What is the "black sky" when using IR film?

[flaviosganzerla]

This may be a very stupid question but... what am I recording in the

sky with IR film? The sky itself isn?t, because it reflects no IR

light, right? Is this the space? Or nothing?

[anupam]

It should record as black. Just as if you photograph a red object with orthochromatic film that isn't very sensitive to red, it records as a very dark shade.

[grinder]

Flavio, You are correct the sky is not reflecting IR and you are capturing alot of IR so the sky has no light as the film is concerned so it does not expose or does so minimaly. The clouds if any are present are higly relective so the turn almost pure white heavily exposed. ,Grinder

[patrick_gainer]

It is black because the sky is blue. I am presuming you are speaking of prints made from those negative.

[stephen hazelton]

The black on your print simply means there wasn't much light on that part of the negative. You're not photographing darkness, you're failing to pick up any light.

[william_john_smith]

Since infrared emulsions are sensitive to the blue region of the spectrum as well as to part of

the red and to the near infrared regions, filters are needed for infrared recording. The sky

will not become black unless you use a filter, at the very least a Wratten#25. A #87 will make

it very dark.

[flaviosganzerla]

Sorry if I haven?t made it clear, it may be my bad English. When we use a infrared filter with an infrared film we record only infrared light. Since in the sky there is no IR light, can I say I am photographing the space (not the sky) and the Earth with this combination of film and filter? As analogy, would it be like seeing trought the clothes of a person with the filter on the lens? In this case clothes = sky.

Is it well explained now? Thanks!

[doremus_scudder1]

Flavio, Your film is not "seeing" space, it is seeing nothing. IR film is "blind" to the light in the sky (when you use the IR filter you show the film only IR light, there is none in the sky). When you close your eyes, you see black, because no light reaches your eyes. The same is the case for your film, it simply cannot see the sky.

[william_john_smith]

In addition to the light which we can see, there also exists invisible electromagnetic radiation

similar to the visible radiation called light. Invisible radiation manifests itself at both ends of

the visible spectrum. Beyond the violet is radiation which is called the ultraviolet, and which

is of relatively short wavelength. It is invisible but has strong action on photographic

materials, making it easy to detect by photographic means. At the other end of the

spectrum, at wavelengths longer than the red, there exist the infrared, meaning "below the

red". This is what infrared film, with the proper filter, records.<P>Just because we don't see

it doesn't mean it doesn't exist.

[lex_jenkins]

I usually try to avoid any mention of the dreaded words "color" and "digital" on this forum because it often leads to trouble, but this will help give an approximation of how color filters affect monochrome images...

 

Download a copy of the freebie image viewer/editor Picasa. It includes an effect called "Filtered B&W". You can get the same effect in almost any image editor, but this one's free and produces the same effect as using the dropper tool in Photoshop or Elements.

 

The first example is in color. That's Jay. He works at Arlington Camera in Arlington, Texas - great shop. We met something like 15 years ago while doing a theatre play, "The Hot L Baltimore". That particular performance was too jokey for the author's intentions (I was among the guiltiest, hamming it up over the top), but it was fun and fed my continuing passion for live theatre. Jay was terrific, too - he had the right attitude of bleak humor for the role.<div></div>

[lex_jenkins]

Conversion to monochrome. Blue filter. Note the dramatic effect on the blue shirt and skin tones. It's as inaccurate in tonality as infrared film. While IR can be useful for portraiture, a heavy blue filter seldom is. However it might be useful for certain other special effects with b&w film.<div></div>

[lex_jenkins]

Red filter. Equally inaccurate and rather unpleasant, especially the skin tones. This is why a red filter with ordinary panchromatic film seldom works well for portraiture, unless you're photographing grizzled mountain men.<div></div>

[lex_jenkins]

And, just for giggles, a green filter. Some of us prefer a green filter to yellow for portraiture. It works well with some skin tones, enhancing rather than distorting them.<div></div>

[william_john_smith]

Thats nice Lex but the topic is black sky and infrared film. Plus those images are a poor

excuse for IR, haven't seen a digital conversation yet that comes close to film IR. The best

way to see what IR looks like is shoot the film with the different filters.

[pgwerner]

As William said, the black skies in infrared photography are from the filter, not from the

film. Specifically, its because IR filters are an extremely thick red filter and red is exactly

on the opposite side of the color wheel from cyan (the color of a clear sky, or water that

reflects the sky), hence it blocks cyan. Even a standard Red #25 filter will darken skies

considerably and IR filters will have even a more pronounced effect. I suppose the fact that

the sky and bodies of water are more absorbers than reflectors of near-IR radiation may

also play a role in the blackness of those objects in IR photography. - Peter

[pgwerner]

I forgot to mention this webpage in my previous reply:

 

http://www.pauck.de/marco/photo/infrared/comparison_of_films/

comparison_of_films.html

 

It shows how different filters interact with IR film.

