Philips Electromagnetic Lamp manual Stroboscopic effect and striations

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3.17 Stroboscopic effect and striations

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.

100Hz fluorescent light ripple

20ms

normal shutter time

Fig. 139. Using the automatic shutter and with the camera locked to mains frequency, it is possible to shoot stable and white pictures.

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.

light integration

short shutter time

mains lock fase

20 ms

3.The movement of the arc on the electrode(s) (flicker) has no fixed frequency and will only become noticeable in exceptional cases. It

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