Modern Panchromatic Film and IR

[Fragomeni]

Hi all. I've made a few posts touching on the use of IR in the darkroom and those conversations have opened up more questions that I think would be best dealt with in their own thread.

As a primer, I'm interested in using IR technology in the darkroom. Not at all a new idea. I am trying to figure out the details of the materials and tools that I will be using and I could use some help understanding the things that I am unable to figure out or grasp hence my posts which so far have been very helpful.

I have a pair of IR goggles on the way (Nyte Vu). From my research, I believe these use IR LEDs which output at 850nm. This is low enough that I believe the IR LEDs will produce a red glow. I have some IR LED (from RadioShack) that I've been messing with to see some differences in what can be found locally. I have an 850nm IR LED and a 940nm IR LED. The 850nm IR LED produces a much more significant red glow then the 940nm IR LED. The packaging describes the 850nm and 940nm measurements as being "wavelength at peak emission" which I take to mean is the farthest into infrared that the light reaches. Is there a way to measure the range of wavelengths emitted (low to high)? Do I need to find someone with an infrared spectrometer? Basically, I'd like to know the lowest wavelength of light emitted so that I can compare it to the data in various film datasheets defining the film's spectral sensitivity. For example, if I'm reading it correctly Ilford's datasheet for Delta 100 (Wedge Sensitivity to tungsten light (2850K)) says that the film tapers off and becomes basically insensitive around 660nm. I want to know if the lowest wavelength produced by the red glow of an 850nm IR LED is anywhere near that 660nm mark. If I'm understanding things correctly, I know that 850nm is the max wavelength of this IR LED but I do not know what the min (red glow) wavelength is and if it is approaching the film's sensitivity range.

Can anyone offer any insight into this? Thank you very much!!

[johnielvis]

hey dude, I'm thinking the cheapest and best sensor is the film itself

do a test strip exposure at the time levels you'll be using with the equipment at the ranges you'll be using...you know...Tmax is the longest you'll be developing

do--darkslide open 1" for .1 Tmax
darkslide open for 2" for .2 Tmax....etc....

then develop with another sheet that's been unexposed (or a strip) for the same amount of time...same tray,tank, whatever....then compare...see if there's any difference..

I just did a FOUR HOUR exposure in regular roomlight due to the reciprocity....a 15' exposure would have done little I could see maybe--intensity of light makes a big difference for sensitivity. particulary in a relatively insensitive area....

take you 1 hour say to do the whole thing as soon as you get your equipment

[Oren Grad]

..."wavelength at peak emission" which I take to mean is the farthest into infrared that the light reaches.

No, this is the wavelength at which the light emitted is most intense - in other words, the wavelength at which the emission curve peaks. There will be emission both below and above this wavelength.

[Fragomeni]

hey dude, I'm thinking the cheapest and best sensor is the film itself

do a test strip exposure at the time levels you'll be using with the equipment at the ranges you'll be using...you know...Tmax is the longest you'll be developing

do--darkslide open 1" for .1 Tmax
darkslide open for 2" for .2 Tmax....etc....

then develop with another sheet that's been unexposed (or a strip) for the same amount of time...same tray,tank, whatever....then compare...see if there's any difference..

I just did a FOUR HOUR exposure in regular roomlight due to the reciprocity....a 15' exposure would have done little I could see maybe--intensity of light makes a big difference for sensitivity. particulary in a relatively insensitive area....

take you 1 hour say to do the whole thing as soon as you get your equipment

Thats exactly my plan when the goggles arrive. I'll just test it basically exactly as you described. I was posting here to see if anyone had any insight ahead of time or if anyone had any extensive knowledge of the LEDs and the various wavelengths.

No, this is the wavelength at which the light emitted is most intense - in other words, the wavelength at which the emission curve peaks. There will be emission both below and above this wavelength.

Thanks for the clarification Oren. So that leaves me with the same question as before if I want to know the high to low range of wavelengths.

[Jim Michael]

It might have been Leigh in the other thread that noted that LED emission is pretty pure, e.g. if they rate it at 850 nm then it should be pretty close to that, so the curve should be pretty narrow. The exception would be where the emitted light is used to excite a phosphor layer like they do for 'white' LEDs.

Another factor I don't know if you've looked into but which someone might have touched on is the output of the LEDs you are planning to use. IR LEDs are often used for things like remote controls where the intensity may not need to be very high. The idea of using IR sources intended for security system illumination would be worth looking into.

[BetterSense]

Is there a way to measure the range of wavelengths emitted (low to high)?

Yes

Do I need to find someone with an infrared spectrometer?

Basically. Or you could just do a test with film. By the way, all Lee/Roscoe filter gels are transparent to IR, so you could stack some of those up to block the red glow.

[cyrus]

Well I was under the (mis?)impression that LEDs produce light over a very narrow (so narrow that some sources say 'single') wavelength (which is why there are no true white LEDs) but if you're seeing the red then obviously that can't be true since by definition IR is supposed to be invisible to the human eye.

This guy compared LED output from various bulbs and charted the results for comparison. Based on my eyeball calculations, it shows that each bulb's output has only a 100nm spread, from low to high (the Radio Shak 940nm LED actually peaked at 925nm.)

[John Schneider]

each bulb's output has only a 100nm spread, from low to high (the Radio Shak 940nm LED actually peaked at 925nm.)

For deep red LEDs (I can't put my hands on any IR LED spectral curves right now), the spectral half-width is 20 nm and the curve peak has a tolerance of about +/- 5nm. This based on the info put out by Philips-Luxeon. Shorter wavelength LEDs have a much greater halfwidth for some reason I don't understand.

[Peter De Smidt]

I agree with simply testing film when your goggles arrive.

Recently, I tried using Acros, a not very red sensitive "regular" bw film with a Hoya RM72 IR filter, which I believe filters out light bellow 720nm. I did get a nice IR image, complete with the Wood effect, i.e. white grass and leaves, but I had to give 15 more stops exposure than normal in bright daylight. That's a lot of light.

[David R Munson]

Recently, I tried using Acros, a not very red sensitive "regular" bw film with a Hoya RM72 IR filter, which I believe filters out light bellow 720nm. I did get a nice IR image, complete with the Wood effect, i.e. white grass and leaves, but I had to give 15 more stops exposure than normal in bright daylight. That's a lot of light.

I would really like to see your results...