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HMI color variation over one mains cycle?


Marc Roessler
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Hi guys,

 

while taking photos of a scenery illuminated with a HMI lamp (magnetic ballast) I noticed the following:

 

#1

each picture has a different brightness; kind of expected, since the camera shutter is not synchronized with the mains frequency (50 Hz here, i.e. 100 Hz light pulses)

 

#2

Every picture has a different color tint to it. The effect is very obvious, color casts vary between white, greenish/yellowish, blueish and magenta/pink.

 

The digital camera was set to a fixed color temperature (daylight), so this is no "pumping" effect of an auto white balance or anything.

 

 

So my question is:

Does HMI light color really vary over one cycle of mains frequency when using magnetic ballasts? Or are my lamps/ballasts damaged? I tried this with two different HMI fixtures/lamps (with new lamps), same behavior for both of them.

 

 

Followup Question: why doesn't this have an effect on color uniformity with digital (non global shutter) or film cameras with mirror shutter? After all, different parts of the image are exposed at different times, right? Or does it have an effect sometimes?

 

KInd regards,

Marc

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Hi guys,

 

while taking photos of a scenery illuminated with a HMI lamp (magnetic ballast) I noticed the following:

 

#1

each picture has a different brightness; kind of expected, since the camera shutter is not synchronized with the mains frequency (50 Hz here, i.e. 100 Hz light pulses)

 

#2

Every picture has a different color tint to it. The effect is very obvious, color casts vary between white, greenish/yellowish, blueish and magenta/pink.

....

 

Followup Question: why doesn't this have an effect on color uniformity with digital (non global shutter) or film cameras with mirror shutter? After all, different parts of the image are exposed at different times, right? Or does it have an effect sometimes?

 

KInd regards,

Marc

 

This may be significant for your #2 question.

 

from http://www.lovehighspeed.com/lighting-for-high-speed/

---

HMI and fluorescent lights are generally fine for speeds under 100fps as long as they use electronic ballasts and are set to flicker free. Although HMI lights do not suffer from the flicker which effects tungsten, HMI’s can suffer from “Arc Wander,” whereby a plasmatic “hot spot” moves within the bulb, causing an amorphous shifting movement in the light output. The most common side effect to this is a rapid colour shift in a shimmering effect. No HMI light with a normal electronic ballast can be guaranteed against some form of flicker, no matter how big the lamp is.

---

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