Calibrate a transmission densitometer

[wskmosaic]

I have just purchased and am awaiting delivery of my first transmission densitometer--a Tobias TBX. All I know about it is that it comes with a power cord and will light up. I didn't pay a lot for it, but probably too much.

I own a 31-step Stouffer wedge. Where do I go from there?

I don't have a manual; I haven't the foggiest notion of what else I need to calibrate the thing or how to go about it.

I photograph with black and white film.

Any guidance or information will be appreciated.

Please don't transmit guffaws!
tnx
Warren

[Bill Burk]

There is a chance you will not need to "calibrate" it. Just use it. If your work doesn't need to be scientifically relied upon then a few points of error will not hurt much. Certainly would not hurt as much as trying to calibrate and messing it up.

Most densitometers have a button to press to zero them. Just press the head down on nothing and zero it. Then check the steps of your 31-step scale. Do not expect the steps to be exactly what the chart says. If the numbers make sense then you can just use it as-is.

Sometimes you'll get a calibrating film (or plaque) with a densitometer. If so, then you can look for steps to calibrate using the reference density.

[Leigh]

Do you expect to get different answers here than at photo.net and APUG?

- Leigh

[Sal Santamaura]

Do you expect to get different answers here than at photo.net and APUG?...Do you expect readers will respect you for behaving that way? The LFPF has no rule prohibiting someone from duplicating their own posts elsewhere. Despite significant membership overlap, there are people who don't participate at all three forums. Asking a snide question which puts down the OP for inquiring in more than one venue doesn't diminish him; it makes you appear petty.

[Ken Lee]

I just had mine calibrated by the manufacturer (X-Rite). It wasn't cheap, but it's certified accurate and they give you a calibration step-wedge with the actual numbers on it, so that you can check against a known reference in the future.

You can purchase a Stouffer step wedge - for more money - that has been checked by Stouffer, and the actual numbers are written alongside. This will help, but it won't actually calibrate your machine, and will only cover the visible portion of the spectrum. (Some densitometers measure UV)

[Leigh]

I did provide a lengthy response to his query on photo.net, including how to tell the
difference between calibrated and uncalibrated Stouffer step wedges.

While writing that response I googled the product and found that the manual was available online.
How many other people wasted their time doing the same thing, when the OP could have done it
himself and not inconvenienced others?

In the time it took him to post the same question on (at least) three different fora, he
could have downloaded the PDF manual, and had complete and accurate information.

Teaching people how to do proper research benefits everyone.

- Leigh

[Sal Santamaura]

Do you expect to get different answers here than at photo.net and APUG?...


...Teaching people how to do proper research benefits everyone...Asking a snide question is not teaching someone how to do research, proper or otherwise.

After many years here, I've concluded that actual attempts at teaching people how to do proper research is futile. With the hope of encouraging newcomers to large format photography, I sometimes spoon feed or reply to previously answered questions, but mostly remain silent. There doesn't seem to be a shortage of others who will post the same information over and over and over again. It's evident by comparing post counts to Join Dates. :)

[wskmosaic]

First, thanks to Bill Bork. That was helpful.

And otherwise, I do apologize for the trouble I mighrt have caused; I was not aware of the size of the overlap. And indeed I did get different answers on each, some more helpful than others.

I invariably do my best to help myself. It's only when I run into blocks that I ask other photographers.

Thanks, and, again, apologies.

Warren

[Sal Santamaura]

...I do apologize for the trouble I mighrt have caused...Don't apologize for causing trouble -- you didn't!

In addition to learning about densitometer calibration, I hope you also pick up some insight about searching. Google can be your friend. At the end of whatever words or phrases you need information on, simply add "site:url" to perform an advanced search. For example, to see what was previously discussed on the LFPF forum concerning the topic of this thread, you'd enter:



calibrate transmission densitometer site:largeformatphotography.info

[JBAphoto]

In practice (with an Eseco TRC-60D) I have found that a cheap uncertified Kodak nr2 step wedge works very well for neg' highlight density calibration to achieve results that print well with my enlarger in my darkroom

I suggest that this crude way of finding a starting point will work if you use the results in your own practice

For the real deal try the Wise and Wonderful Web for a pdf of the book of destructions for your own densitometer

Important - don't let your digital friends jibe you about using it to measure the density of film photographers

John

[Chuck P.]

I have just purchased and am awaiting delivery of my first transmission densitometer--a Tobias TBX. All I know about it is that it comes with a power cord and will light up. I didn't pay a lot for it, but probably too much.

I own a 31-step Stouffer wedge. Where do I go from there?

I don't have a manual; I haven't the foggiest notion of what else I need to calibrate the thing or how to go about it.

I photograph with black and white film.

Any guidance or information will be appreciated.

Please don't transmit guffaws!
tnx
Warren

You bought a densitometer, so why not know that it provides accurate results? Otherwise, why have one? I bought my X-Rite off ebay a few years ago and was able to download the owner's manual for it and learned that I could get a calibration tablet from X-Rite. Maybe you could get one for your model, you never know.

[Leigh]

Generally speaking, calibration wedges are not unique or specific to any particular product.
Any step wedge will work with any densitometer.

It's always possible that a manufacturer will want a particular product to be calibrated at specific points, usually to correct for non-linearities in the response. But the level of error achieved using a general step wedge would be pretty small.

The 31-step Stouffer wedge that the OP mentioned would work fine, even if it's the non-calibrated variety.

- Leigh

[Chuck P.]

