View Full Version : Spectrosensitometer

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10-26-2007, 07:46 AM

Hope you can help.

A while back you mentioned that you used a spectrosensitometer to test sensitivity of emulsions to various wavelengths of light.

Can you recommend a "low budget" spectrosensitometer? Can you point me towards a typical setup or photos of a film spectrosensitometer?

Thanks From,


Photo Engineer
10-26-2007, 07:49 AM
1 constant voltage power supply
1 light source with a repeatable daylight type output
1 Bausch and Lomb monocrhomator
1 step wedge as wide as the output of the monochromator
1 solid mounting plate

That is the high end.

The low end is a strip of Wr 99, 98 and 70 filter mounted over a step wedge. Exposures will show relative R/G/B speeds. This is not quantitative.


10-26-2007, 08:16 AM
Interesting ... that would be a fantastic piece of equipment for emulsion making.

How are those parts put together to be functional?

Photo Engineer
10-26-2007, 08:25 AM
The above should include a shutter mechanism and timer for achieving different exposures.

It is linear as follows:

Light > Shutter > Monochromator > Step Wedge / object being tested.

The optical items are all mounted on a flat plate of metal for a sturdy makeup and to give optical alignment. The monochromator has a range from about 300 nm - 900 nm with an adjustment for which range - ie UV, visible or IR. It also has a filter drawer next to the shutter for any desired filter combination.

I have posted examples here of exposures. I have no good photos of the device itself.


10-27-2007, 03:54 AM

Thanks for your reply. Your answer inspired me and I found that I already had the answer in no less than three photography books (I should have looked before I posted!). One being the 1958 edition of the Ilford Manual of Photography. Attached is a scan from this excellent book showing a spectrograph. Baker also has construction details for one.

Diffraction gratings and lenses can be purchased very inexpensively from "The surplus shed". http://www.surplusshed.com/

These companies sell step wedges.
http://www.stouffer.net/TransPage.htm also http://www.tiffen.com/Kodak%20Professional%20Accessories%20catalog.pdf

Any recommendations on the part number of the step wedge that would be most suitable?

Photo Engineer
10-27-2007, 08:33 AM
You have seen examples of wedge spectrograms that I posted here. There are many in the book by Mees and James, done by Paul Gilman.

You must calibrate it somehow.


10-27-2007, 09:15 AM
Wow, that looks fantastically simple ... it's one of those "Why didn't I think of that?" kind of things.

I think I might build one.

What is involved in the calibration? I'm assuming you'd need to somehow correct for the spectrum of the light source? I know that you'd also probably have to mark off the wavelength scale, but that'd be easy with, say, a fluorescent lamp.

10-27-2007, 04:59 PM
Yes they do look straightforward to construct. Everything is more difficult in the doing though....at least for me :-)

PE raises a good point regarding calibration.

I understand that a fluorescent light source would have many spectrum lines in it. See the picture of a fluroscent light spectrum at:

Baker has some paragraphs on calibration in Photographic Emulsion Technique, 1948 edition. He mentions using an arc lamp (not something widely used today). I was thinking that Light Emitting Diodes (LEDs) might be a more modern alternative.

If I ever get around to making one I would use it for relative differences. For example seeing if the emulsion was now sensitised to green light. Exposing some panchromatic emulsion such as Plus-x etc would give a "reference" spectrograph. This avoids the need to calibrate.

Photo Engineer
10-27-2007, 05:13 PM
Yes, you need to know where UV, visible and IR are positioned and the actual wavelengths. Look at some published spectra in Mees and James or here, that I have posted to see how they look. That is about the best I can say.

Mine is calibrated, and has 2 settings. One is visible and one is IR. If I don't use a UV filter, I get a UV harmonic at 700 nm. This harmonic vanishes with a UV filter and is one of the faults of these particular instruments.


10-29-2007, 01:45 PM
There exist three other forms:

1.sometimes you can find the Lagorio-tableau 16 colorstripes are combined with 13-part grayscales. The tableau is around 24x30cm and was made in ?Czecheslowakia?Poland?. Named VUZORT. The U has a accent. I have heard, that other forms had exist.

2.The Schott variable Interferenzfilter from around 360 to 800nm. It is ca 27 mm x 190 mm, my is combined with a parallel graystep made by grainless bw film. Ideal for 35mm films to control.

3.In older times scientist had have a pocket-spectrometer. This is today in the program of scientific tools, but you will find often on auctions for history of technology and science. A Zeiss Jena with scale 400-700nm is combined with a simple only mechanical construction from ICA, Dresden (famous camera-company) for 6.5 x 9 cm plates, where a 8mm slit will exposed in 7 following positions. In the plate-cassettes it is simple to mount 2 parallel 35mm stripes.

Photo Engineer
10-29-2007, 02:17 PM
Yes, it can be simpler as you say.

I mention above a 3 stripe chart with a 21 step scale behind it which can give you relative speeds of a film at 3 different wavelengths (RGB) once you calibrate it.


