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HMI Flickering

Published : 3rd January 2005

Hi,

I run a production house for tv commercials for 4 years. Three days ago we finished filming a cosmetic commercial. Yesterday during film to tape one-light transfer we were faced with a problem which I never encountered before. I need help.

In one of the C.U. shots, and in this shot only, all takes show flickering. The key lighting was from a 6K HMI fed from a supposedly flicker-free transformer.

Both my overseas DOP and first camera assistant told me that such accident unfortunately sometimes do happen and is generally nearly impossible to monitor during shooting. The lighting man guessed that it's probably due to a faulty flicker-free transformer & not due to the camera. The guy at post production told us that he is unable to fix the shot. The shot in question is what they call the benefit shot (showing the supposed effect of the cosmetics product) which is an important shot. Luckily we caught several beautiful CU’s earlier when the actress was not noticing which in my opinion are better than the shot that flickers.
I've to face the clients in roughly 3 hours to present the rough cut.

Please kindly share your professional knowledge on this subject with me:

(1) Is it more likely due to the transformer & not the light itself nor the camera?

(2) Revisiting the VHS recording of the video assist I do notice a flickering of the image for that shot.

Unfortunately nobody noticed it on the spot or, if somebody did, grasped the significance of this flickering of the tv image. So could it have been observed in the camera finder? Is the video assist image a good monitor of flickering on the spot?

(3) Is there some sort of hand-held meter which could read this HMI flickering on location?

(4) The guy at post production told me that it's nearly impossible to fix this on D1. Is it true?

(5) The same post production man told me that about 2 out of 10 TVC’s he did encounter this problem.

Is it true? Have I been just lucky?

Thank you in advance.

Edmund Lo
Creative Director & Managing Director
TV commercials
COME Productions
Hong Kong based


class="style9">>I've to face the clients in roughly 3 hours to present the rough cut.

If the shot you used to replace the flickering shot matches closely enough, don't even tell the client about the problem. They may not notice.

Art Adams, DP [film|hdtv|sdtv]
San Francisco Bay Area - "Silicon Valley"
http://www.artadams.net/


Hi Ed,

There certainly is a meter that will work for you, a B&S flicker meter see the web page :-

http://cinematography.net/edited-pages/%5Bcml%5D-

Frequency-Meters.htm

Can it be fixed in post? Well I have seen miracles done with flicker by good post houses.

It wouldn't have been noticed in the camera V/F as that's flickering all the time anyway I'd have thought that some of the people watching the assist may have noticed but......

2 out of 10! Bollocks! It is/should be a rare problem.

Cheers

Geoff Boyle FBKS
Director of Photography
EU Based
www.cinematography.net


>>In one of the C.U. shots, and in this shot only, all takes show flickering. >The key lighting was from a 6K HMI fed from a supposedly flicker-free >transformer.

Check the footage closely, is only the Key Light flickering? If you were using HMI's for fill, bounce, backlight, or background light, check to see if any of those sources are flickering with the Key. This will either eliminate or include the camera as the source of the problem.

Was this the last shot of the day, and the last time the 6K was used? If not then why is there no flickering in the other shots?

Was this shot an off-speed shot? Even though you have flicker free ballasts, there is a switch on the ballast for flicker free--or silent mode. If you were recording any dialogue, or if by chance no one made sure the switch went to flicker-free and you were shooting at higher un-matched frame rates--this would explain the flicker for that shot. This would be completely human error and not the fault of the equipment.

There is no way to notice by eye that type of flicker on set.

I have experienced this problem more like 2 times out a thousand plus set-ups--not common at all.

One more thought--If your using generator power always make sure it is crystal sync and running at the proper Hertz Frequency.

Good luck with the client.

Michael Ambrose
Gaffer
Los Angeles, CA.


Michael Ambrose writes :

>>There is no way to notice by eye that type of flicker on set.

