Alexa LF and IR problems.

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Bendersky
Posts: 10
Joined: Fri Jul 14, 2017 3:24 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.
Igor Barbosa
Posts: 25
Joined: Wed Mar 23, 2016 3:07 pm
Location: London

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,
Bendersky
Posts: 10
Joined: Fri Jul 14, 2017 3:24 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
Posts: 10
Joined: Fri Jul 14, 2017 3:24 pm

We saw the problem using ND 1.2 (with no IR).
Frederic Merten
Posts: 41
Joined: Thu Nov 26, 2015 11:49 am
Location: Munich, Germany
Contact:

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
Mathias Lischke
Posts: 7
Joined: Tue Nov 24, 2015 11:32 am
Location: Munich, Germany

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
Bendersky
Posts: 10
Joined: Fri Jul 14, 2017 3:24 pm

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

Thanks
Mathias Lischke
Posts: 7
Joined: Tue Nov 24, 2015 11:32 am
Location: Munich, Germany

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
Bendersky
Posts: 10
Joined: Fri Jul 14, 2017 3:24 pm

Gracias!

Saludos
Jan Heugel
Posts: 459
Joined: Wed Aug 13, 2014 3:15 pm
Location: Munich, Germany
Contact:

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
PSil001
Posts: 1
Joined: Thu Feb 02, 2023 12:42 am

>> 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.


So the Alexa has an internal IR filter on the sensor and that will be working no matter what internal filter is being used (clear, nd)? Is it right?
Jan Heugel
Posts: 459
Joined: Wed Aug 13, 2014 3:15 pm
Location: Munich, Germany
Contact:

Dear PSil001,

yes the cameras have an IR filter integrated in the sensor cover glass, so it is always there.
Nevertheless you cannot use ND filters without IR cut-off (which you used to have when shooting on film). Image sensors are more sensitive to those corner-frequencies and some fabrics will look wrong when you do not use IR cut-off filters like our FSNDs (Full Spectrum Neutral Density Filter).

Kind regards,
Jan
Jan Heugel
Application Engineer
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