Sensitometry with the Tobias Associates Wejex Model R

sanking said:

I may need to qualify my comments about accuracy. [...] In other words, an exposure of 0.1 units should always produce the same amount of light on the film, subject to the precision of the hardware and software.

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Since your exposure times also fall in the several second range, 2.5 seconds is probably a good compromise.

But I would suggest turning the sensitometer lamp on and making some measurments of the light on the film plane with your light meter. (I'm hoping you have a digital one that reads 0.1 EV units.) Make a bunch of readings and see if the readings change any. Any change of more than 0.1 EV would indicate that you may want to make the integrator mods. 0.01 EV precision would be nice, but I suspect that 0.1 would suffice just as well, especially considering all the other variables that you mentioned above.

If the readings do not change, then I think you can be confident that the light output of the Wejex is controlled well enough that you don't need to modify it for the light integrator. Could save a lot of time messing around with it...

jdef said:

Okay, I metered the low density step of my scale at EV 0, which according to my meter's conversion chart equates to .5 footcandles, or 5.5 lux ca., whatever that means. How do these equate to candle seconds?

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Jay - if you type into Google "convert 0.5 footcandles to lux" and hit enter, it will return 0.5 footcandles = 5.381955 lux. Pretty slick, huh? Once you have converted from footcandles to lux, don't worry about footcandles anymore - we're going to do everything in lux, or actually lux-seconds since that is our unit for exposure.

SO rounding that gives a reading of 5.38 lux for your low step. Your expsoure time is 2 seconds, and that gives 13.45 lux-seconds. That's the exposure for the lowest step. If you next step is 0.3 from the previous, then you can simply divide the exposure for the first step by 2 for the next step, if you don't have actual densities for the steps.

If you do have densities for each step, you have to convert your exposure units to the log of the units. So do a log(exposure in lux-seconds). Now both the exposure units and the density units are in logs and they can be added and subtracted directly.

So if your first step is 13.45 lux-seconds, then log(13.45) = 1.129. If your second step is actually 0.30 denser than step 1, then you get 1.129 - 0.300 = 0.829. As a check, if we take the antilog of 0.829: 10^0.829 we get 6.74. 6.74 * 2 = 13.48. That's the number of lux-seconds we started with (please ignore the rounding errors here).

If your second step is only 0.25 denser than step 1, then you get 1.129 - 0.250 = 0.879. If step 3 is 0.55 denser than step 1 then 1.129 - 0.55 = 0.579.
After a while, subtracting out the density of the steps from the lightest one, you will eventually get negative values for expsoure. That's fine, because we are using logs and negative numbers in logs only mean that the original value was less than 1. So some of the values will be negative for the exposure. If we got up to Step 6, and let's say the value was -0.321, well the antilog of that is 10^(-0.321) = 0.47 lux-seconds. So we aren't giving negative exposures, they are just exposures that are less than 1 lux-second.

Check out this graph for 100TMX to see where we are heading:

http://www.kodak.com/global/en/professional/support/techPubs/f4016/f009_0438ac.gif

By the way - can you tell me what meter you are using? I'm not sure about making a snout for your meter and expecting to get resonable values from it. Depends on the meter. Do you have a spot meter you can use, perhaps with close-up lenses to narrow in on each step of the wedge?

Jay,

Just curious, when you adjust the collar on the resistor to decrease or increae light output, where is it greater, with the collar up or down?

I am assuming that since the use of the sensitometer was for exposing X-Ray film I should set the light for minimum output.


Sandy

jdef said:

I'm using a Gossen Luna Pro F, which has a dome diffuser that prevents the measuring cell from lying flat on the surface of the scale, so I made a little black paper tube to isolate the measuring cell from stray light, and to restrict the measurements to the individual steps of the scale. I don't know if this method is accurate in absolute terms, but I'm encouraged by the linear readings.

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Jay, I dodn't think I would put a lot of confidence in your readings with the Luna Pro F. Even if it is the new digital one where you can read 0.1 stop readings, your readings will certainly be low due to the geometry from the paper tube shoot.

I would suggest you find someone nearby with a spot meter, hopefully a digital one for get 0.1 stop resolution. That, or bust the cover glass out like Sandy did and use your densitometer!

Kirk