Spectrophotometer as densitometer?

[DocFrankenstein]

Being the science nerd, I want to know the density of my negatives to make sure the development time is right.

I do not have access to a densitometer for film specifically, but microbiology labs do have a spectrophotometer I can use.

Basically it's the same thing, but it measures light at a particular wavelength instead of the whole spectrum.

How should I approach this? Should I measure at a particular wavelength or take an average in say 50nm increments from 200nm to 600?

Anybody?

EDIT: for spelling

 

[ShutteredEye]

Well, folks in the ink and dye industry use them to create color profiles. They general use 10 nm increments to profile a color to match them.

Are you going to use a uniformly exposed negative? How are you going to standardize it?

 

[Hertz van Rental]

The wavelength shouldn't make a difference for B&W - you are measuring the density of the neg and so it isn't a problem (as long as it is in the visible range).
If you pushed me I'd say try to match the wavelength measured to the base colour of the film. If you are reading in the blue and the film base has a pinkish tinge you could get some inaccuracy.
Colour neg will pose different problems - leave that one with me for now.
You might also have a problem if it is a double beam SPM because of the way it works.
Your main problem is calibration. Off the top of my head I believe the measurement units are different. You will need to get hold of a master step wedge used for sensitometry to do this.
It's late here so I'll do a bit of reading tomorrow and see if i can come up with some more useful advice.

 

[DocFrankenstein]

Are you going to use a uniformly exposed negative? How are you going to standardize it?

I am not too sure how I would standardize it, since I do not have a lightmeter yet.

1)take a grey card and the meter in my canon A1. Then I'll make a few bracketed shots of a grey card.

2) develop them using my normal dev (agfapan in D76, for 13 minutes).

3) Measure the negatives. Adjust development time until I have the grays in place.

 

[DocFrankenstein]

Thanks Hertz

I expected you to chime in

 

[ksmattfish]

Have you looked on Ebay at used densitometers? I was checking them out about 8 months ago, and they were going cheap. Particularly the ones that would only work for BW film. I saw several go for around $30.

 

[DocFrankenstein]

Have you looked on Ebay at used densitometers? I was checking them out about 8 months ago, and they were going cheap. Particularly the ones that would only work for BW film. I saw several go for around $30.

I just might get one from there. Thanks

 

[ksmattfish]

Do some research before you buy. If all you are using it for is BW film then there are some that are smaller and cheaper that will work fine. Some are huge, and do color analysis, and probably lots of other stuff that may not be vital to you.

 

[Hertz van Rental]

If you go looking for a densitometer, remember they come in different kinds:
Reflection, Transmission B&W, Transmission Colour, Transmission/Reflection Colour.
As you go up that list they get more expensive.
Reflection densitometers are used on print material.
Transmission does negs.
Also remember that if you are going to do anything remotely serious with it you want a Lab version. One of the best models is the Macbeth range (they appear to be made by a number of different companies). Most will handle a 10x8. You can expect to pay $200 and upwards secondhand so think if you really need one.
Densitometers are primarily used for checking film characteristics and for seeing how accurate your processing is. If you use one on a normal neg it just tells you the density of the bit you measure - but compared to what?
At best you will be able to find D-max and D-min, giving you a contrast range. How does that help you?
There are ways of using a densitometer to control processing and to give a 'compensation factor' for your method of Photography, as well as some other things. But the methods are complex and involved so I won't go into them here.

As for the spectrophotometer. A single beam should work OK for approximate use.
Units are not a problem as you use relative values (unless your machine measures in lux-seconds).
Measuring density means you are measuring the amount of light transmitted.
You find this by comparing the amount of light transmitted to the amount of incident light.
Transmittance=Transmitted light/Incident light=%
Using this method the transmittance decreases with density so it is always less than one. Much better to use the reciprocal:
1/Transmittance=Incident light/Transmitted light=Relative value.
The result will always be bigger than one but can go over a range of 1:10,000.
Best:
Density=log(1/Transmittance)=log(Incident light/Transmitted light)
This will give you a relative value you can plot on a graph.
All you need to do then is calibrate using a sensitometry master wedge.
Simple ;-)

 

[Rob]

If you go looking for a densitometer, remember they come in different kinds:
Reflection, Transmission B&W, Transmission Colour, Transmission/Reflection Colour.
As you go up that list they get more expensive.
Reflection densitometers are used on print material.
Transmission does negs.
Also remember that if you are going to do anything remotely serious with it you want a Lab version. One of the best models is the Macbeth range (they appear to be made by a number of different companies). Most will handle a 10x8. You can expect to pay $200 and upwards secondhand so think if you really need one.
Densitometers are primarily used for checking film characteristics and for seeing how accurate your processing is. If you use one on a normal neg it just tells you the density of the bit you measure - but compared to what?
At best you will be able to find D-max and D-min, giving you a contrast range. How does that help you?
There are ways of using a densitometer to control processing and to give a 'compensation factor' for your method of Photography, as well as some other things. But the methods are complex and involved so I won't go into them here.

