatic shutter switched off, compared
with the 100 Hz fluorescent light ripple.
Fig. 139. Using the automatic shutter and
with the camera locked to mains
frequency, it is possible to shoot stable
and white pictures.
3.17 Stroboscopic effect and striations
20 ms
100Hz fluorescent
light ripple
normal shutter time
light integration
short shutter time
mains lock fase
20 ms