Kirk Keyes
10-29-2007, 02:53 PM
This harmonic vanishes with a UV filter and is one of the faults of these particular instruments.

It's not really a fault, it's just a function of using a diffraction grating. A spectometer using a prism will not have this issue. Prisms have their own issues...

I bought a 4 inch spherical mirror with a diffraction grating on it to make one of these. Someday I'll do it!

Colored LEDs should be an easy way to calibrate any spectrosensitizer now that red, green, blue and even UV LEDs are readily available.

Photo Engineer
10-29-2007, 03:00 PM
Yes, as you say Kirk.

We have big instruments at EK without this problem. They are used for major work.


10-30-2007, 01:44 PM
In the 1960's I worked for a company that produced spectrometers. For most purposes we supplied xenon lamps as the light source. I do not remember the range thes lamps cover but we normally worked with the uv and the near uv regions. All mirrors were first surface. All the lenses and windows were quartz or equivlant. Most calibrations were done using varions compounds that had known adsorbance lines or floursed (spelling) at known wave lengths. Most of the equipment we produced was used in research labs, either in universities or medical research labs.

06-19-2012, 01:05 PM
Some talk and inklings of a DIY spectrosensitometer from 5 years ago; did anyone ever get around to doing it?

So let's break down what the optical path might look like. It's hard to tell from the diagrams what exactly is happening in the 3rd dimension.

So we have a light source and a lens (simple meniscus?) to concentrate the light onto a slit. The slit will presumably create a thin "bar" of light that next must go to a diffraction grating. I think the "science class" type gratings that are mounted in 35mm slides would be ideal, as has been noted above.

Now, does the angle of the grating affect how the light is diffracted, and how much it fans out?

How important is the collimator in front of the grating? It looks like the purpose of that is to get the light to come at it perfectly parallel. But wouldn't a horizontal bar of light (from the slit) hitting the grating still make a reasonable projection of the spectrum?

Then the diffracted beam of light encounters the step-wedge, placed in front of our test film.

What I'm having trouble imagining is the interaction between the projected spectrum and the step wedge. Most step wedges that we know are arranged in a long thin strip, not wider than a centimeter or two. How on earth are we supposed to project the spectrum onto this and achieve a 21-step gradation at all spectral frequencies? We need each spectral region (400, 500, 600, etc.) to go through the whole range of steps. This seems obviously impossible with the thin step wedges that we're all used to.

Now, Kirk, you say you have a spherical mirror with a grating in it; how would that design work? It sounds like that would be fundamentally different from the above diagram posted by Emulsion.

Thanks all,

06-19-2012, 01:07 PM
Hi Holmgurgers,
Now that several people are seriously looking at sensitizing dyes, in the words of an old New Yorker " It can't hoiht." I would like to be able to look at the spectrums of my emulsions. It comes down to the cost of such a spectrometer.
Someting that I have not tried, but your post brought to mind. Shooting color charts through cut off filters may be more informative than just shooting the color charts in sunlight.

06-19-2012, 01:30 PM
I agree Bill.

I'm sure that color charts and just shooting the film and interpreting the results can be informative, but not nearly as illustrative as a wedge spectrograph. A series of those can be compared against each other almost immediately, and without much deduction. Their look says it all.

I'm convinced we can make something very inexpensively. All we're doing is projecting a spectrum onto a piece of film, through a step wedge. It's just a matter of buttoning it all up and working within some parameters.

First though, I think we really need to wrap our heads around the design and understand that (in 3 dimensions) more clearly. Hopefully some Q&A will get us there fairly quickly.

06-19-2012, 01:57 PM
How about buying a set of LEDs at different wavelengths and soldering them side-to-side? I know, it's not perfect, but it would be easy. The set could be: IR, 660 nm red, 625 nm bright red, orange, yellow, traditional dim yellowish-green "green", high-brightness 520-530 nm green, blue-green, blue (470 nm), deep blue (450 nm), UV (405 nm). Add trimmer pots to every LED and calibrate using a commercial panchromatic film with a known, published sensitivity curve.

Photo Engineer
06-19-2012, 02:09 PM
Someone posted the name of a company in Buffalo that makes inexpensive monochromators. I did not save the information as I already have one! Sorry. However, IIRC, they ran under $50.

In my unit, it is a fully straight line path from the light to the "output". The monochromator has an opening and is box like with an opening on exactly the other side for the exit beam. There are probably many twists or turns inside the "black box" but I am loathe to open it. It works you see. So, what ain't broke, don't try to fix.

There are 2 photos of the unit in the book and a sequence of the unit in operation in the DVDs.


06-19-2012, 02:11 PM
A brilliant idea, but I'm going to say that personally, I think it's too much trouble... at least, it's certainly outside my abilities.

I'm gonna be thinking in terms of the above design (the diagram), with a simple halogen or tungsten bulb. It's the optical path that I really wanna get a grip on.

edit: Thanks PE, gonna have to look for that Buffalo co.