IR flash/speedlight photography. Experience, caveats, gotchas and ideas appreciated

[Helge]

I’m about to venture into (near and actual) infrared, indoor evening flash photography (Superpan 200 and Ikonta).

I bought a pack of Congo Blue and Primary Red LEE filters that I am planning to put over the flash.
I’ll probably start off with only two or three layers of the Congo Blue over the flash since I am also planning on putting a red filter over the camera to get a more general cutoff of any higher frequency light (Congo Blue lets through both UV and IR with a big cut in-between).

I’m probable going to either do off camera, remote triggered flash, or open flash where the camera is left in bulb on a tripod, and I’ll selectively flash the room.

I am of course curious to hear what kind of experience others have on the the subject?

What exposures?
How is a filtered Xenon flash to be rated?
Does multiple flashes with bulb mode on the shutter from different directions work well (careful not to aim at the lens)?
How does flashbulbs IR spectrum extent with and without blue coating (I only found up to 700 nm which looks promising, but you should never assume)?

Doing a search, one of the major and only hits is Weegees kitschy 50s IR-bulb photos.
He did it mainly to remain unseen, while I am more in it for the effect.
And also IR bulbs are impossible to find in larger quantities today.

One of the major questions is the amount of filtering of the flash that is required.
Too much and you were going to be left with too little light.
Too little and it’s not really going to be IR.

Another question is glass and acrylics. How much will it reflect and will I be able to shoot through it?

 

[ntenny]

I tried this some years ago with two layers of Congo Blue and one of Primary Red, on a flash with a guide number of 148 (feet). The filter looked the same as an 89B to my eye. After filtration, I estimated the guide number at about 16. Rollei IR film was usable; Efke IR wasn’t.

It didn’t produce super-strong IR effects, but people had a little bit of the “zombie eyes” look.

By all means try it out. I think you’re going to need to burn a roll or two on bracketed exposures to work out the filter’s effect on your particular flash.

-NT

 

[Helge]

I tried this some years ago with two layers of Congo Blue and one of Primary Red, on a flash with a guide number of 148 (feet). The filter looked the same as an 89B to my eye. After filtration, I estimated the guide number at about 16. Rollei IR film was usable; Efke IR wasn’t.

It didn’t produce super-strong IR effects, but people had a little bit of the “zombie eyes” look.

By all means try it out. I think you’re going to need to burn a roll or two on bracketed exposures to work out the filter’s effect on your particular flash.

-NT

Thanks for the answer! Some useful details and pointers there.

I’m hoping to cut most of the non deep red and IR out by using a red filter in front of the lens, instead of as a gel on the flash.
In addition to using an X-sync leaf shutter I should be able to control ambient contribution quite well.

Why not just use a real IR filter then and just fire the flash naked?
Well, I still want to retain some types of strong existing light, like bulbs, fluorescent surfaces (mainly CRTs, neon signs and paint) which I’m hoping the UV window of the Congo Blue will pump (failing that, I’ll use M-sync, in a double exposure, with an unfiltered flash, to pump the phosphors the tenth of a second before M-sync does, and use the decay phosphorescence).