Reading stouffer step wedge with densitometer - which side?

Discussion in 'Darkroom Equipment' started by Usagi, Oct 31, 2009.

  1. Usagi

    Usagi Member

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    Normally you need to read density of the film from the emulsion side.

    Now I need to read densities of my uncalibrated stouffer step wedge, but from which side? I assume that the dull side of the wedge is emulsion and the more glossy is the base.

    But by following that assumption I got clearly too low readings, the glossy side gave almost dead on readings (which is good as the uncalibrated wedge has tolerance of 0,10 density near the dense end).

    Now I am a bit confused. Which side is which :confused:
     
  2. Mike Wilde

    Mike Wilde Member

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    I am guessing we are talking transmission measurements of the step wedge. I have never considered which side up. I think I have done it with text readable, which I think is base side up, the same way I would project a negative.
     
  3. Photo Engineer

    Photo Engineer Subscriber

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    With a good densitometer, the readings should be pretty much symmetrical.

    PE
     
  4. Usagi

    Usagi Member

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    Perhaps I should not call my old MacBeth to good densitometer :wink: Big, heavy, ugly, old, noisy, ...

    With normal film there's small difference with readings so I have used emulsion side reading.
    Now as I realized that readings from step wedge are more accurate from base side (glossy side), then perhaps I should read film too from base side?

    Althought difference does not have any practical meaning...


    Here's result from stouffer TP4x5-21 (text version is as attachment)

    [​IMG]
     

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  5. Dan Henderson

    Dan Henderson Member

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    I am interested in knowing why there should be a difference since the measurement is taken through the film. My first guess was that more light gets scattered off of the shiny base side instead going through the film to be read. But the OPs table seems to refute that hypothesis.
     
  6. Denis K

    Denis K Member

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    Warning: this is only my theory.

    Another theory is that when the beam or source light hit the grain structure of the emulsion on the film it gets scattered. The density distribution of the scattered light falls off as the angle away from the normal gets larger. Remember, the attenuated light is now heading for a small aperture leading in the the optical detector. The further the light travels from the point of scattering, the wider the scattered beam will be, causing less to go through the fixed aperture. When less light enters the aperture it reads as increased attenuation, thus higher density. If you make a reading with the emulsion next to the fixed aperture the scattering distance is reduced and more light enters the aperture thus reading as reduced attenuation, resulting in a lower density.

    If the above is true, it begs the question. Which is more correct? For photography, I would say the emulsion side reading would be the better reading because an enlarger lens will focus all of the scattered sight to the correct place on the printing paper and thus should be reflected in the density reading. In my opinion, only the light that is absorbed by the emulsion or gets scattered backwards should add to the density reading.



    Denis K
     

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  7. Photo Engineer

    Photo Engineer Subscriber

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    Well, due to light scatter, matte papers have lower dmax than glossy papers. The same can be said of film, but in the case of film I would not expect such a difference because you have a source of light and a detector and the path is straight through. With paper, the source and detector are on the same side, and therefore the multiple angles of return can affect density.

    Here, we may see an old instrument which is reflecting some light back to the source depending on orientation and due to slight misalignment. OTOH, the instrument may need a slope adjustment for Dmax and Dmin and the problem will go away. Slope just means recalibrating both Dmin and Dmax to equal a certain predetermined value. In this case, you would have to know the factory "settings" for the step wedge produced by Stouffer.

    At EK, we had ceramic blocks that were calibrated for reflection density, and we had cast carbon blocks for transmission density. These were precisely measured and allowed us to preset slope and calibrate the entire density range.

    OTOH again, we read with emulsion towards the light source IIRC.

    PE
     
  8. Kirk Keyes

    Kirk Keyes Member

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    Keep in mind that there are 4 types of transmission densitometry measurements that can be made:
    diffuse light source|diffuse sensor
    specular light source|diffuse sensor
    diffuse light source|specular sensor
    specular light source|specular sensor

    As the light source or sensor or the light source moves away from the film, they become less diffuse and more specular.

    That said, which is right? Well, neither, really. It all depends on what one needs/wants.

    In general, most film densitometers are diffuse|diffuse in design. A microdensitometer could have either a diffuse or specular light source, but it will need to have a specular sensor.
     
  9. Usagi

    Usagi Member

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    As I got more accurate readings from step wedge when read with emulsion towards the light source, I guess that it's best to standardize my measurements for that and read all films always that way.

    I have calibration patches and they give indentical reading regardless of side. But theyre not film based.