IR blocking filter

BetterSense · Aug 27, 2009

I built a spotmeter and it totally works, but it's too IR sensitive. I knew the absorption spectrum of the silicon sensor was biased toward IR but I figured it might work anyway since lots cameras use Si sensors, plus I hoped that my glass lens would absorb some IR and of course it will be OOF on the measurement plane compared to visible light. However grass makes the sensor respond much more than a sheet of white paper even though the paper is much whiter. So clearly I have terminal IR problems at this point.

The only solution I can think of is to get some kind of filter that will filter out IR and pass visible light, but I don't know what material to use. Most common "IR filters" are designed to pass IR and block visible, such as for remote controls and stuff.

How do cameras that use silicon photosensors get around this problem?

Robert Kerwin · Aug 27, 2009

Do a search on the B&H website for "hot mirror," you should get a number of IR blocking filters in various sizes. Another possibility would be to salvage the filter in front of the sensor from a dead digital camera.

BetterSense · Aug 27, 2009

The digital camera idea sounds like a good one. It should be just about the right kind of filter after all.

Ed Sukach · Aug 27, 2009

BetterSense said:
I built a spotmeter and it totally works, but it's too IR sensitive. I knew the absorption spectrum of the silicon sensor was biased toward IR but I figured it might work anyway since lots cameras use Si sensors, plus I hoped that my glass lens would absorb some IR and of course it will be OOF on the measurement plane compared to visible light. However grass makes the sensor respond much more than a sheet of white paper even though the paper is much whiter. So clearly I have terminal IR problems at this point.

Not clear. It may be that the IR refected from a "white" target may, or may not, be different than that from a green target. The central frequency of "white" (~ ...oh squint your eyes - more or less) is probably somewhere close to the central frequency of "green"... and a measurement of the radiated eneregy would indicate overall brightness - rather than energy at a specific (or random) blend of color.

Try the same with a RED and a BLUE card. Still rough, but possibly closer - there really is no way to tell how much IR is being tossed around.

Run-of-the mill window glasses do not have much of an effect on Infra Red radiation. The other end of the spectrum - Ultra Violet - IS absorbed rather efficiently - that is why it is difficult to get a sun tan sitting in front of an ordinary window. Other materails, plasics, quartzes are used in te manufacture of UV lamps.

Your best bet would be to visit the fiter manufacturer's catalogs - B&W in particular publishes spectral transmission graphs of their filters, and choose from there.

BetterSense · Aug 27, 2009

Not clear. It may be that the IR refected from a "white" target may, or may not, be different than that from a green target.

I don't think it was that the grass was green...but that it was grass. Vegetation always reflects lots of even near-IR; notice how it is aways white in IR photographs.

I've also verified this effect by shooting an electrical service box and the parking lot nearby...both of which look identically "bright" and register the same exposure settings on commercial lightmeters and TTL camera meters...but the parking lot was several stops brighter than the electrical box on my homemade meter.

My hand is also apparently "brighter" than an open notebook, even though the white notebook paper is lighter for exposure purposes.

domaz · Aug 27, 2009

Just curious- what sensor did you use? I've been thinking of making a meter as well using the Dead Link Removed.

BetterSense · Aug 27, 2009

That's what I used. I placed it at the focal plane of a simple glass lens. At f/2 it oscillates at a couple hundred Hz at EV_100=0. A 50mm focal length gives a quite narrow spot and a 25mm focal length gives a 'spotty enough' spot, which is what I have now because my case is small. I use at Atmega168 microcontroller and a 9v battery.

Mr Bill · Aug 27, 2009

Cool project, for fun at least!

I don't know too much about the electronics, but have some ideas about the other parts. I'm sure you have an IR issue, simply because you didn't use a filter. If trying to scavenge a filter, not only digital cameras, but any color video camera should also have an IR cut filter in it. Although it's probably part of a blur filter package, you can probably just use the whole thing without a problem.

In the meantime, here's another idea. If you have an IR-pass filter (visually opaque), you could take two readings, with and without filter. The difference between "everything" and "IR only" = visible light. You don't have such a filter? What about several layers of processed leader (opaque) from color neg film? For some reason, the dyes used in such film (or E6 slide film) don't seem to be able to block into the IR range, so they would probably serve as decent IR pass filters.
If you wanted to check on this, first see if you can read the output of an IR remote control. If you are able to do so, then block it with the dark color neg film and read again. I doubt that the black film will affect the reading, at least more than about 10% or so.

For another area to play with, I think you'll find that both fluorescent lights and LED flashlights have virtually all of their output in the visible range. If so, the black color neg film should stop virtually all their light (with respect to what your meter can see).