A trivial suggestion, and something I've never tried, but has anyone tried looking through the blades of one of those little hand held cooling fans and turning it on? As it runs up to speed it sweeps through a wide visual frequency range like a ramping shutter, so it might actually reveal flicker from some fixtures.

In Scotland we don't tend to use these little hand held fans much, since they tend to blow the snow in our face.

Clive Mitchell
http://www.bigclive.com


Graham Rutherford writes:

>> HMI Flicker I think is badly named

No more so than "Flicker Free Ballasts"

Seriously, you can have everything perfect: the electronics of the HMI, the speed of the Generator-- perfect 60.00Hz, the speed of the camera, perfect at 24.000 fps, and still get flicker -- if the HMI globe has too many hours. That is, if the electrode gap becomes too wide. Since this is a function of hours of usage not frequency, the unit may leave the rental house in perfectly acceptable condition and develop flicker in the course of the shoot. The older the bulb, the worse it gets.

What's more, this flicker may or may not be noticeable on film depending on circumstances of the shot. For instance, a lamp with significant flicker can be completely unnoticeable on a sunny exterior, when it is one among many. However, if this same light is used as a single source on an interior, it can be a disaster. Frequency meters, and CineChecks only detect off frequency conditions, they will not detect variations in light output; only a B&S meter will detect the minute variations in light output caused by an aging lamp. There's tons of info on this subject and on the B&S meter in the archives.

>>a more apt description would be HMI strobing. It can't be seen by eye >unless you are looking through the camera with its rotating >shutter/mirror.

I for one would very much like to meet the person who can detect HMI flicker and/or strobing by looking through camera.

Brian Heller
IA 600 DP


1/ : It can be The transformer (Ballast), Light or camera.

Camera : If the camera is shooting variable speed or variable shutter or ramping & your A/C's haven't informed your Gaffer then this could be the problem(Your Gaffer would need to set his ballasts {transformer} to Flicker Free/Silent). Also there is a minute possibility that it could be the stock (Once in 20 + years of shooting we had a problem with emulsion layering).

Another issue is that if your camera was set for USA 25 frames, & your generator set @ 50 hertz, & everybody thinks everything is Ok but as above its not.

Light / Lamphead : As bulbs age or due to manufacturing faults / rough handling in the field, there is the possibility of flicker.

Ballast (Transformer): As stated above if your Gaffer is not informed or switched on enough to know that the camera is running at anything other than the normal settings, then the ballast may be switched to the wrong mode.(Assuming that, as you state it was a flicker free ballast & not magnetic). The other cause is the service/maintenance of the electronic ballast, you pay for what you get(Purchase & maintenance). This goes for your lamphead & feeder cables.

Generator : As already stated in another post if your generator is not running @ synch this can cause a problem. If not synch locked it will cause a flicker.

2/ : The only flicker I've ever noticed by eye was never on a video assist, but on set because of the generator or faulty lighting fixture/dodgy bulb. The image you see on your video tape is normally split from your viewfinder & therefore has the flicker from the shutter.

3/ : Again as stated in a previous post there is a meter manufactured by B & S (who produced Arri's electronic ballasts),I'm unsure of how many variations of these meters came out. I have one the earliest ones, it measures how much the light source varies from a steady constant light ie. the Sun or a battery torch, point it at either of these & it will read 0%, the output varies with the light source ie Tungsten can read @ 10%, Magnetic HMI's 40%. However I have heard that some later versions (& cheaper versions) were not as reliable.

4/ : This I can't comment upon as you don't give enough detail, is it just one fixture out of many; or just one.

5/ : With the existence of the internet & these great forums, in this day & age, & also with accessibility almost worldwide, there is no reason for these problems to reoccur, I don't mean to preach on this one, this should be a rare problem, the info is out there. All you have to do is read it.