How can you calibrate a densitometer from a non-calibrated step density? Seems that in order for a determination to be made that the unit is reading accurately and within it's tolerance, then it has to read a density of a true and known value. For my X-Rite for example, the step density on the calibration tablet that is used for calibration is 1.97-----if the reading is within +/- 0.02 density units, then I can consider it properly calibrated. If not, then I can adjust the densitometer to within that tolerance.

[Curt]

Do you expect readers will respect you for behaving that way? The LFPF has no rule prohibiting someone from duplicating their own posts elsewhere. Despite significant membership overlap, there are people who don't participate at all three forums. Asking a snide question which puts down the OP for inquiring in more than one venue doesn't diminish him; it makes you appear petty.

...and how many people go to the library and look in just one book. Sometimes it helps to get the question out there for several results.

[Helen Bach]

...and how many people go to the library and look in just one book. Sometimes it helps to get the question out there for several results.

Would it be courtesy, however, to at least mention that the question has been asked elsewhere? When you go to a library and refer to more than one book all the effort is on the person who wants the answer. The books, being lazy, idle objects, make no effort. When you ask in a forum you are asking people to make some effort. Isn't it courteous to give them the chance of finding out what has already been written in answer?

[Leigh]

How can you calibrate a densitometer from a non-calibrated step density?
Hi Chuck,

It's a question of terminology and accuracy.

The "non-calibrated" Stouffer step wedges are provided with nominal values only.
The "calibrated" wedges have each value measured individually, with the actual values written on the storage sleeve.

Both types are produced to the same level of accuracy. It's just a question of how closely the true value is presented.

As with any calibration process, accuracy is a sliding value. You use whatever level is required for your purposes.

- Leigh

[Chuck P.]

I guess I see it this way. If you handed me your uncalibrated step tablet and said, here, calibrate your densitometer with this, I would then ask, but how do I know that the reading I get is true and accurate if I don't know before hand what the reading is supposed to be? And to that you would then say.................

[peter ramm]

The response of a densitometer is highly dependent upon the optical path. The film itself is part of that path and scattering within the film will materially affect density measurements.

That is why absolute density calibration is done under a restrictive set of conditions (both illumination and collection angles are 180 degrees, scattering in the step wedge material is defined). Under these conditions, 0.90 D is exactly 0.10 D less dense than 1.00 D in the wedge and 0 D means that no light is coming through.

That will not be true under your measurement conditions and your densitometer will not be completely linear in response, both because of instrument and reading conditions. However, it's the best that we can do in practical application and by calibrating your instrument to the step wedge you get an approximation of true density with the main difference being that it is on a sliding scale. There is no absolute zero.

Going any further than this gets complicated. For example, precision densitometers use an internal calibration standard to provide a fixed reference point, usually on every scan. You are unlikely to have this so you won't know if your instrument is varying from moment to moment (with line or battery voltage for eg). However, you can characterize the precision of your measurements by making repeated readings of ROD. Do you get the same ROD value every time you measure a step on the wedge (precision) and is the amount of variation from measurement to measurement constant at all intensity values (linearity)? Essentially, you are measuring signal to noise across the relevant density range.

Or maybe you just use the thing.

[Leigh]

And to that you would then say.................
You DO know what the densities are supposed to be. Each step is numbered, and its nominal density is printed on the sleeve.

You know the value within the nominal accuracy of the wedge, which value is given in the product specification. For example, an uncalibrated wedge may have an accuracy spec of ±3% while its calibrated version is spec'd at ±2%, or similar.

A calibrated wedge is individually measured and the value of each step is written next to the nominal value, so it's accuracy is tighter.

You don't actually "calibrate" a densitometer, other than setting one upper endpoint, perhaps 2.5 or 3.0 or some such.
All other values are derived from that one setting.

You use the various steps to check the linearity of the instrument, which you can't change.

- Leigh

[Leigh]

... and 0 D means that no light is coming through.
???

This is a transmission densitometer.
0 D means that there is no density in the light path, just air, so all light gets through.

One common usage for the zero setting control on the instrument is to select between absolute and relative density
measurement modes. In photography, we usually want relative densities, so set "zero" as looking through an empty
area of the negative, like the non-imaging edge area.

- Leigh

[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. :D

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/uploadedFiles/US_plugins_acrobat_en_motion_education_sensitometry_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):

http://www.stouffer.net/graphics/21steppromo1.jpg

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):

http://www.stouffer.net/graphics/21steppromo1.jpg

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 (http://www.stouffer.net/Productlist.htm); 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.

[ic-racer]

PM me if you need the Tobias repair manual on PDF.

Not sure how we get 30 posts on this when the calibration procedure is a single paragraph in the manual.

[Leigh]

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.
Sorry if you got the wrong one. The nominal values with tolerances are given on the Stouffer website.

If you're satisfied with the performance of your meter, why did you order the step wedge in the first place???


- Leigh

[Chuck P.]

Sorry if you got the wrong one. The nominal values with tolerances are given on the Stouffer website.

If you're satisfied with the performance of your meter, why did you order the step wedge in the first place???


- Leigh

to carryout personal film speed tests and development time tests as described here (http://www.amazon.com/The-Ansel-Adams-Guide-Photography/dp/0821219561); to test for relative ISO Range Numbers of my VC paper as described here (http://www.amazon.com/The-Variable-Contrast-Printing-Manual/dp/0240802594). Although my meter does not require a step tablet, both of these testing methods that I use do, and I'm quite pleased with them.