Maybe there's potential application for the people shooting IR film; it seems to be something of a problem knowing how much exposure to use.

Ed Sukach · Aug 27, 2009

BetterSense said:
I don't think it was that the grass was green...but that it was grass. Vegetation always reflects lots of even near-IR; notice how it is aways white in IR photographs.

Not NECESSARILY true. The brightness recorded would depend - a whole lot - on the amount of IR radiation reaching and reflected from the subject.... and there is NO way to determine that without a using an IR sensitive 'meter' of some sort.
"Both LOOK ... ". Not relevant. By definiton you can NOT see IR - that is what makes it "Infra".

Do you have information about the sensitivity (@ given wavelength/s) of your IR cell?
If you do, you might be able to select a (~) standard filter/s that would enable you to match the sensitivity of your meter to that of the film you intend to use.

polyglot · Aug 27, 2009

Can I suggest that you make the hot-mirror removable from your meter? If you can have the option of installing either a hot mirror or a low-pass (eg R72) filter, you can use this as an IR meter for shooting near-IR film. No more guesswork and bracketing!

BetterSense · Aug 27, 2009

Cool project, for fun at least!

Fun??!! I've resoldered and traced point-to-point 22ga solid core wire hundreds of times and written 700 lines of C. I'm doing this because I need a spot meter!

In the meantime, here's another idea. If you have an IR-pass filter (visually opaque), you could take two readings, with and without filter. The difference between "everything" and "IR only" = visible light.

That would work and be a good diagnostic, but it wouldn't be acceptable for the meter to actually work that way and have to take 2 readings or use 2 chips. I'm aiming to have a usable meter here; I already have EV, aperture priority, shutter priority, and film speed adjustments coded and printing on a 2x16 character LCD panel. I don't have any fancy averaging routines, but the meter lets you shoot the target and cycle through applicable camera settings

"Both LOOK ... ". Not relevant.

Of course it's relevant. If two things appear to be the same tone but one of them measures several stops brighter then you know something is up. Especially when it's something known to emit lots of IR (grass).

Do you have information about the sensitivity (@ given wavelength/s) of your IR cell?

Yes, and using a complementary colored filter to "even out" the red-leaning response would be a good idea, but I don't know where to find color gels or information on their transmission spectra.

In the meantime I dissected my wife's old cell phone and pry'd the IR filter off the CCD. Unfortunately I broke it getting it out, but the biggest piece barely covers the sensitive area on my light sensor. I'll have to wait till tomorrow to recalibrate and see if the IR anomolies are still present. Surprisingly the IR filter sucked down almost a stop under mixed CFL/incandescent interior lighting.

Ed Sukach · Aug 27, 2009

OK -

I visited the "Sparkfun" website and found information in TSL230R-LF-e3.pdf. A quick (actually a little more than that) viewing of the included "Photodiode Spectral Responsivity" chart indicates sensitity close to the "Extended Red" IR films we have today - starting at 300nm and ending ~ 1100nm, with peak at 700nm. That is close enough (from memory of film sensitivity) for use without filtration.
As a matter of fact, I can't think of a filter MILD enough to improve the "matching" - at all. Even a protective pice of glass MIGHT detract from th operation of this sensor.

With this in mind, I would trust information from this sensor - MUCH MORE than I would put faith in my own perception - remember, IR can NOT be seen.

Now ... "Sparkfun". What a web site!!! An Electrical/ Microprocessing Geek could overload here!!
I thought I was into some pretty "heavy" stuff in the Companies I have worked for - but these guys....

Great Googly Moogly ...!!!

Ed Sukach · Aug 27, 2009

BetterSense said:
Of course it's relevant. If two things appear to be the same tone but one of them measures several stops brighter then you know something is up. Especially when it's something known to emit lots of IR (grass).

I do NOT want to start a battle here, I'll only TRY to repeat, once more:

I will NOT accept, as some sort of proof, assumptions about IR radiation (Infra = "unperceivable") based on VISUAL PERCEPTION.

"Something known ..." (deleted).

Grass doesn't "emit", anyway. It only reflects... and if there is not much IR to start with, not much will be reflected.

Yes, and using a complementary colored filter to "even out" the red-leaning response would be a good idea, but I don't know where to find color gels or information on their transmission spectra.

What purposee would this "evening out" serve? You are close to Extended Red IR film.... "Evening out" would only be useful if you are to use conventional black and white, or color film.

As I've said, try the Tiffen or B&W (preferred) catalogs.

