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  1. #11
    BetterSense's Avatar
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    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.
    f/22 and be there.

  2. #12
    Ed Sukach's Avatar
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    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 ...!!!
    Carpe erratum!!

    Ed Sukach, FFP.

  3. #13
    Ed Sukach's Avatar
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    Quote Originally Posted by BetterSense View Post
    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.
    Carpe erratum!!

    Ed Sukach, FFP.

  4. #14
    BetterSense's Avatar
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    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.
    f/22 and be there.

  5. #15
    BetterSense's Avatar
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    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.
    f/22 and be there.

  6. #16

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    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.

  7. #17
    Ed Sukach's Avatar
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    Quote Originally Posted by BetterSense View Post
    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/Ecomm...D=704&IID=5306 ],

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

    Hope this help!
    Carpe erratum!!

    Ed Sukach, FFP.

  8. #18
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    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.
    f/22 and be there.

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