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2.The light ripple can also have an effect on the quality of camera
pictures.This phenomenon may become apparent when CCD colour
cameras operate in auto-shutter mode and the lighting of the area
is predominantly with fluorescent lamps,
The auto-shutter mode is normally selected when cameras are
equipped with manual or fixed iris lenses and the automatic light
response is controlled by an electronic shutter system in the camera.
The more light is exposed to the camera, the shorter the shutter
time.This means: the shorter the light integration in the sensor takes
place. For example, with a shutter time of 1/1000th of a second the
light integration of the CCD sensor is 1 msec only.Within the normal
CCIR scanning period of 20 msec (50 Hz) the 1/1000th of a second
the light intergration time is just a snap-shot in the normal frame
scanning period. In this manner the sensitivity of the camera is reduced.
As described before, the light output of fluorescent lamps varies
continuously from minimum (at zero crossing) to maximum during
the positive and negative phases of the mains voltage, twice during
one mains voltage cycle. In other words:the fluorescent lamp is
flashing 100 times per second. Due to the lag of our eye, viewing a
scene illuminated with ‘TL’ lamps, gives the impression of a white
and continuous light output.
At the dip of the light output, the excitation of the fluorescent
powders is with minimum energy.At this point, the light output is
therefore not white, the colour depending on the properties of the
non-saturated excitation of the fluorescent powders in the lamp.
As the human eye works as an integrator, this effect cannot be
noticed.The light ripple of a ‘TL’ lamp is illustrated in Fig. 137.
When the automatic shutter in the camera is switched off, the two
light ripples of a ‘TL’ lamp are integrated during the normal 20 msec
frame integration time of the sensor and consequently the light
impression is white.This is illustrated in Fig. 138.
Using the automatic shutter in sufficiently illuminated scenes, the
shutter speed increases and consequently light integration in the
sensor takes place during a short period of time. Depending on the
position where the light integration (snap-shot) takes place with
respect to the mains phase (light ripple), it is now possible that a TV
5
143
Fig. 137. Colour shift during the 100 Hz
light ripple of a fluorescent lamp
3.17 Stroboscopic effect and striations
white light
yellowish light
reddish light
momentary light output
non-saturated
phosphors