higher CRI? flicker?

[dvdguy]

Hey guys,

I was wondering why is there an occurrence of "flicker" when the CRI is over 90+ or so I read. Just checking if anyone knows any theory behind this. Why are the ballasts needed and what happens on camera if you DONT use ballasts? I mean what do they actually mean by flicker?

Thanks in advance

 

[chilipie]

Hey guys,

I was wondering why is there an occurrence of "flicker" when the CRI is over 90+ or so I read. Just checking if anyone knows any theory behind this. Why are the ballasts needed and what happens on camera if you DONT use ballasts? I mean what do they actually mean by flicker?

Thanks in advance

I've never heard of that problem before (and this page appears very quickly when searching for it), but I'd be interested to see a link if you have one…

Ballasts turn DC (Direct Current) power from a mains supply, and convert it into AC (Alternating Current) power for the lamp, while also controlling the flow of power to the lamp. Not exactly sure what would happen if you didn't use a ballast, but I suspect you'd either have no light or a broken lamp. I wouldn't recommend doing any ballast modifications unless you know what you're doing.

Flicker is… well, this graph is the waveform of AC power:

(x-axis is time, y-axis is current/voltage)

As the current rises and falls, so does the brightness of a lamp. Have you ever looked at a tungsten bulb as it's turned off, and noticed it glowing for a moment while it has no power? The brightness in fluorescent lamps tends to decay much faster, so the flicker you see with your eye is often a cheap ballast is failing to regulate the current correctly, leading to the lamp dimming noticeably between waves.

When it comes to cameras, you see flicker when the camera's shutter is not moving in sync with the sine wave of an AC power supply. Each time the shutter opens and a frame is recorded, the lamp has reached a different point in the wave and so the quantity of light rapidly goes up and down. Flicker is usually avoidable by selecting a shutter speed that matches the frequency of the mains power supply - 1/50 or 1/100 in Europe, and 1/60 or 1/120 in the USA - so that every time the shutter opens, the lamp is at the same point in the wave.

(Take all of the above with a pinch of salt… my electrical knowledge is a bit patchy.)

 

[dvdguy]

I've never heard of that problem before (and this page appears very quickly when searching for it), but I'd be interested to see a link if you have one…

Ballasts turn DC (Direct Current) power from a mains supply, and convert it into AC (Alternating Current) power for the lamp, while also controlling the flow of power to the lamp. Not exactly sure what would happen if you didn't use a ballast, but I suspect you'd either have no light or a broken lamp. I wouldn't recommend doing any ballast modifications unless you know what you're doing.

Flicker is… well, this graph is the waveform of AC power:

(x-axis is time, y-axis is current/voltage)

As the current rises and falls, so does the brightness of a lamp. Have you ever looked at a tungsten bulb as it's turned off, and noticed it glowing for a moment while it has no power? The brightness in fluorescent lamps tends to decay much faster, so the flicker you see with your eye is often a cheap ballast is failing to regulate the current correctly, leading to the lamp dimming noticeably between waves.

When it comes to cameras, you see flicker when the camera's shutter is not moving in sync with the sine wave of an AC power supply. Each time the shutter opens and a frame is recorded, the lamp has reached a different point in the wave and so the quantity of light rapidly goes up and down. Flicker is usually avoidable by selecting a shutter speed that matches the frequency of the mains power supply - 1/50 or 1/100 in Europe, and 1/60 or 1/120 in the USA - so that every time the shutter opens, the lamp is at the same point in the wave.

(Take all of the above with a pinch of salt… my electrical knowledge is a bit patchy.)

Wow, thats a good, thourough explanation. But Id appreciate if someone could give an explanation with more exampling. I understand most of the text, but not the entire thing.

For instance, why do I always see a Ballast wired with a HMI, why dont I see for example a Ballast wired to a Tungsten Continous Fresnel light? What that makes that so special? I dont know, why doesnt every light have that?

And arent Ballasts JUST meant for still Photography? I mean nobodys gonna even notice any quantity of light going down. Cause to the camera the light is always the same (in terms of video, not photography).

Thanks

 

[chilipie]

These answers are taken from this ballast and fluorescent tubes FAQs.

What is a ballast, and how does one work?
A ballast is an electrical component used with a fluorescent bulb (or mercury vapor lamp or arc lamp) to conduct electricity at each end of the tube. It supplies the initial electricity to the bulb that creates light, and then it regulates the amount of electricity flowing through the bulb so that it emits the right amount of light.

Why do fluorescent light bulbs need a ballast?
Fluorescent light bulbs need a ballast because they use a gas to create light. Regular light bulbs (also known as incandescent bulbs) create light by heating a filament inside the bulb. The heat makes the filament white-hot, producing the light that you see. In a fluorescent bulb, when the gas is excited by electricity, it emits invisible ultraviolet light that hits the white coating inside the fluorescent bulb. The coating changes the ultraviolet light into light you can see. (A lot of the energy used to create the heat that lights an incandescent bulb is wasted. Because fluorescent bulbs don't use heat to create light, they are far more energy-efficient than regular incandescent bulbs.)

The combination of gas, electricity, and coating in a fluorescent bulb is so effective at producing light that, without something to regulate the electricity flowing into the bulb, the light will continue to gain intensity until the bulb stops working. That's where a ballast comes in — it supplies the initial electricity that creates the light, and then it regulates the amount of electricity flowing through the bulb so that the right amount of light is emitted.

As far as I understand it, a similar principle applies to HMI lights - a ballast supplies the larger, initial current to strike a cold lamp, and then prevents the light from drawing more and more power as the light gains intensity.

Not sure what you mean about stills vs. video, but the use of a ballast is determined by the type of light, not its application.