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RED and IR


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#1 Mike Brennan

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Posted 03 April 2008 - 05:35 PM

RED users are reporting IR issues, namely not being able to filter enough of the IR out so it doesn't effect the visible light spectrum. It is ost noticible when using NDs that knock but a greater percentage of visible spectrum than IR.

I assume that the Prism in a 3 chip camera should absorb a lot of the IR spectrum, is this accurate? I can't recall a single mention of IR being a problem in 3 chip cameras.

We recall that Panavision changed their IR filter on f900 to allow a little more IR and red wavelengths onto the chips and even under that scrutiny there wasn't a suggestion that IR was causing a problem. Or was it being balanced out of the system in the OHB matrix?


Any info or science about digital stills cameras IR sensitivity in comparison to RED?




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#2 John Sprung

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Posted 03 April 2008 - 05:54 PM

... there wasn't a suggestion that IR was causing a problem. Or was it being balanced out of the system in the OHB matrix?

The matrix is in the electronics downstream from the chips. At that point, there's only a red channel, no way to tell what parts of the stuff in it come from visible light and what parts from IR. Filtering out IR has to happen optically, where we have access to the wavelengths of the photons.

I'm not sure whether the optical glass of the block would necessarily absorb more or less IR than the glass used in lenses.



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#3 Keith Walters

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Posted 03 April 2008 - 07:07 PM

RED users are reporting IR issues, namely not being able to filter enough of the IR out so it doesn't effect the visible light spectrum. It is ost noticible when using NDs that knock but a greater percentage of visible spectrum than IR.

I assume that the Prism in a 3 chip camera should absorb a lot of the IR spectrum, is this accurate? I can't recall a single mention of IR being a problem in 3 chip cameras.

We recall that Panavision changed their IR filter on f900 to allow a little more IR and red wavelengths onto the chips and even under that scrutiny there wasn't a suggestion that IR was causing a problem. Or was it being balanced out of the system in the OHB matrix?


Any info or science about digital stills cameras IR sensitivity in comparison to RED?




Mike Brennan

With film, the emulsion does not react to infrared at all. This is not deliberate a design aspect, it actually took many years of research to produce emulsions that even react to RED, so making film infrared sensitive would require more work!

As a consequence, filters originally designed for film cameras do normally not address IR performance, as it has never been a concern.

With a three-chip camera, dichroic prism surfaces reflect the red and blue light toward the red and blue pickup devices. The dichroic mirrors by their very nature are only designed to reflect red and blue, not IR.
The remaining green component travels through the prism along with any IR present. However it is easy to add an IR filter to the green light path because a filter designed to stop IR it will have little effect on green.

So filters designed for film use are usually OK for 3-chip cameras as well.

With single-chip colour cameras this is a much biger problem becasue it is very difficult to produce band-stop filters that remove all the IR while not appreciably affecting the visible red channel.

However, from what I have read about this, the people having problems were using an extraordinarily large amount of ND, usually trying to maximize DOF in daylight. Like the rolling shutter problem, it is something to be aware of, but hardly a show stopper.
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#4 Keith Walters

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Posted 03 April 2008 - 07:44 PM

Oops, I meant to say "maximize the shallowness of DOF":-)
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#5 Sam Wells

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Posted 04 April 2008 - 10:49 AM

Any info or science about digital stills cameras IR sensitivity in comparison to RED?

Mike Brennan


Hi Mike

Can't speak of other cameras but from all reports - I haven't tried to get "IR pics" - my D3 is quite insensitive to near IR -- iow users wanting to do IR photography would be frustrated using this camera (unless getting the IR cut filter removed I suppose)

I haven't tried any ND on this yet (I've got an ND 1.5 here I could tape on !) maybe I will...

I wonder where in the spectrum one (might) need to be worried about...

-Sam
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#6 Mitch Gross

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Posted 04 April 2008 - 11:27 AM

RED cameras are no more sensitive to IR than any other single chip sensor, it is simply a function of how these people are choosing to use it.
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#7 Michael Peploe

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Posted 04 April 2008 - 11:29 AM

Hi Mike

Can't speak of other cameras but from all reports - I haven't tried to get "IR pics" - my D3 is quite insensitive to near IR -- iow users wanting to do IR photography would be frustrated using this camera (unless getting the IR cut filter removed I suppose)

I haven't tried any ND on this yet (I've got an ND 1.5 here I could tape on !) maybe I will...

I wonder where in the spectrum one (might) need to be worried about...

-Sam


Situations where the color temperature is very low and you are using ND filters. Especially stacking ND filters. Diimmed tungsten lights, fires, those beautiful susnsets.
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#8 John Sprung

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Posted 04 April 2008 - 01:53 PM

It would be interesting to take a look through the problematic filter stack with a dark adapted eye. We generally think of the visible range as about 400 to 700 nm, but the human eye still has some limited sensitivity all the way out to 830 nm.



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#9 Keith Walters

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Posted 04 April 2008 - 06:11 PM

Hi Mike

Can't speak of other cameras but from all reports - I haven't tried to get "IR pics" - my D3 is quite insensitive to near IR -- iow users wanting to do IR photography would be frustrated using this camera (unless getting the IR cut filter removed I suppose)

I haven't tried any ND on this yet (I've got an ND 1.5 here I could tape on !) maybe I will...

I wonder where in the spectrum one (might) need to be worried about...

-Sam

This reminds me of the great Sony Handycam Nightshot Plus Scandal of 2005 :lol:

Many Handycams have the "Nightshot Plus" feature where you can activate an inbuilt infrared LED which allows you to take pictures of sleeping children and so on. Then it was discovered that if this feature was was activated outdoors in strong daylight, a lot of women's underwear could be seen glowing brightly through their clothes, because most synthetic fabrics strongly reflect infrared.

Due to unprecedented outrage from the usual suspects, Sony were forced to modify the cameras so that Nightshot Plus could not be activated above a certain light level!

Being familiar with the results of inadequate IR filtering on CCD cameras, for the life of me I could not work out what change to the processing could possibly make the sensor receptive or non-receptive to IR light at will. The answer was obvious when I saw a wrecked handycam - they simply have a little motor that swings the IR filter in or out!

Edited by Keith Walters, 04 April 2008 - 06:16 PM.

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#10 Sam Wells

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Posted 10 April 2008 - 12:52 PM

http://provideocoali...ir_filter_test/

-Sam
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