Alexa LF and IR problems.

Wed Feb 06, 2019 2:09 pm

Hello, yesterday i was shooting with an Alexa LF and i had some IR problems.

The camera have an IR filter infront of the sensor no?

The IR filter is a single filter or is IR, Low pass and UV all toghether in one filter?

This camera have no LF IFM Optical Clear Filter. Could this be a problem? Or have sone relation with the IR issue?

Thanks,
Martin Bendersky.
Bendersky
 
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Re: Alexa LF and IR problems.

Wed Feb 06, 2019 2:46 pm

Hi Martin,

The cover glass in front of the sensor is indeed an IR filter. Could you tell me a little more about your setup, were you using any ND filters? What can you tell me about the light sources?

Best,
Igor Barbosa
 
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Location: London

Re: Alexa LF and IR problems.

Wed Feb 06, 2019 2:55 pm

Supremes lens. 4k uhd pro res xq. 5600k

It was a mix of daylight with 2k tungsten open face with full ctb and some diffusion.

I saw the problem in a black velvet.

Thanks
Bendersky
 
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Re: Alexa LF and IR problems.

Wed Feb 06, 2019 4:42 pm

We saw the problem using ND 1.2 (with no IR).
Bendersky
 
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Re: Alexa LF and IR problems.

Wed Feb 06, 2019 5:20 pm

Hello Martin,
please send images where the effect occurs and a link to this thread at service@arri.de.
Our colleagues from the service department will take a look at it.

Thank you,
Fred
Frederic Merten
Product Management Camera Systems
Frederic Merten
 
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Re: Alexa LF and IR problems.

Wed Feb 06, 2019 6:06 pm

Hi Martin,

>> The camera have an IR filter infront of the sensor no?
All ALEXAs have an IR filter in front of the sensor. It is part of the cover glass, the first glass surface seen from outside. It has a light green-ish tint.

>> The IR filter is a single filter or is IR, Low pass and UV all toghether in one filter?
The cover glass is an IR filter, the next glass layer behind it is a combined low-pass / UV filter.

>> This camera have no LF IFM Optical Clear Filter. Could this be a problem? Or have sone relation with the IR issue?
The optical clear filter is just used to adjust for the change in flange focal distance introduced by using internal FSND filters. It has no effect in reducing IR exposure.

From your previous messages I understand, that you used a pure ND filter without IR filtering component (IRND), is that correct? If so, the IR pollution you saw may have well resulted from that. While a classic ND blocks the visual range of light, it will not block much of the IR range. So the IR percentage will be higher than without ND filter.

A rough example:
No ND: 100% visual, 100% IR
Classic ND 2.1: 6.25% visual, ~80% IR

Thus its important to use IRND filters (or our FSND) which block both visual light and IR equally.

Best regards,
Mathias
Mathias Lischke
 
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Location: Munich, Germany

Re: Alexa LF and IR problems.

Wed Feb 06, 2019 7:16 pm

Hey Mathias, but the IR filter in front of the sensor should not solve the IR problem?

Thanks
Bendersky
 
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Joined: Fri Jul 14, 2017 3:24 pm

Re: Alexa LF and IR problems.

Thu Feb 07, 2019 11:34 am

Hey Martin,

I'm afraid it can't. The IR level needs to be balanced together with the visual light. Filters that block visible light but let most of the IR pass, will cause IR pollution.

Best regards,
Mathias
Mathias Lischke
 
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Re: Alexa LF and IR problems.

Fri Feb 08, 2019 5:16 pm

Gracias!

Saludos
Bendersky
 
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Re: Alexa LF and IR problems.

Tue Feb 19, 2019 12:19 pm

A word form the workflow guys here: up to ND0.9 you're okay using regular NDs, the IR spill is not that visible. All the above should/must be IRND's cause you will have an uneven reduction of the spectrum of light hitting the sensor.

It's like a blanket, if it is too small you'll get cold feet.


Cheers,
Jan
Jan Heugel
Application Engineer
Jan Heugel
 
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