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wildbillbugman
06-19-2012, 03:23 PM
Hi hrst,
Your idea is workable assuming the "technician" who assembles the thing has at least one hand which is in some way connected to his brain. I fear that I would solder each LED to one of my fingers. I think that I may look into PE's source for a $50 ready made thingamajig.
Bill

Photo Engineer
06-19-2012, 03:25 PM
At EK we used an R/G/B filter combination to mimic a spectrosensitometer to judge the relative speeds in the 3 regions of the spectrum. It worked just fine. I build one when our old one wore out. And, I had to calibrate it!

PE

holmburgers
06-19-2012, 03:31 PM
On page 175 of this book is the basic outline of a DIY spectroscope for telescopes. It could be easily adapted to photography methinks.

http://books.google.com/books?id=XDE_yHZT2iIC&pg=PA175&dq=diy+diffraction+grating+spectroscope&hl=en&sa=X&ei=qNLgT8-qGajA2gW8y8i7Cw&ved=0CEEQ6AEwAQ#v=onepage&q=diy%20diffraction%20grating%20spectroscope&f=false

wildbillbugman
06-19-2012, 05:40 PM
Hi All,
Holburgers,
Your diagram scares me! "Simple" to you maybe. I'll bet you were one of those nine-year-olds who built his own rockits,radios and computers.
PE,
Any chance that you could sketch a crude diagram of your tri-filter gizmo?
Bill

Photo Engineer
06-19-2012, 06:03 PM
Well, Bill, I have posted a picture and a diagram here before. I tried and cannot find the picture I posted, but it was the one used in the book and I have "retired" them all since the book's completion. I can repeat the diagram here in text.

Constant voltage > Light > shutter > Monochromator > step wedge.

The input to the electronic shutter is a precise timer. The whole optical path is on a heavy metal plate to keep it in alignment. It is nothing fancy and could be cobbled together by anyone. A prism or diffraction grating could work. It is all straight line from one end to the other, and about the length of a counter top (~24") and about 10" wide.

PE

hrst
06-19-2012, 06:07 PM
Okay! How about this;

I ordered some surface-mount LEDs from Ebay/China, the wavelengths I am missing. Unfortunately, they always sell at least 10 to 20 pieces, whereas I only need one of each ;). And I have hundreds or thousands of some wavelengths already.

I happen to make PCB's all the time and it would be wasteful to make just one of those tiny little boards. So, I will make several of them at the same time. And once I need to calibrate mine, I can just apply that calibration to all of them. I am thinking about fixed resistors.

So, those will be tiny little units with a series of LED's: for example, IIRC, 660, 625, 600, 580, 550, 520, 470, 400 nm. As they are LED's, they are not sharp but show some response to +/- 25 nm so there is a bit uncertainty there, and the calibration won't be perfect either, but at least it will be cheap, small (something like a credit card) and simple and quickly done. It will need some kind of diffusor, but I have a solution for that ready too. It will work from a 9V battery and cost something like 5-10 USD.

I can include a simple timer to the design with practically no extra size or cost.

Anyone interested? What about the dimensions? I was thinking the LED "bar" could be something like 5 cm (2") long.

I will build this one for myself anyway and this is easy to "mass produce" in a range of 1-10 units.

wildbillbugman
06-19-2012, 06:52 PM
hrst,
Yes , for such a price, or a little more, I am absolutely interested. Anything I do not have to build myself. I have built things from wood, glass and plastic. But never anything involving wires and solder.
Bill

Bill Burk
06-20-2012, 12:37 AM
hrst,

Maybe you could stack them in a grid... 10 LED's of each frequency for example. Use fixed resistors to attenuate each LED a little more than the one above to it to create a wedge of that part of the spectrum. Then maybe with a dozen stacks like this...

Use a few more components this way. But you wouldn't have to place it on a dozen Stouffer scales.

Photo Engineer
06-20-2012, 10:12 AM
Bill;

If you turn the wedge 90 degrees, you only need one step wedge. It has to be wide enough for the entire spectrum though.

PE

holmburgers
06-20-2012, 11:48 AM
hrst, I would definitely love to get my hands on such a gizmo. Looking at your past electronics skills, I have no doubt you can make one! Nevermind my previous neigh-saying... :whistling:

Ok, so attached is a depiction of a typical step wedge like we all use (Stouffer), and then an ideal form for a spectrosensitometer. We need something that's more "squat". Hamburger as opposed to hotdog... if you will. Right?

Now why is a thin slit so important? That spectroscope article suggests that it should be 2-10 mils wide. Why does the light have to be collimated exactly?

I've found 500 lp/mm and 1000 lp/mm diffraction gratings on eBay for very cheap; like the attached image. I think I'm going to just have to buy these and play around with them to see how they act in different circumstances. Again, in every diagram the diffraction grating or the focusing lens is oriented at an angle; is this the only way to get the diffraction grating to "do its stuff"?, that is, to spread out an image of the spectrum.

