Printable View
Much simpler: use your enlarger and your lab meter, as I have suggested initially.Quote:
Originally Posted by Jorge
BTW: these densitometers do exist and are called "micro densitometers", which in this case does not mean that their probe size is very small.
Well my home made meter uses my enlarger for a light source. So what does that mean?
Well Robert, we must have been very technical. Your densitometer is perfect for this case, because it has exactly the same light source as your enlarger...Quote:
Originally Posted by Robert
micro densitometers are used in thin film applications and this is a totally differnt field from photography. they cost thousands of dolars and you never see one on the market...they are made specially for people like Dupont, Dow Agfa...etc...Quote:
Originally Posted by Thilo Schmid
Robert, be careful. It is easy to get a miss reading with a 'projection' densitometer. There can be spacial and/or temporal variation in the illumination that can throw off your null reading. Spacial variations can be eliminated by not moving the sensor and by moving the negative instead. Temporal variations are probably not an issue unless you are using coldlight. If so, make sure it is warmed up and that you check it at the beginning and end.
Good luck.
I'm guessing a projection densitometer would be one that puts the negative in the enlarger? I'm just using the enlarger to project a circle of light on the baseboard. The sensor is sitting on the baseboard with the negative right on it . It's a condensor enlarger.
Robert, one more thing. What does your detector look like? I used to have the transmission densitometer standard around here, but I can't find it. Anyway, in that standard it defines how the negative shall be illuminated and the light from the sample is measured (the angle). Basically, if the light source is collimated, as through an aperture, the sensor should read as much of the resulting light coming from the negative, i.e. the sensor should have an 'integrating sphere' design. If the light source is diffuse, then the sensor should 'see' only a narrow angle. All angles on one side, narrow angle on the other.
A different density results if this is not followed, because of the Callier effect. This difference only matters if you want to be able to compare numbers with those in books.
It sounds like your light is collimated, therefore your sensor should have an integrating sphere or some kind of integrating quality. It needs to be able to see light coming from all angles of the negative. If the negative is in direct contact with the sensor, this works. Or if you put a piece of white Plexiglas in contact with the negative and then the sensor looks at the back of the white Plexiglas, this works too. Unfortunately, the Plexiglas attenuates the signal.
You could turn this around and do it on top of a light table. Mark a small area on the light table, put the negative there and put the sensor on top looking down. This is good if your sensor is not an integrator and sees only a narrow angle.
Hope this makes sense. Maybe someone out there has the standard and can correct any inaccuracies.
It's this one.
http://www.apug.org/site/main/viewtopic.php?t=1450
The negative sits in contact with the sensor.
I think you're on track. Since you have a home brew densitometer, I be a little careful and check the readings it gives you. I've read the nice section on building one in Phil Davis's "Beyond the Zone Sytem" book. Sounds good but has some pitfalls that he talks about. The acid test is to measure a transmission reference with your densitometer and see if it agrees with what another one said. I have a 31 step transmission wedge from Stouffer (http://www.stouffer.net) that I sometimes use to check my densitometer. Pay the extra for them to put the acutal measurements of each step on the sleeve. It's still under $30 for the calibrated step wedge, I think.