Regards,

James McGuire
Gaffer Dublin


Hi,

Many thanks to Geoff Boyle, Michael Ambrose, Anders, Art Adams, Clive Mitchell & James McGuire who so kindly share their professional views on my problem of a flickering HMI shot.

Yesterday this shot was replaced by a similar shot in the rough cut & presented to the client who loved the cut & never knew a problem ever existed.

This is a great forum. A shout for help can call up so many enlightening responses from such a group of seasoned professionals all over the world.

I read all your posts carefully. Unfortunately the rental house was unable to supply us the serial no.s of the HMI lamp & the ballast so we could not run a test. The shot is such a C.U. shot. (the model applying a brand of a rather well-known cleansing oil to her face, filmed at slightly high speed) The 6K HMI appears in the background as well as in the bounce, so we could not tell if the fault is from the sync of the Arriflex 435.

A new bit of information : The ballast in question was actually a replacement of a broken down ballast. It seemed to arrive on the set roughly during the time of the shot although nobody could know for sure.) Strangely this is not the last shot of the day while none other shots show any flickering.

While we still try to hunt down the culprit, one thing is for sure : I'll suggest my DOP of the following shoot to use a B&S meter.

My sincere thanks again to you all. I welcome any further input on the subject.

Edmund Lo
Creative Director & Managing Director
TV commercials
COME Productions
Hong Kong based


Hi Ed

My commiserations, I also had flicker on a commercial shooting various shutter angles and camera speeds 75/150fps at 45/90degree shutter etc.

I never really nailed what it was except that it was a camera fault of some kind.

HMI Flicker I think is badly named it ... requires a pulsing A.C. driven lamp and a rotating camera shutter... a more apt description would be HMI strobing. It can't be seen by eye unless you are looking through the camera with its rotating shutter/mirror.

It is not common and can usually be fixed by the Post House providing they have the appropriate software for Inferno etc.

Flicker free ballasts for HMI's turn the input sine wave into a square wave output which turns the lamp on longer thereby eliminating the strobe.

On silent mode(modified square wave) electronic ballasts should still be OK up to 30 frames or so and on flicker free to around the 1000 frames, because they are switch mode they can accept a wide range of both voltage and frequency inputs without effecting their output.

I listen for the frequency change of the globe oscillation when switching between low noise and flicker free. There is a definite change of pitch in the lamp oscillation as the lamp is driven harder by the square wave.

Just a simple little check to reassure me the ballast has switched to flicker free mode.

A lot of the new ballasts are high frequency and are permanently flicker free mode.

Graham Rutherford
Gaffer
Australia


Just a note on electronic ballasts, on silent mode the input voltage is changed from AC to DC. However the output voltage in silent mode is basically a carbon copy of the input frequency wave(just slightly out of phase), & therefore the usual rules apply.(The H bridge switching configuration turns the DC back into AC across the bulb).

When you switch to flicker free that's when you get the benefits (The dips between voltage peaks properly filled in, etc).

I would personally treat an electronic ballast running on silent as a magnetic unit, with the same due cautions.

Regards,

James McGuire
Gaffer Dublin


Hi James

I agree about treating a electronic ballast on low noise the same as an inductive ballast.

The Lightmaker/LTM electronic ballasts had a '22-32 frames’ switch position and 'All' position for flicker free. The DC-AC converter driving the output IGBT's and lamp is a 60 cycle electronically generated square wave with no tie in to the input power to the ballast, but yeah if I was to shoot 30 frames I would definitely switch to 'All' (flicker Free).

I have seen people caught switching the HMI ballasts to flicker free but having domestic fluorescent lights or MEI type street lighting in shot. Which are of course strobing like crazy at non HMI safe speeds.

The mains frequency here is 50Hz and with the 435 cameras 150 frames is regularly used. Everyone was used to shooting 50 and 100 frames with no problem with fluoro’s etc. but at 150 frames there is no window... A few red faces however.

Regards

Graham Rutherford
Gaffer
Australia.


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