Calibrate a transmission densitometer

[peter ramm]

You are absolutely right Leigh. Musn't mix up my transmittence and D values. D-uh.

Anyway, the general argument is that 0 D on a non-standard transmissive densitometer is actually 0 OD, and that once calibrated it becomes 0 ROD. There is no absolute scale as it is still calibrated only with reference to that specific instrument. What we want is for that instrument to give us sensitive and repeatable values, even if those values differ from some other instrument - and they will.

[Leigh]

Anyway, the general argument is that 0 D on a non-standard transmissive densitometer is actually 0 OD...l.

I don't know what a "non-standard transmissive densitometer" is.

0 D absolute is air, by definition.

It's more meaningful in the context of photographic negatives to set a relative zero at base+fog
and read deltas rather than absolute densities.

Densitometers are calibrated at a single high-density point, the exact value depending on instrument design.

The TD response is linearized by design, with perhaps internal adjustments that are set and locked at the factory.

- Leigh

[Chuck P.]

Each step is numbered, and its nominal density is printed on the sleeve.

When I purchased a 1/2" x 5" uncalibrated tablet 3 or 4 years ago, it did not have any density readings on the sleeve when it arrived. Well then, I guess someone goofed when the 1/2" x 5" one was sent. Hmmmm. I read the densities myself and wrote the readings on the sleeve, I trusted them because I knew my densitometer was calibrated from the X-Rite calibration tablet that I had.

I've sense purchased a calibrated 4x5 tablet that did have readings, and my densitometer did check out well with it.

[Leigh]

OK. That's quite possible.

My comments quite specifically addressed the Stouffer product, which is what I have.

The nominal values are printed on the storage sleeve, in a table with blanks to be filled in with the actual values.
Those blanks are blank for the uncalibrated product, and filled in for the calibrated version.

In the general case, if you buy a wedge that's advertised as being 0.1D steps or whatever,
you can pretty easily figure out what each step should be.

The accuracy of any wedge should be stated in the specs, regardless of whether it's individually calibrated or not.

- Leigh

[Shen45]

Calibration of a densitometer and checking it against a "calibrated Stouffer" are two different tasks. I have an Xrite and a Techon, both have very different requirements for actual calibration based on various internal software/hardware steps. Both units provide factory "standard" transmission and reflection patches that are read and then other steps are taken to fine tune to the known value written on the patches. Both manufcturers provide patches that are very different in density and are not suitable for use with the other machine for the purpose of calibration, however once calibrated as per the manufacturers patches and instructions both machines will provide the same reading from either a Kodak or stouffer step wedge I have. If readings are all over the place it is usually an indication that internal electronics require replacement or adjustment. Beyond most, not all people.

[peter ramm]

I don't know what a "non-standard transmissive densitometer" is.

0 D absolute is air, by definition.

Pretty close, and it doesn't matter in photography - unless you make film. Kodak used to publish some nice little pamphlets on density measurement. I'll see if I still have copies. This one is about photographic applications http://motion.kodak.com/motion/uploa...y_workbook.pdf

It's more meaningful in the context of photographic negatives to set a relative zero at base+fog
and read deltas rather than absolute densities.

I agree. However, the OP seemed to be quite anxious about absolute density deltas, as opposed to relative deltas.

Densitometers are calibrated at a single high-density point, the exact value depending on instrument design.

There are many different kinds of densitometers. Calibration grade devices (standard densitometers) use double integrating spheres for illumination/collection and yield ISO standard diffuse density values. I call them standard because we know that one will respond in the same way as another. Perhaps standardized would be a better term.

The TD response is linearized by design, with perhaps internal adjustments that are set and locked at the factory.

That works if you do not need to compare densities across machines or if you have low precision requirements. Photographers get both breaks. If you want higher precision, though, you need either a repetetive calibration to internal references or an external calibration. The step wedge is a convenient way to do the external. Never mind. Quantitative densitometry has little relevance to film photography, though there are other uses of film that are very quantitative indeed

[Leigh]

Peter,

Please put your response text outside the quote delimiters. It's impossible to follow your foregoing post.

As you said, "There are many different kinds of densitometers." Very true.

This is a photographic forum. We're discussing densitometry as used for evaluating photographic negatives.
Discussion of other applications or other types of densitometers is off-topic.

I should clarify that I'm talking about normal user-level calibration of the instrument, which is done
at the beginning of every measurement session, and perhaps every hour or two during the session.

This involves setting the zero, then setting some high value using the available density standard.
There should be panel controls to set both of these points, since they're routine operator adjustments.

I am NOT talking about instrument calibration via internal adjustments as is done at the factory or a certified cal lab.

- Leigh

[Chuck P.]

OK. That's quite possible.

My comments quite specifically addressed the Stouffer product, which is what I have.

This was a Stouffer 1/2" x 5" strip.

[Leigh]

OK. Here's what I'm talking about (current product, from the Stouffer website www.stouffer.net):



Perhaps your specific product is not available in a calibrated version, so no need for the sleeve imprint.

As I said earlier, figuring out the nominal value of each step is not rocket science.

- Leigh

[Chuck P.]

OK. Here's what I'm talking about (current product, from the Stouffer website www.stouffer.net):



Perhaps your specific product is not available in a calibrated version, so no need for the sleeve imprint.

But it is---as is shown here; there are only two products that are 21-step and 1/2" x 5", one calibrated (T2115) and one not (T2115C). The one not had no values with my purchase, I've no idea if nominal values were supposed to be shown. My uncredentialed opinion is that I only care that the value my densitometer returns is accurate to within the stated tolerance of the machine, and I do that by calibrating it as per the manufacturer's recommendation. That, so far, has been quite sufficient.