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3.17 Stroboscopic effect and striations
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
Fig. 137. Colour shift during the 100 Hz light ripple of a fluorescent lamp
non-saturated phosphors
momentary light output
white light
yellowish light |
reddish light |
