# The Effect of Near-Infrared Filters on Visible Spectrum



## Timsky (Sep 7, 2014)

I have a technical question regarding B&W film photography and how heavy red/near infrared filters and IR sensitive films effect the color red in the visible spectrum. Do they lighten or darken or red or is it left alone?  I've been searching the web like crazy and I can't find the answer or any before & after comparisons that contain red- most people seem to use IR for landscape photography which rarely contains red.

Does anyone have knowledge of this or can point me to further info and examples?


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## snowbear (Sep 7, 2014)

I can't say definitively but it might make a difference in how the subject reflects IR.  That being equal, I suspect red would be a bit darker.


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## timor (Sep 7, 2014)

Get a roll of Rollei 80S and shoot it with red (Wratten 25 or 29) and you will see it for yourself. The sky will be much darker and the foliage much brighter. 
Wratten number - Wikipedia, the free encyclopedia


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## petrochemist (Sep 7, 2014)

It will depend on a great many variables. Different near IR filters transmit different amounts of visible light, which further varies by the wavelength of that light. Different 'visible light' cameras have different (trace) sensitivity to the near IR being transmitted, and different 'red' subjects will reflect different amounts of IR (as will the non red parts of the image).

With a 720nm filter (the standard R72 IR filter) most visible light cameras will record reasonable IR images though the exposure may be very long.
'High colour' infra red filters transmit down to 650, 600nm or even 550nm. With normal cameras these will allow through most of the red whilst blocking nearly all the other visible light. (The trace IR response of normal cameras becomes irrelevant - these filters are intended for modified cameras). This doesn't actually lighten the reds, but it darkens the rest significantly, which if the exposure is increased brightens the reds relative to other colours in the final image.

BTW many of the red/orange filters I use for IR photography are old visible light ones - I've not yet found one that blocks IR significantly, even a recently purchased Cokin 'blue' filter transmits 90+% of most NIR wavelengths.

There's more information on the transmission of filters on the infra red forum I'll add my latest spectral information (some normal Hoya/cokin filters) there when I get the chance.

Update The IR forum linked to above has gone. A new one formed by the old members can be found at Home Global Infrared Photography Village


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## Timsky (Sep 7, 2014)

Thanks Snowbear. In the absence of any other information I opened up Photoshop to look at their B&W photo tool which has presets for different color filters, incl. an infrared setting. When you look at the color slider settings (reds,yellows,greens,cyans,blues,magentas), for the High Contrast Red and Infrared presets there's a disparity with the reds.   The High Contrast Red preset increases the brightness of red 120% whereas the infrared filter reduces it -50%. I know these are just an approximation to mimic the behaviour of filters on analog film but it's confusing none the less.




snowbear said:


> I can't say definitively but it might make a difference in how the subject reflects IR.  That being equal, I suspect red would be a bit darker.


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## Timsky (Sep 7, 2014)

BTW, I have previously shot black & white film using red contrast filters and Rollei Infrared Film using a B+W infrared filter (I forget which model as it was a few years ago) but I can't remember what the effect was on red coloured objects


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## timor (Sep 7, 2014)

Red objects will be rendered white as abundance of red light will overexpose those parts of the negative. This work especially well with Tmax films (extended red sesitivity).


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## compur (Sep 7, 2014)

Timsky said:


> I have a technical question regarding B&W film photography and how heavy red/near infrared filters and IR sensitive films effect the color red in the visible spectrum. Do they lighten or darken or red or is it left alone?



Red/near infrared filters lighten red on B&W film including on IR film. 

All B&W filters lighten similar colors and darken complementary colors. 

But, with IR films there is the added factor of IR light intensity of the subject. That is why green subjects like leaves, grass, etc often appear lighter with IR photos when they would normally be darkened by a reddish filter when using conventional B&W film. They appear lighter on IR film because of their high IR reflectance.

Now, let's say you had a red subject which has little or no IR reflectance shot through a red/near infrared filter. It would still be rendered lighter on IR film simply because of its color being similar to that of the filter.


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## Ridgeway (Sep 16, 2014)

A couple of comments.

Filters NEVER lighten anything. They only subtract light and so darken. The only thing that will cause something to appear lighter is increased exposure.

Complementary colours have no place in discussions about filters. The effects of a filter should be determined only by reference to its spectral absorption curves. If you use the rule that filters darken complementary colours then we would all be using yellow filters to darken a blue sky whereas we all know an orange or red filter darkens it more.

To the OP I would say, look up the absorption curves for the filters you are interested in and they will tell you by how much the filter will reduce light in the red area of the spectrum. No filter will 'lighten' anything. Here's the curve for a typical 720nm filter.







Similarly, check the sensitivity curves of the films you are interested in and they will tell you to what extent they are sensitive to red light - but most IR films have good sensitivity in all of the visible spectrum which is why it's necessary to use an IR filter if you want a true IR effect. Here's the spectral response to Rollei IR400 for example, which shows good sensitivity right down into the blue region.


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## compur (Sep 18, 2014)

Ridgeway said:


> A couple of comments.
> 
> Filters NEVER lighten anything. They only subtract light and so darken. The only thing that will cause something to appear lighter is increased exposure.



This is not always true in the context of film photography. Reducing exposure "lightens" negatives and increasing exposure "darkens" them, for example. The same is true of darkroom printing of negatives. Less exposure lightens the print and more exposure darkens it. So, it is not true that "filters never lighten anything."

Since your post appears to be a comment on my post above I will clarify my meaning of that post.