As for the spectrophotometer. A single beam should work OK for approximate use.
Units are not a problem as you use relative values (unless your machine measures in lux-seconds).
Measuring density means you are measuring the amount of light transmitted.
You find this by comparing the amount of light transmitted to the amount of incident light.
Transmittance=Transmitted light/Incident light=%
Using this method the transmittance decreases with density so it is always less than one. Much better to use the reciprocal:
1/Transmittance=Incident light/Transmitted light=Relative value.
The result will always be bigger than one but can go over a range of 1:10,000.
Best:
Density=log(1/Transmittance)=log(Incident light/Transmitted light)
This will give you a relative value you can plot on a graph.
All you need to do then is calibrate using a sensitometry master wedge.
Simple ;-)

or you could just guess

 

[Hertz van Rental]

or you could just guess

Note to self: Don't accept Rob's offer to process films.

 

[DocFrankenstein]

If you go looking for a densitometer, remember they come in different kinds:
Reflection, Transmission B&W, Transmission Colour, Transmission/Reflection Colour.
As you go up that list they get more expensive.
Reflection densitometers are used on print material.
Transmission does negs.
Also remember that if you are going to do anything remotely serious with it you want a Lab version. One of the best models is the Macbeth range (they appear to be made by a number of different companies). Most will handle a 10x8. You can expect to pay $200 and upwards secondhand so think if you really need one.
Densitometers are primarily used for checking film characteristics and for seeing how accurate your processing is. If you use one on a normal neg it just tells you the density of the bit you measure - but compared to what?
At best you will be able to find D-max and D-min, giving you a contrast range. How does that help you?
There are ways of using a densitometer to control processing and to give a 'compensation factor' for your method of Photography, as well as some other things. But the methods are complex and involved so I won't go into them here.

I am going to be working strictly with BW. My initial goal is to standardize and find the correct development time in D76.

I'd probably then compare the densities with the "tonal response curves"? agfa provides for their films and see if it's worth using Agfa developer over D76 or maybe other developer.

By the time I'm done, Agfa will be discontinued and I'll have to start over again. :lmao:

As for the spectrophotometer. A single beam should work OK for approximate use.

What if I average the readings? What spectrum do the Macbeth densitometers use?

Units are not a problem as you use relative values (unless your machine measures in lux-seconds).

We're doing exactly what you've described, except we use the word absorbance instead of density for some reason.

Thank you for the info

 

[Hertz van Rental]

My initial goal is to standardize and find the correct development time in D76.

It's been done for you by Kodak. The info is in the tech sheet providing you use Kodak film.
Manufacturers only publish sensitometric data for their films in their dev. Data for one manufacturers film in another's dev is like rocking horse poo.
You would do better to work on standardising your processing so it matches the published data - quicker and easier.

I'd probably then compare the densities with the "tonal response curves"? agfa provides for their films and see if it's worth using Agfa developer over D76 or maybe other developer.

By the time I'm done, Agfa will be discontinued and I'll have to start over again. :lmao:

There is a fair bit of work there.

What if I average the readings?

I believe a twin beam measures the difference in transmissivity between two points. Using it as a densitometer you would be measuring two points and finding the difference in density. Without knowing one of the densities to start with you won't get very far. I think getting it to work would be problematic.

What spectrum do the Macbeth densitometers use?

For B&W? Good question and one I don't have an answer to. I should imagine that it is the same as the wavelength used by light meters.
Off the top of my head (as usual) a white light source - with B&W colour temp isn't a problem - and a photodiode or possibly phototransistor as receptor. Most of those have peak spectral sensitivity towards the blue end, if I remember my electronics. You should be able to get data from a components manufacturer but if you check up in something like the RadioSpares catalogue it should tell you.
http://rswww.com/cgi-bin/bv/rswww/home.do?cacheID=uknetscape

We're doing exactly what you've described, except we use the word absorbance instead of density for some reason.

I suspect you are analysing dyes in liquid suspension or on a clear carrier? In dyework I believe you check how much light is absorbed at different wavelengths to define hue.
Sensitometry used to use the term 'opacity' but it was deemed too imprecise. Different branches use different nomenclature - and keep changing it.
Chemists have changed the naming convention on a lot of organic compounds some three times since I did Chemistry.

 

[DocFrankenstein]

I suspect you are analysing dyes in liquid suspension or on a clear carrier? In dyework I believe you check how much light is absorbed at different wavelengths to define hue.

Yes. Protein concentrations mostly.

I believe a twin beam measures the difference in transmissivity between two points. Using it as a densitometer you would be measuring two points and finding the difference in density. Without knowing one of the densities to start with you won't get very far. I think getting it to work would be problematic.

I could use an unexposed part of the film as a reference.

It just might be cheaper for me to buy the agfa developer and just use the recommended times... :meh: Especially counting the manhours I'll have to put into it...

 

[Hertz van Rental]

I could use an unexposed part of the film as a reference.

It just might be cheaper for me to buy the agfa developer and just use the recommended times... :meh: Especially counting the manhours I'll have to put into it...

Yes. I think that might work. You would need to check the density of the substrate to get a base level plus fog reference point, otherwise your readings would be lower than those given by the manufacturer. You might be better off nulling one beam reading just the light with nothing between.

Manufacturers recommended times are nothing more than guides. There are many variables that actually affect the results you get. If you do have access to a densitometer it can help a great deal. However there are ways to get optimum results without putting in a lot of lab work. It involves using the manufacturers published data - the down side is that Kodak is the only film manufacturer who publishes enough info to be able to do this.
I used to keep the secret close to my chest but seeing as I'm not being a pro any more and digital has changed the game I guess I could spill the beans. I've only got data for Panatomic-X, Plus-X and Tri-X in Kodak developers I'm afraid, but it was more than I ever needed.
Give me some time to sort it out.