IR Pass Filters

[Dave Liquorice]

Playing with a cheapo LED IR illuminator and webcam sensitive to IR. Works well enough but the illuminator at night is a very visible red. I've played with a bit of old floppy disc, that cuts the visible red completely when a handful of yards from the illuminator. Trouble is it also reduces the IR substantially as well.

You can get screw fitting IR pass filters for cameras(*) costing from £30+ to under £10 and various start pass wavelengths. Has anyone any

experience with the cheaper end filters? Is the transition from stop to pass fairly sharp or spread out? Optical precision is not a requirement. A

4" square bit of Lee 87 polyester filter can be had for £15 that has a published spec but is a

[John Rumm]

Might be worth trying a bit of exposed slide film... looks black the visible wavelengths, but lets some IR through.

(I use it placed over the built in flash on the camera when I don't want any direct flash illumination, but need to optically trigger a slave flash)

[Peter Parry]

Kodak Wratten used to do gel filters to go on floodlights to convert them to IR, a 500W halogen could barely be seen behind one. The optical quality was very high and I used one filter and a pair of scissors to make a number of camera filters. No idea what the frequency band they passed was but they worked with IR film. I've also used LEE Poly (polyester) filters in a Cokin mount (and black bodge tape to keep the light out) . Apart from the fact they are quite delicate (expensive sweet wrapper) they were fine. A friend put them between two UV filters to create a more robust device, that worked on most lenses but double banking the filters did produce some masking at wide angles.

[Capitol]

I'd expect the IR to be close to a single wavelength, hence filtering is unlikely to be very successful. The normal spectral bandwidth is between 20 and 50nM AIUI for infra red diodes.

[Dave Liquorice]

Exposed and developed or just exposed? What ever I don't have any to hand. Would the dark bit you sometimes get at the begining of a set of negs work?

[Dave Liquorice]

Still do but have you seen the price? £130+ for a 3" square bit! The IR Illuminator only cost a tenner and could have got one for about £7 if I was willing to wait for it to come from China...

I wonder if I'm looking at the same stuff?

They are a more reasonable price at about £17 for a 4" square.

Been playing a bit in the meantime, plain white paper isn't bad, lets through a fair bit of IR and diffuses the red glow. Hadn't any black paper to try. Remembered I had some lighting gels, a range of ND and some colours for making anaglyph 3D glasses. They all transmit IR, hardly surprising, stuck in front of a 2kW tungsten bubble they'd melt even quicker if they absorbed IR...

With rather more reduction in IR than I'd like I've got the red glow down to below being really noticeable, that's with 4 layers of 0.9 ND and 7 of a blue, might be 132 Medium Blue. Of course it's a nearly full moon and clear tonight so the camera is doing pretty well with available light.

The IR is for when it's dark and stormy so people can see how full the road is getting with snow...

[Dave Liquorice]

Difficult to tell with the lighting gels if the visible reduction is the same as the IR, I don't tink it is.

That's sort of the idea behind getting a filter that transitions fairly quickly as close to the bottom of the visible red as possible. I'm not sure where the bottom of the visible red is, the 'net produces a range from 650 to 750 nm. It'll obviously vary by individual but 100 nm is at least 1/4 of the whole visible range...

[John Rumm]

Exposed and developed...

An unexposed and developed section of negative might - although that is not as "black" as an exposed and developed section of colour transparency film. If you have a box of slides produced commercially, you often get one or two black slides from the ends of the roll.

[Richard]

Might be worth talking to these guys:

[Vortex11]

Under a tenner on eBay. You just need to be patient with dellivery from China.

[Martin Brown]

If you still have some lying around unexposed slide film tails is as good as anything. It even works with human eyes if you make a good seal and allow about two minutes to dark adapt. The scene reappears in pseudo false colour with live plant foliage very light coloured.

I'd go for 780nm if you want to be reasonably sure of eliminating obvious visibility. Even then some might see it. Schott RG780

These are colloidal glasses with quite sharp low pass.

715nm definitely will not cut it. Wratten87 should be OK.

[Brian-Gaff]

They seem to be able to make remote controls cheaply enough, where do they get the stuff from? Brian

[The Natural Philosopher]

decent IR LEDS.

The OP obviously has LEDS that are at the bottom end of visible, not truly in the IR spectrum

[Peter Parry]

IR trailcams use one of two types of LED emitter, the 850nm LED emits a noticeable dull red glow but have good range, the 940nm emit no visible light at all but tend to have a shorter illumination range.

940 are used in covert illumination systems and IR remote controls.

[Martin Brown]

No you really want unexposed slide film developed end tails. Or modern dye based negative film exposed to light and then developed.

I suspect the latter is not as dense.

[Martin Brown]

You can just about see the dim emission from nominal 750nm IR LEDs when dark adapted. I suspect their FWHM is +/-10nm.

Never seen even a hint from 820 or 850nm ones.

[John Rumm]

In fact, swap those around, ;-)

Slide film will be black when totally unexposed and developed, and negative black when fully exposed and developed.

[Peter Parry]

The ones in the trailcam I have are 850nm and clearly visible as a faint red light in the dark. The image here

is pretty close to how they appear when viewed in the dark.

[Adam Aglionby]

Security cams usually other way, solenoid operated IR CUT filter drops in during daylight and gets pulled out at night allowing illuminators to work.

Visble light generally thought of as 400-700nm.

[Dave Liquorice]

The brighter spots are how the LEDs on this cheapo illuminator appear in room lighting... At night from 20m away it's very obvious. Don't know what the LEDs are, LEDs don't have a part No. stamped on them.

Put it back tonight and an awful lot of the IR has been filtered but there is just enough to light things. On the other hand I've probably over done the filtering, I could hardly see the red glow and I know where to look! Also there is a lot of back scatter from the gels so losses are probably higher than a single filter.