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  1. #1
    BetterSense's Avatar
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    IR blocking filter

    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?
    f/22 and be there.

  2. #2
    Robert Kerwin's Avatar
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    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.
    "Photograph more, worry less"

  3. #3
    BetterSense's Avatar
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    The digital camera idea sounds like a good one. It should be just about the right kind of filter after all.
    f/22 and be there.

  4. #4
    Ed Sukach's Avatar
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    Quote Originally Posted by BetterSense View Post
    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.
    Carpe erratum!!

    Ed Sukach, FFP.

  5. #5
    BetterSense's Avatar
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    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.
    f/22 and be there.

  6. #6

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    Just curious- what sensor did you use? I've been thinking of making a meter as well using the TLR230R.
    Last edited by domaz; 08-27-2009 at 04:55 PM. Click to view previous post history. Reason: no space

  7. #7
    BetterSense's Avatar
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    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.
    f/22 and be there.

  8. #8

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    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.

  9. #9
    Ed Sukach's Avatar
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    Quote Originally Posted by BetterSense View Post
    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.
    Carpe erratum!!

    Ed Sukach, FFP.

  10. #10
    polyglot's Avatar
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    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!

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