 

Peter

[raczoliver]

Lex, you seem to have attached the wrong pictures for the "red filter" and the "blue filter", if I am not mistaken. If I am, please correct me.

 

Flavio, the black thing you get on your photographs is not the space, it is the sky. What is happening is simply that IR film is sensitive to a different range of colors than panchromatic (what we normally use), and one thing it is not sensitive to is blue. A black thing absorbs most of the visible spectrum of light, hence very few light is reflected from it onto the film. IR films are not sensitive to the full visible spectrum of light. They "see" different things than the human eye. They see some colors that the human eye cannot see, but the human eye also sees some colors that IR film cannot see, namely blue.

[johndc]

I believe the order of those image with regards to filtering is: <BR>

 

B&W Image #1 - Red filter (err channel, mmrhmm) <BR>

B&W Image #2 - Blue filter <BR>

B&W Image #3 - Green filter <BR>

<P>

Notice the roughness of the skin with the blue "filter". Personally, I like that kind of effect sometimes, particularly on subjects with darker skin.

<P>

With regards to the original topic, what others have said is absolutely correct. You are not seeing outer space with the IR film (damn that would be seriously friggin' cool). You are, instead, not seeing anything because there is nothing there to see. With "normal" film, something appears black because it is not reflecting or emitting any visible light. With IR film, something appears black because it is not reflecting (or emitting) any infrered radiation.

The sky does not reflect or emit any IR radiation, so it appears black.

[william_john_smith]

Say what? White light from the Sun is dispersed as it travels through the sky into a spectrum, whithin the spectrum is IR and IV. Think of the sky as a giant prism. Read what I wrote re: invisible electromagnetic radiation. That is why you use a red filter, to block IR and the visible blue (IV) of the sky.<P>BTW, you don't need IR film and filters to get a black sky. For a fine example of conventional black and white film and a filter to darken the sky see Ansel Adams "Moonrise, Hernandez, New Mexico, 1941". This image was taken using a #15 Green filter. <P>Bottom line it is a filter that is doing the work creating a black sky.

[jonas_gustavsson]

I really still don't see the answer to the question as I interpret it: "Once we take out the Rayleigh scattering of a clear blue sky, will we be able to see beyond the atmosphere (in the IR, naturally) just like at night?" I would say that the answer must be yes and that we should be able to see e g stars and planets emitting or reflecting IR radiation out there. This sounds like a fun experiment, but aren't IR films pretty grainy, esp in the IR range and most celestial objects will still have stronger radiation in the visible range, meaning that maybe scattering from atmospheric particles may still swamp any celestial sources even if everything <750 nm or whatever is taken out.

[jordan_w.]

Jonas, your suggested experiment is pretty interesting. Ultimately I don't think it'd work for the simple reason that although the sky scatters mostly short visible wavelengths (i.e. blue) from the sun, there's still some IR scatter going on. It's small compared to the blue scatter, but I think it'd likely swamp any IR from starlight by orders of magnitude. Cool idea, though.

[stephen hazelton]

I think I see what you're getting at now.

 

The air scatters blue light around, and makes the sky appear blue. However, starlight should be shining through just as much in the daytime as in the night. The blue scattering doesn't diminish the starlight, just totally overpowers it.

 

I have never seen an IR photo of the daytime sky that showed any stars. Of course, generally, IR photos show sky, but they show lots of non-sky, too, and if exposed for that non-sky, then any starlight is too dim to be seen.

 

It might make an interesting test to see how long of an exposure you could make with the dark sky, and whether you could detect stars. I'm guessing not.

[lex_jenkins]

Right you are, folks. I mislabeled the "red" and "blue" filtered photos. Thanks for the heads up. I can't relabel them (moderator's access is limited in handling most photos) so, hopefully, folks will take note of your correct info.

 

William, my illustrations are applicable at a general level for panchromatic films and can help some folks to visualize the effects of filters on b&w films. And it's relevant to this thread because red has pretty much the same effect on blue whether it's panchromatic or IR film or a scan or digital capture. The main way in which a simplistic digitally created example fails to mimic IR is in representation of greens. Everything else is roughly comparable in color-to-tone representation. So, yeh, this is relevant.

[flaviosganzerla]

I think that Jonas, Jordan and Stephen got what I was trying to ask. If someone have anything to say, please do it.

[johndc]

<i>"That is why you use a red filter, to block IR and the visible blue (IV) of the sky."</i>

<P>

Really? So you use a red filter to block the IR when using IR film? Other than the rush you get from paying $12/roll, what, then, is the point of using IR-sensitive film?

<P>

<i>"For a fine example of conventional black and white film and a filter to darken the sky see Ansel Adams "Moonrise, Hernandez, New Mexico, 1941". This image was taken using a #15 Green filter."</i>

<P>

I think the fact that the picture was taken in the evening has alot more to do with the dark sky than the filter.