It'd be ideal to use plastic lenses so that we don't alter our UV transmission much. Cheap acrylic lenses abound (http://www.scientificsonline.com/acrylic-lens-set.html), in all different types of powers (http://www.edmundoptics.com/optics/optical-lenses/aspheric-lenses/molded-acrylic-aspheric-lenses/2734).

Photo Engineer
06-20-2012, 12:52 PM
What we need is the step wedge on the right and a grating that produces an "image" as high as the step wedge. In simpler terms, the wedge should be about 4x5 and the grating should supply uniform illumination over that area. Otherwise, the result will be useless. Stouffer Iand EK) made wedges with different size steps and in different size sheets. Kodak sheets were up to 8x10, and the smallest step was 0.3 inches (IIRC). Steps were in 0.15, 0.3 or 0.6 increments.

PE

holmburgers
06-20-2012, 01:21 PM
So projecting a nice, big and evenly illuminated spectrum is the crucial thing here.

hrst
06-20-2012, 01:38 PM
Wouldn't one with reduced number of steps still be quite useful? Even with just one mid-density, we could measure the relative sensitivities. AFAIK, there should be no contrast difference between wavelengths in a BW material, so density difference at mid-density should be directly comparable to shadow speed at that wavelength. That being said, of course a step wedge helps to find the speed directly without using a densitometer (or scanner).

I actually was thinking about this project two years ago and have a prism for this purpose. But I think that with a prism, there is a risk of unwanted reflections in the prism itself and in the optical system, too, risking some white light leak-through which would be detrimental for the measurement. I suppose that the diffraction grating has smaller risk if done right, but it is hard verify by eye that you are not having any leak. You should have a sharp-cutting red, green or blue filter and place that filter on a region with supposedly no such wavelength to verify that. So, I'm going the LED way first just because it's so simple; but I'm going to make it small. Maybe I can fit 4-5 steps.

Photo Engineer
06-20-2012, 01:42 PM
You may have a situation where you did not add enough dye. With a limited scale, you would not see the tiny bump of the sensitivity region with that tiny amount of dye added. So, we used a big scale relatively speaking.

And, a variation in contrast as a function of wavelength is not unusual. After all, you are using white light for exposure and the curve is the integral of all wavelengths but with a spectrosensitometer you actually can see these variations.

PE

hrst
06-20-2012, 02:07 PM
OK, I was thinking something like this. Any comments appreciated!

For situations where more steps are needed, 2 or more subsequent exposures with different exposure times could be used side-to-side.

holmburgers
06-20-2012, 02:37 PM
The exposure timer and the time recorder are really awesome; great ideas!

You mentioned a diffuser, and will this be integrated with a step wedge?

Presumably the LED will be in a cavity of sorts, and the diffuser placed some distance away; perhaps along the bottom surface where it will make contact with the test film? The "cells" will have to be isolated well of course, to prevent any cross-talk between spectral regions.

Honestly, it's a brilliant little device you're dreaming up here... LEDs open up so many possibilities.

Photo Engineer
06-20-2012, 02:49 PM
OK, I was thinking something like this. Any comments appreciated!

For situations where more steps are needed, 2 or more subsequent exposures with different exposure times could be used side-to-side.

The scales you show are just fine. You must remember that for negatives you must go from Dmin to Dmax. This, of course, is the raw form of the emulsion.

If you do not go to Dmax, then you can miss the true extent of sensitivity.

What you show appears to be a reversal scale.

PE

hrst
06-20-2012, 03:18 PM
Presumably the LED will be in a cavity of sorts, and the diffuser placed some distance away; perhaps along the bottom surface where it will make contact with the test film? The "cells" will have to be isolated well of course, to prevent any cross-talk between spectral regions.

Yeah, you got the details perfectly right! Then, the step wedge will be mounted to the diffuser and the step wedge is in direct contact with the film being exposed.

I'm probably just going to print custom step wedges. Those digital photography people who also occupy our club darkroom, have bought there a $2000 pigment inkjet that is GREAT for printing everything except photographs; for example, PCB exposure masks. I measured a Dmax of 2.8 when printed on a OHP film, and when printed at 1200 dpi, pixels can be seen with a microscope, so this is a great tool for making good enough custom step wedges. (Naturally, a spectral response of the black pigment ink is not guaranteed to be perfectly even, unlike those high-quality and expensive step wedges.)

hrst
06-20-2012, 03:37 PM
More ideas:

Making it smaller, simpler & cheaper by leaving out the step wedge (or multiple leds) completely and making the LED spots smaller, and make it blink N times with increasing exposure, so that you move it along the material and get a step wedge-like exposure.

hrst
06-20-2012, 04:07 PM
Like this.......