Cinematography Mailing List - CML

Frequency Meters


What do people think of the B&S frequency meter that Arri represents?

Are there other units out there that are more useful?

The functionality of the B&S unit seems to be quite nice but I haven't used it enough to form a qualified opinion.

David Perrault

It is an indispensable piece of equipment if you are going to do "off speed" or high speed photography using "flicker free" HMI lighting.

The proper utilization of the B&S meter can save your job from a reshoot caused by supposed "flickerfree" HMI lighting flickering badly and ruining a shot, or the whole day!

A must have item if you are going to vary from "standard" sync sound frame rates.

Bill Bennett Los Angeles

I love it and don't go out without it


Can any Angelenops tell me if there is anyone who is currently renting this meter? I should probably add one to the arsenal, but I would like to play with one a bit before spending the $$$ Luddite that I am, I have been surviving without one for a while, but that may reflect luck more than skill

Mark H. Weingartner

Lighting and VFX for Motion Pictures

Having embraced web technology in the last 5 minutes, I have discovered that Photosonics rents them. I will probably go over there in the next couple of days and play with one. At $20/day to rent, it is probably worth the 2/10 mile drive


What is the difference between the B&S meter and the Cinematography Electronics frequency meter?

Arri in NY doesn't seem to know.

Brian "Frequing Out" Heller

The Cinematography Electronics, Inc. (CEi) freq meter is intended to be used with HMI lights and magnetic ballasts that have light output that pulses 120 times per second when on a 60hz supply or pulse 100 times per second when on a 50 hz supply. The CEi meter can accurately measure the frequency of the light pulses and determine if the generator or mains frequency is within tolerance to allow shooting with a crystal controlled camera that is operating at one of the "HMI legal" speeds. In a 60 hz situation that would be 10, 10.909, 12, 13.333, 15, 17.143, 20, 24, 30, 40, 60 fps. 120 fps can be used with care, but since there is only one light pulse per frame and the light pulses have a half sine wave shaped peak, you need to phase your camera so the image in the viewfinder is a dark as you can possibly make it after you start up for each take, that way each light pulse is going onto the film, not into the viewfinder. It also takes some explaining to the Director, who sees his video assist image get quite dark after camera startup.

Camera speeds slower than 10 hz have been calculated and published, but I personally feel that you can shoot any speed below 10 fps, and certainly below 6 fps, and not see any appreciable flicker on the film. This is because there are so many complete HMI light pulses recorded per frame at those long exposures that the *difference* between the remaining last part of a pulse on one frame when compared to a remaining last pulse on another frames is insignificant. The CEi meter can also verify the accuracy of the speed of the camera, you point the camera at a constant light source, put the meter into the FPS mode, point it into the camera's viewfinder and start the camera. The shutter interruptions are used to calculate the speed of the camera.

The B&S meter on the other hand is another animal altogether. Like the CEi meter, it can measure the mains frequency when you are using magnetic ballasts, but B&S meter's true usefulness is preventing reshoots caused by high frequency flickering during high framerate shots using "flickerfree" HMI electronic ballasts. Yes this can and does happen. It usually happens with older bulbs, and gets worse the older the bulb becomes. It is impossible to know how old a bulb is in rental kit, and you CAN NOT see this flicker with your eye, if it is there, but the film will see it, if it is there.

What the meter measures is the duty cycle of the lamp output. It reads out in "Percentage of Flicker" The sun, which is a continuous source has zero percent flicker. You can use it to test the meter. The tolerance for a electronic ballast "flickerfree" HMI lamp and ballast is a maximum reading of 3 percent on the meter when in the "Percentage of Flicker" mode. My gaffer says that he sees a typical reading of "good" lamps of around 1.9 to 2.0 percent. When it get up to around 2.6 to 2.8 percent, he replaces the globe.

My gaffer warns that really high intensity light confuses the meter, so he reads the flicker off a piece of white card or the back of his hand, which is illuminated by the light being tested.

We had some troubles on a job where we were doing some speed ramping using "flickerfree" HMI lamps and looked into the solution and found that you have to use this meter if you want trouble free usage from "flickerfree" HMI lamps and ballasts.

Bill Bennett Los Angeles

Is this true on every time you start camera? What about magnetic ballasts? Any problem with mercury vapours at 120 FPS? This is a little new to me, I've never had to do it on any other show. (Been at it for 15 years)

Thanks Bill. We have had a similar problem with "flicker free" HMI lamps that we could never pin down -- everything checked out OK with the CE meter. Thanks to your explanation, we are looking into getting the local lighting company to get one.

Brian Heller-- soon to be truly flicker free.

michael bratkowski wrote: > Is this true on every time you start camera?

Yes, you must phase the camera to the light pulses every time you start the camera.

> What about magnetic ballasts?

I am talking about magnetic ballasts when I talk about phasing a camera operating at 120 fps. With "flickerfree" you can run at any frame rate you choose, but to be certain of suitable results, you need to use the B&S "percentage of flicker" meter we were talking about in my last post.

> Any problem with mercury vapours at 120 FPS?

Mercury vapors are the same as HMI lamps that utilize magnetic ballasts, you need to phase the camera if you are going to shoot at 120 fps in a 60 hz environment.

Bill Bennett

It is worth mentioning that if you are shooting under merc vapor in an arena or other large public space, there is a very good chance that they are being fed on three phase power, and if so, at any given time, some are on and some are off. If they are far away and blending together (and not in shot) they may mush together to a uniform stop. If this is the case, you will have trouble seeing any variation to phase to. Individual highlights and kicks caused by individual fixtures may be brighter or dimmer than you see through the eyepiece.

Mark H. Weingartner

Lighting and VFX for Motion Pictures

ARRI USA is carrying the frequency meter Bill Bennett mentioned. It's part of the ARRI USA lighting division product line, so you need to talk to a lighting person. Part number is zbs-101. List price is $650.


Marc Shipman-Mueller,

Technical Representative

Arriflex Corporation; 1646 North Oakley Ave, Suite #2, Chicago, IL 60647-5319, USA

David Wrote :

>What do people think of the B&S frequency meter that Arri represents?

I am not familiar with this particular meter. I can tell you to buy a meter that records, they can be very helpful when trouble shooting. My personal favorite is "The Line Gap Meter" manufactured by Linegap.

> My personal favorite is "The Line Gap Meter" manufactured by Linegap.

I have the Line Gap meter. It will not solve the problem that Bill Bennett addressed, but the thing I like about it is that you put it in the distro line somewhere, and it continuously monitors frequency/voltage and has a chip in it that "remembers" the highest and lowest points of departure. So you can plug it in, go away, and come back and it will tell you the high and low frequency and voltage while you were gone. It also has an "idiot light", that glows red if the voltage/frequency gets out of generally accepted range. This, you can see on the set, without having to pick up the meter and look at it.

-- Jim Dollarhide Director/Cameraman Jackson, MS


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