If, for example, a photographer wishes to lighten the green leaves of a tree in relation to the brown tree bark then this can be achieved through the use of a green filter. The green filter will tend to block more of the light from the bark then from the leaves so, in the final print, the leaves will appear lighter in relation to the bark than they would if shot without the filter.

This is what is mean by "filters lighten like colors" in the context of photography. It isn't meant to be taken literally but only as a rule of thumb to help in selecting which filter to use to achieve what effect as far as manipulating the relative tonality in B&W photography is concerned.



> Complementary colours have no place in discussions about filters.



Sorry, but I don't recognize you as an authority who can determine what anyone can or can't discuss in relation to anything. 

Again, I am speaking of a rule of thumb that is well known and widely used in B&W photography to choose filters for specific tonal effects. In effect, filters are used to lighten relative tonal representations of like colors and to darken relative tonal representations of complementary colors in B&W photography.


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## Ridgeway (Sep 21, 2014)

compur said:


> This is not always true in the context of film photography. Reducing exposure "lightens" negatives and increasing exposure "darkens" them, for example. The same is true of darkroom printing of negatives. Less exposure lightens the print and more exposure darkens it. So, it is not true that "filters never lighten anything."



I think you're clutching at straws here. Whatever.


> If, for example, a photographer wishes to lighten the green leaves of a tree in relation to the brown tree bark then this can be achieved through the use of a green filter. The green filter will tend to block more of the light from the bark then from the leaves so, in the final print, the leaves will appear lighter in relation to the bark than they would if shot without the filter.



Now you're talking about contrast. Yes, filters may increase contrast but they won't 'lighten' anything without an associated increase in exposure.


> Sorry, but I don't recognize you as an authority who can determine what anyone can or can't discuss in relation to anything.


Ditto.


> I am speaking of a rule of thumb that is well known and widely used in B&W photography


Not widely used anywhere that I'm familiar with.


> to darken relative tonal representations of complementary colors



Have to refer you back to my previous example. If you can explain to me, in terms of complementary colours, why a red filter will darken a blue sky more than a yellow one (yellow being the complementary colour of blue) then I'll be happy to believe it.


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## compur (Sep 21, 2014)

If a B&W photographer wishes to lighten a certain color in relation to other colors in a scene then the way to do that is with a filter of like color and vice versa.

See the Ansel Adams books for more information.

This simple rule of thumb is not meant to be taken as some ultimate literal law of physics and your game of semantics is growing tiresome. Yes, I know what you mean and I think you know what I mean (though you seem loathe to admit it). Either that or you have no B&W darkroom experience. Or, maybe you just like to argue for argument's sake.


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## Ridgeway (Sep 22, 2014)

Yes, I know what you mean but did the OP? You gave a rule of thumb that might be misleading and I just cleaned it up for you. It actually seems to be you who are loathe to admit someone may have given a more accurate answer than yourself. You haven't questioned my answer at all but have merely spent your time trying to justify your own. Maybe you need to just accept that my answer was perhaps more accurate and move on. My only concern is that the OP gets an accurate answer to their question rather than some vague rule of thumb that we experienced photographers may understand but they may not.

And for the record, yes I have many years B&W darkroom experience though I don't see the relevance as we haven't been talking about darkroom work at all. There you go, just confusing things again.


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## compur (Sep 22, 2014)

If the OP asked you this question:

I am shooting a tree with green leaves and a brown trunk with B&W film. When I shoot the tree with no filter the leaves turn out darker than I would like in relation to the tree trunk. What color filter should I use so that the tone of the leaves are lighter in relation to the trunk in the final print?

What would your answer be?


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## timor (Sep 22, 2014)

compur said:


> If the OP asked you this question:
> 
> I am shooting a tree with green leaves and a brown trunk with B&W film. When I shoot the tree with no filter the leaves turn out darker than I would like in relation to the tree trunk. What color filter should I use so that the tone of the leaves are lighter in relation to the trunk in the final print?
> 
> What would your answer be?


 I would spray the leaves with near white paint, water soluble so next rain will wash it off and off course environmentally friendly so no bush is poisoned.  Probably I would have to do that before you guys resolve your dispute. 
Here, little table;


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## timor (Sep 24, 2014)

And for deeper penetration of the subject additional info about actions of filters (by Wratten number) here:
Frequently Used Filters for Black & White
Not all is, what seems to be.


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## petrochemist (Sep 24, 2014)

Timsky said:


> Thanks Snowbear. In the absence of any other information I opened up Photoshop to look at their B&W photo tool which has presets for different color filters, incl. an infrared setting. When you look at the color slider settings (reds,yellows,greens,cyans,blues,magentas), for the High Contrast Red and Infrared presets there's a disparity with the reds.   The High Contrast Red preset increases the brightness of red 120% whereas the infrared filter reduces it -50%. I know these are just an approximation to mimic the behaviour of filters on analog film but it's confusing none the less.



Photoshop filter tools are very different to actual physical filters.
The 'infra red' filter is trying to mimic the results you would get shooting with infra red film. 
Like all such fake infra red treatments, It doesn't do it very well. Not surprising as there is NO information in a standard shot showing the infrared reflectance of objects. I suspect it's main function is to assume things that are green are foliage and to brighten them (Healthy foliage reflects very high amounts of infra red). Items painted the same shade of green as a tree will often be difficult to pick out in a visible light photo yet very obvious in infra red. What it's trying to do is similar to estimating the red channel by looking at the blue and green channels.

FWIW many visible light filters transmit significant infra red (including a blue filter I got recently) which can give interesting effects if the camera has been modified to record infra red.


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## timor (Sep 24, 2014)

Quick check by experiment: Rollei Retro 80S and red or even orange filter and voila.


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