IR Cutoff filter

I've extracted what I guess is infrared filtering glass from a photocopier. It's light blue colour seem just like a dilute solution of copper sulphate, which I once saw used as an infrared filter to prevent heating of liquid helium while it's photographed. I guess it's intended to give a response closer to the eye.

Scrim

By far, the cheapest way to do this is to pick a photo diode that incorporates such a filter. What diode are you using. Perhaps someone knows of a similar one that includes the filter.

As to filter materials, Schott KG series filter glasses are made for this purpose.

BG series has more red attenuation.

photocopier.

The heat filters used on slide projectors are made of a phosphate glass. They might be the best source, if you can find an old one.

A light blue colour doesn't sound very promising, but you never know. If you can get it put on a spectrophotometer and check the absorption spectrum in the visible and near IR, that will tell you if it's any good.

Remember rubylith sheets? Charettes still sells them.

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Not sure what wavelength you really need to pass. If you are on a tight budget, colored plastic or colored glass works well. The bandpass is fairly wide, but probably not from 420 to 660 nm. It does block IR well.

Any hobbly shop will sell colored glass (sometimes sold in broken pieces for those doing stained glass windows). They also sell colored plastic for the same purpose.

Cellophane is even cheaper, but it's thin and has more loss.

Are you doing free space laser communication??

A

I have read that over-exposed films can be used as an IR filter Or use (a piece of) the reflector of 'cold' halogen lamp.

Wim

I agree with John, with this addendum: If you're trying to improve signal to noise ratio by reducing photocurrent due to ambient light, this often proves to be an elusive goal. The reason is that shot noise is proportional to the square root of bias current. So, you will typically need to reduce ambient photocurrent by more than the square of the passband transmittance of any filter. An example will make this clear. Supposing that you reduce ambient photocurrent by a factor of 2, for a shot noise reduction of about 40%. If your passband transmittance is 60%, you will not have gained anything.

Paul Mathews

I'm trying to reduce the IR response, so that the sunlight IR doesn't mask the visible light I am trying to measure.

Modulation and all that will not work, I need to know the amplitude of each Red, Green, and Blue device.

I'm using a reversed biased BPW34, and a the speeds I need to measure at, it's 50pf (or whatever the datasheet said) is wonderful, it helps me maintain 400nS rise response, to make amplitude measurements 100 nS later, of the R, G, & B sources rather easily. It only lacks a filter.

Something like the SFH2430 (?), has too much capacitance and internal resistance for the method I am using.

The BPW21 isn't bad, but it's cost of 4.55 is a bit high for me, plus it is not in a surface mount package.

I found one device from Hamamastu that might possibly work, it's marginal, I'll have to wait for it to arrive to actually test it.

Any advice is greatly welcomed, thanks.

Sorry, don't recall rubylith.

I do miss the 5 cent bottles of p>

Over exposed film is used to block visible and pass IR. I need the opposite.

Why not use the IR reflective film designed for automotive window applicaitons? Pick some up at PepBoys.

Marc

gives the spectral properties of copper sulfate solution as well as other dye solutions.

Its a red masking film sold in art supply stores. You can get a 30"x40" sheet for about $6.00 (us). Used to use it to make pcb long ago...

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Spectra in solution and spectra in glass are not necessarily the same, mainly because there could be other species present. Cu2+ is likely to absorb too much of the visible for him. If it was a pale green it would be more promising.

(snip)

Have you looked at any photo diodes that have narrow band visible filters built in like:

I doubt it would be a stable IR filter. Its chromophores aren't designed for long-term stability.

Unless I've slipped up here, strange that it should be red - kind of implies it lets that end of the spectrum through. An infra red blocker whose range extends into the visible will have a lack of red in its passed light, which makes it look blue.

Scrim

You can make a cheap optical low pass filter for testing purposes out of photographic film negative. Take some Kodak Kodacolor ASA

100 film, remove it from the roll, expose it to light for a minute or so, then wind the film into the roll again. Tell the person doing the developing that you just want negatives and that you overexposed it on purpose (you don't want them trying to lighten the negative/darken the print!) The processed color negatives can be used as a low-pass optical filter. It should stay stable for about as long as negatives last unless you hit it with a lot of light (direct sunlight or a laser). It's fragile and easy to scratch so I ususlly use two layers with the scratchable sides facing each other.

Also see:

There are many kinds of 'modulation'. The simplest, in concept, involves sampling just before a pulse, sampling during a pulse, then subtracting the first sample from the 2nd. This works wonderfully well, and it can be done with analog or digital electronics, in hardware or software. Of course, it's best to optimize the optics first. One of the easiest ways to reduce the ambient contribution is to narrow the field of view, using lenses and/or baffles, to just what you need and no more. Perhaps contrary to intuition, lensing can actually reduce ambient photocurrent if it narrows field of view (and, if your optics don't 'look' into the sun or an image of the sun).

Paul Mathews