i´ve got the opportunity to buy a used sensitometer. Do I need an extra step wedge or does this device produce the steps on my film by itself?
Depends on the sensitometer. Which one are you looking at?
The Wejex has the step wedge glued in place between glass. OK, as long as it is the model with the 21 step wedge. EG&G just has a glass plate and you tape whatever step wedge you have on there.
If you have to buy a step wedge, (and you already have a calibration target for your densitometer) you can get the cheaper non-calibrated one, as you are going to want to know all the values as measured by YOUR densitometer anyway (for your x-axis).
It really should not matter if it's measured by YOUR densitometer or someone elses. As long as your densitometer follows the guidelines for specifications of densitometry design. And by that, if you have a commercially made densitometer that's not more than 50 years old, it's in good operational condition, you know how to calibrate your densitometer and you have good calibration standards for your densitometer, you should get similar, if not identical results as the manufacturer of your step wedge did when they measured it.
Originally Posted by ic-racer
That said, get an a good calibration standard for your densitometer, calibrate the densitometer, then get an uncalibrated step wedge, and calibrate it yourself.
it´s a Kunze (Germany), built to calibrate X-ray films in radiology surgeries.
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I'd be interested if anyone's 'home darkroom' densitometer matches all 21 steps of a 'calibrated' wedge. With respect of consumer densitometers, I don't know of any densitometer 'standards' for power supply regulation, light source measuring aperture, color source, intensity distribution and luminous flux. These will vary from brand to brand and machine to machine and will influence the linearity.
Originally Posted by Kirk Keyes
There are ANSI specifications for densitometry. And I suspect that most of the major commercial (Xrite, Macbeth) ones do follow them.
ANSI PH2.17: American National Standard, Density Measurement – Geometric Conditions for Reflection Density.
ANSI PH2.18: American National Standard, density measurement – spectral conditions. This standard defines the Status responses for densitometers, such as Status T, and Status A.
ANSI PH2.19: American National Standard, conditions for diffuse and doubly diffuse transmission measurements (transmission density). This standard defines the Geometric and Spectral Conditions for transmission densitometers.
Yes, and light meters are presumably made to ANSI ANSI-PH2.7 but I wouldn't expect your light meter to read the same as mine.
Originally Posted by Kirk Keyes
Also, (quote from the Tobias web site):
So, either the standards are not that strict or the MFGs don't follow them strictly.
Densitometers from different manufacturers have different optical and electrical characteristics, therefore they do not always display the same density values across their full operating range.
Last edited by ic-racer; 09-13-2008 at 06:08 PM. Click to view previous post history.
Since X-ray films are frequently exposed with a fluorescent enhancement screen of either blue or green, the X-ray sensitometers frequently expose film to either green or blue light.
Originally Posted by luxikon
One of these days I'm going to test an X-ray (blue/green) sensitometer with a Wejex White light sensitometer and an EG&G xenon sensitometer so see if there are any significant differences. Maybe someone will lend or donate an old blue/green sensitometer....
A ong time ago I've bought second hand a used (well used) X-Rite 339 densitometer. It it a transmission only densitometer for visible light and UV light. It came without the calibrating tile and I had not enough money to buy one from X-Rite.
So, as I had to order things from Stouffer, I bought a calibrated transmission step wedge.
At first, it was not good. But upon retouching the slope trimmer, the thing became very good into visible measurement. UV is another matter, as I do not know an UV step wedge of accessible price...
IMHO, for personal use, one needs only a densitometer which is linear and able to repeat itself regularly. The absolute measurement are not of paramount importance.
P.S. the curve I get is like a sinus curve around the correct densities (if the Stouffer calibration is accurate). Retouching the slope trimmer reduce the amount of swing but does not suppress it. I don't know if it is a design flaw or if I'm not as good and clever than I thought.....