BetterSense · Aug 27, 2009

With this in mind, I would trust information from this sensor - MUCH MORE than I would put faith in my own perception - remember, IR can NOT be seen.

It's not my perception. I have cameras with TTL meters that I have compared it with. Under multiple circumstances I have verified that the silicon sensor is just wrong, because of how it departs radically from what other exposure meters (which have proved to be working and accurate) indicate, for certain targets which are known to emit (reflect, whatever) IR well.

Like I said in my original post, by reading the TLR2030 datasheet, I didn't think spectral response would end up being a problem, but it clearly is. I will consider the problem solved when the TLR2030's output tracks reasonably well with other exposure meters that are proven accurate, instead of departing wildly for certain targets.

BetterSense · Aug 27, 2009

What purposee would this "evening out" serve?

It would serve the purpose of giving the meter a spectral response at least approximately the same as the film I am shooting.

You are close to Extended Red IR film.... "Evening out" would only be useful if you are to use conventional black and white, or color film.

I don't care about IR film. I have never stated that I intend this meter to work for IR film. I don't shoot IR film or plan to in the future. I shoot TriX and Neopan 400.

Mr Bill · Aug 28, 2009

Fun??!! I've resoldered and traced point-to-point 22ga solid core wire hundreds of times and written 700 lines of C. I'm doing this because I need a spot meter!

Ok, sorry. I didn't think it could be a serious attempt since you started out, with Si sensor and hoping you didn't need an IR filter. Here's why you need one: human vision covers roughly from 400 to 700 nm wavelength, and Tri-X film falls off a bit sooner on the red end, at about 650 nm. So anything your sensor can see towards IR, from 650 or 700 on, MIGHT be problematic.

If every light source you used for photography had the same spectral signature, this would be ok. However, if you sometimes want to meter in daylight, and sometimes under tungsten light, big problems! Compared to a nominal 5000K daylight source, a tungsten source, at nominal 2800 is drasticially deficient in blue (yet still loaded with IR). So if your meter can see some IR, and is calibrated in daylight, it would barely notice the missing blue of tungsten light. Tri-X, on the other hand, would suffer greatly from this loss. So the result is properly exposed Tri-X under daylight conditions, but heavy underexposure under tungsten lighting. Of course, the issue is more complicated, but I hope this is a convincing argument.

For serious work, I think you need to know more about your combined spectral response. However, if you can match a commercial meter under both tungsten and daylight, you might be ok. Otherwise, various "heat absorbing glasses", such as Schott KG-5(Edmund Optics cat# K49-092), might be players.

Don't overlook the need for light baffles in your optical path; flare light can really distort the readings.

Ed Sukach · Aug 28, 2009

BetterSense said:
It would serve the purpose of giving the meter a spectral response at least approximately the same as the film I am shooting.
I don't care about IR film. I have never stated that I intend this meter to work for IR film. I don't shoot IR film or plan to in the future. I shoot TriX and Neopan 400.

I stand corrected. There has been interest here on APUG concerning the construction of a device for measuring the instensity of the spectral mix connected to IR film ... and I was drawn into a conclusion "jump". My apologies.

There is a semantic problem here, or close by: An "IR Filter" could be taken as a "Filter for Use with IR Film", OR a "Filter that removes IR".
B&W (Schneider) seems to recognize the problem, and differentiates between the two by adding "CUT" to the description of ones removing IR.

Now ...

You might be interested in B&W's "Item Stock Code 65-040761 - 30.5 E IR CUT (489) , found at
[ http://www.schneideroptics.com/Ecommerce/catalogitemDetail.aspx?CID=704&IID=5306 ],

or browse through the "IR CUT" filters, 486. 489, etc.

Hope this help!

BetterSense · Aug 28, 2009

I stand corrected. There has been interest here on APUG concerning the construction of a device for measuring the instensity of the spectral mix connected to IR film ... and I was drawn into a conclusion "jump". My apologies.

No problem; I should have been clearer. If I was interested in making a light meter for IR film, it looks like I'd be on the right track with this sensor, depending on the film used.

There is a semantic problem here, or close by: An "IR Filter" could be taken as a "Filter for Use with IR Film", OR a "Filter that removes IR".

I know; that makes it really hard to google, because mostly you get "IR pass filters".

I'm 90% sure a digital camera sensor-filter is going to work. I checked it on the electrical box this morning and it agreed with the camera this time, although the sun wasn't totally up yet. Plus, it's free. It's not worth spending very much on a homemade light meter before you would be better off buying one. So far my total parts costs are about $35.

IR blocking filter

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