Spectrophotometer as densitometer?

Discussion in 'Beyond the Basics' started by DocFrankenstein, Sep 8, 2005.

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  1. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    Being the science nerd, I want to know the density of my negatives to make sure the development time is right.

    I do not have access to a densitometer for film specifically, but microbiology labs do have a spectrophotometer I can use.

    Basically it's the same thing, but it measures light at a particular wavelength instead of the whole spectrum.

    How should I approach this? Should I measure at a particular wavelength or take an average in say 50nm increments from 200nm to 600?

    Anybody?

    EDIT: for spelling
     
  2. ShutteredEye

    ShutteredEye TPF Noob!

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    Well, folks in the ink and dye industry use them to create color profiles. They general use 10 nm increments to profile a color to match them.

    Are you going to use a uniformly exposed negative? How are you going to standardize it?
     
  3. Hertz van Rental

    Hertz van Rental TPF Noob!

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    The wavelength shouldn't make a difference for B&W - you are measuring the density of the neg and so it isn't a problem (as long as it is in the visible range).
    If you pushed me I'd say try to match the wavelength measured to the base colour of the film. If you are reading in the blue and the film base has a pinkish tinge you could get some inaccuracy.
    Colour neg will pose different problems - leave that one with me for now.
    You might also have a problem if it is a double beam SPM because of the way it works.
    Your main problem is calibration. Off the top of my head I believe the measurement units are different. You will need to get hold of a master step wedge used for sensitometry to do this.
    It's late here so I'll do a bit of reading tomorrow and see if i can come up with some more useful advice.
     
  4. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    I am not too sure how I would standardize it, since I do not have a lightmeter yet.

    1)take a grey card and the meter in my canon A1. Then I'll make a few bracketed shots of a grey card.

    2) develop them using my normal dev (agfapan in D76, for 13 minutes).

    3) Measure the negatives. Adjust development time until I have the grays in place.
     
  5. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    Thanks Hertz :)

    I expected you to chime in
     
  6. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Have you looked on Ebay at used densitometers? I was checking them out about 8 months ago, and they were going cheap. Particularly the ones that would only work for BW film. I saw several go for around $30.
     
  7. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    I just might get one from there. Thanks
     
  8. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    Do some research before you buy. If all you are using it for is BW film then there are some that are smaller and cheaper that will work fine. Some are huge, and do color analysis, and probably lots of other stuff that may not be vital to you.
     
  9. Hertz van Rental

    Hertz van Rental TPF Noob!

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    If you go looking for a densitometer, remember they come in different kinds:
    Reflection, Transmission B&W, Transmission Colour, Transmission/Reflection Colour.
    As you go up that list they get more expensive.
    Reflection densitometers are used on print material.
    Transmission does negs.
    Also remember that if you are going to do anything remotely serious with it you want a Lab version. One of the best models is the Macbeth range (they appear to be made by a number of different companies). Most will handle a 10x8. You can expect to pay $200 and upwards secondhand so think if you really need one.
    Densitometers are primarily used for checking film characteristics and for seeing how accurate your processing is. If you use one on a normal neg it just tells you the density of the bit you measure - but compared to what?
    At best you will be able to find D-max and D-min, giving you a contrast range. How does that help you?
    There are ways of using a densitometer to control processing and to give a 'compensation factor' for your method of Photography, as well as some other things. But the methods are complex and involved so I won't go into them here.

    As for the spectrophotometer. A single beam should work OK for approximate use.
    Units are not a problem as you use relative values (unless your machine measures in lux-seconds).
    Measuring density means you are measuring the amount of light transmitted.
    You find this by comparing the amount of light transmitted to the amount of incident light.
    Transmittance=Transmitted light/Incident light=%
    Using this method the transmittance decreases with density so it is always less than one. Much better to use the reciprocal:
    1/Transmittance=Incident light/Transmitted light=Relative value.
    The result will always be bigger than one but can go over a range of 1:10,000.
    Best:
    Density=log(1/Transmittance)=log(Incident light/Transmitted light)
    This will give you a relative value you can plot on a graph.
    All you need to do then is calibrate using a sensitometry master wedge.
    Simple ;-)
     
  10. Rob

    Rob TPF Noob!

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    or you could just guess :lol:
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Note to self: Don't accept Rob's offer to process films.
     
  12. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    I am going to be working strictly with BW. My initial goal is to standardize and find the correct development time in D76.

    I'd probably then compare the densities with the "tonal response curves"? agfa provides for their films and see if it's worth using Agfa developer over D76 or maybe other developer.

    By the time I'm done, Agfa will be discontinued and I'll have to start over again. :lmao: ;)

    What if I average the readings? What spectrum do the Macbeth densitometers use?
    We're doing exactly what you've described, except we use the word absorbance instead of density for some reason.

    Thank you for the info
     
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