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HMI flicker with Arri-S?


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#1 Luke Jensen

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Posted 26 November 2006 - 01:40 PM

Hi everyone,

I'm shooting a project pretty soon with an Arri-S and I heard that there can be flicker problems when using HMIs.
I'll be using a 1200 with a magnetic ballast, and the camera has the Arri-brand variable speed motor
(shooting at 24fps).

Thanks,
Luke
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#2 Tim Carroll

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Posted 26 November 2006 - 02:13 PM

If you are using the Arri variable speed motor, flicker from the HMI lights will not be the problem. Those motors are considered "wild" motors because the speed fluctuates quite a bit. You will not get a constant film speed with that motor.

I'm not an expert at shooting with HMI lights, so I will let others help you there. If you are definitely shooting with an Arriflex 16S, and you want a constant film speed, the best bet is one of Clive Tobin's crystal sync motors. If memory serves, there were two variation (TXM10-S and TXM-17) that could be hooked up to his Milliframe Controller, which can be adjusted to deal with HMI, TV screen phase, etc.

-Tim

PS: I did find this on Clive's site:
Filming Under HMI Lights. For shooting under discontinuous (flashing) illumination there are only certain speeds that will give flicker-free footage. Such sources include HMI lights, fluorescents, and discharge type street lights. On 60 Hz and 50 Hz current, these lights flash 120 or 100 times per second respectively. To get even exposure from one frame to the next there must be a whole number of flashes per frame.
Safe speeds can be calculated by dividing the flashing rate by 1, 2, 3, 4, 5 and so on.
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#3 Kevin Zanit

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Posted 26 November 2006 - 03:33 PM

You will have flicker if you use the magnetic ballasts. You may not have flicker with the electronic, it depends how much the camera speed varies, though I would say you would probably not have a problem.
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#4 Olex Kalynychenko

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Posted 26 November 2006 - 04:02 PM

Hi everyone,

I'm shooting a project pretty soon with an Arri-S and I heard that there can be flicker problems when using HMIs.
I'll be using a 1200 with a magnetic ballast, and the camera has the Arri-brand variable speed motor
(shooting at 24fps).

Thanks,
Luke


What frequency of main power you will use ? 50 HZ or 60 Hz ?

HMI lighters have hard pulses volume of light with double of frequency of main AC power.
AC 50HZ - winking 100 hz
AC 60 HZ - winking 120 hz.
The next generation of HMI lighters with " flicker free" electronics ballast have special electronics for pulses smoothing

You can make simple calculation.
If you will use HMI magnetic and 50Hz power.
100 hz / 24 fps = 4.1666
The nearest integral value 4 x. ( four pulses of light will inside of exposing time of one film frame)
100/ 4 = 25 hz.

The calculation of beat frequency
25 hz -24 fps = 1 hz.
You will have beating of density on footage with frequency 1 hz or every 24 frames.

If you take HMI and 60 hz power.
120/24 = 5
You will have flicker free mode.

But, all this calculation will have good result, if the frequency of main power will have 50.00 Hz or 60.00 hz, not other volume and, if the camera will fixed speed 24 fps.

You wish use cine camea with rheostat speed control.
I know not about Arri-S camera, but, i can tell you a some about Konvas-1 with rheostat speed camera motor.
The Konvas-1 have electronical motor and speed meter with telerance +/- 5%.
If you see 24 fps, The camera will have real speed 22.8....25.2 fps.

If the camera will have 22.8 fps and 60 hz HMI lightes, you will have beat frequency
24-22.8 = 1.2 hz.

That's why, the many filmmakers, who shoot of Olympic games on modern stadiums with big HMI lightes, had bad quality of footages. Because had fliskers on footages.

What you can do.
You can use 3 phase alternator and 3 pcs of HMI lighters, connected with every phase.
You can set crystal sync speed motor.
The very good result have camera with crystal sync motors and outside synchronization of speed.
This is motors have outside adapter for generate of pulses from frequency of main power.
The pulses transfer on motor and motor adjusting a speed with real frequency of main power.
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