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holmburgers
06-21-2012, 10:18 AM
I think a step wedge is important. Any kind of "dynamic" exposure (where it has to be moved manually) is going to be really troublesome methinks. It's going to destroy the whole continuity of having an identical-looking pattern that can be easily compared to others at a glance.

I also think you should consider switching from a Rick Astley song to, say, an Oingo Boingo song. Thoughts?

Bill Burk
06-21-2012, 11:26 PM
I think holmburgers is right moving the device would be hard, and make it less reliable.

But I agree with PE, if I understand the idea, a grid pattern to outline where you are looking sounds like a good idea.

I still like the fixed resistors to calibrate and multiple LED's of each wavelength idea. If necessary you could make different ones blink a specific number of times but there would be intermittency effects.

How about, keeping it credit card size but spread the small spots out so they are unlikely to interfere with each other. At regular intervals place a white LED to provide a benchmark grid.

Might as well add a "white" LED column as well to provide for exposure test - even if it is not sensitometrically perfect light it may add to the usefulness.

As PE said, you really need to go to DMax so 10 full stops of exposure range from dimmest to brightest (or shortest to longest exposure).

I am intrigued by the possibility it can be done without a Stouffer scale, with the LED's in direct contact with the emulsion. Then you don't have to make a diffusion chamber or other manufacturing invention. It can be a bare surface mount electronic device with one part and a battery. I almost want to recommend using a "123" battery. They are not uncommon these days and provide a good amount of 3v power.

Photo Engineer
06-22-2012, 09:30 AM
Bill;

There were small desktop spectrosensitometers all over Kodak Park at one time, along with the big units for precise work in B-30 and B-57 for film and paper. IDK what happened to them. Results using them are found in Haist, Mees and also in my book.

PE

Kirk Keyes
06-27-2012, 06:24 PM
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.

Sorry for taking so long to get back - I'll had a bad cold the last week...

Yes, a spectrometer with a mirror (by using a reflection grating) does have quite a bit different layout than one that uses a transmission grating. The reflection gratings, both flat and with curvature, make spectrometers that can be more compact then transmittion gratings as the light path can fold back over itself.

The reason I found a spherical reflection grating is that I was going for a design that's called a "Rowland circle spectrograph". With a Rowland circle spectrograph, if the inlet slit and the grating all lie on a circle, then the diffracted spectrum will also lie on the same circle. The diameter of the circle equals the radius of curvature of the grating blank. So if the circle is large, then you can make a pretty big spectrogram with no extra lenses or mirrors.
See:
http://gratings.newport.com/library/technotes/technote3.asp

The drawback is that if you have to use film to measure the spectrum, then you need film that's on a flexible support, like paper or acetate. I was looking at testing variable contrast papers at the time that I bought the mirror, so that would not have been an issue. Glass plates would have been a bit of a problem...

I also bought a flat relection grating as well, just for fun and in case I wanted to use plates.

I was going to use colored LEDs as a way to light sources to calibrate my spectrograph, but I like the idea of using LEDS at various wavelengths to make "spot tests" at those wavelenghts by contacting emulsions directly on the LEDs. I like the idea of making an array - "columns" of various wavelengths and then "rows" at differing light intensities.

One thing to think about is that LEDs can have fairly wide emission spectra - some designs like "superirradiant" LEDs are a bit more like LASER diodes and have more narrow spectra emision.

Side note, my second year physics professor at Reed College in 1983 told that class that if any of us figured out how to make a commercially viable blue LED, we would be rich! Too bad I dropped out of that class before I had a clue about how to make a blue led... (Well, I knew, you had to make a diode with a higher energy gap). Now I see that someone started selling blue LEDs in 1989.

Photo Engineer
06-27-2012, 06:43 PM
But good, durable blue LEDs just recently (Blue Ray).

PE

Kirk Keyes
06-27-2012, 07:16 PM
Some talk and inklings of a DIY spectrosensitometer from 5 years ago; did anyone ever get around to doing it?

Still on my list to do someday...



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.

The image of the slit continues on through the spectrometer unless it gets blocked out. So the projected image with the spectrum will be as tall as the slit.



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.

Those cheap, 35 mm slide diffration gratings are typically not very efficient. A "replica" grating is going to be a better solution than the cheap holographic ones. Best are original or master ruled gratings, but who can afford those?


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

Yes, the ruling angle controls the angle of diffration, The more lines per inch or mm controls the range of wavelengths that can practically be diffrated. Replica gratings will have the same diffration angle as the ruled grating that they were copied from.



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?

Collimated light will give a sharper image of the spectra. If you have your light source a LONG ways away relative to the dimensions of your spectrograph, then you would not need a collimator. If your light blulb is right next to close to the slit, then you will definitely want to collimate the light source.



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.

As I said above, the image of the slit will project though the spectrometer unless it is blocked by something. It's hard and expensive to get gratings of any great width, so you want to maximize what you have going through the spec.

I was planning on getting a centimeter or two at the most of spectra. So I was going to make multiple exposures on the film/paper at various light intensities to get the spectral info for the film. To change the intensity, I was planning on keeping the exposure time at 1 second, and then using Wratten ND filters between the light source and the slit. Since I can measure the spectral characteristics of the Wratten ND gelatin filters with a spectrometer that I have (or look them up online as Wratten/Kodak publishes this info) then I could apply a correction to my measured spectrograms.

Scan through this book - it's pretty good and has all the math and more that you'd need to calculate anything with gratings.
http://gratings.newport.com/library/handbook/toc.asp

Kirk Keyes
06-27-2012, 07:27 PM
Bud good, durable blue LEDs just recently (Blue Ray).

Blu-Rays use LASER diodes at 405 nm - funny thing is that is considered a "violet" wavelength!

I see Sony unvieled a Blu-Ray prototype in Oct. 2000 - I'm not sure when the diodes were invented, but it's certainly a few years before they showed the Blu-Ray.

Speaking of random LASER devices, here's something I plan on making some day - a Laser Harp:
http://youtu.be/sLVXmsbVwUs

I love when the laser fans out after first being powered up!

Photo Engineer
06-27-2012, 07:31 PM
That harp reminds me of the sound track from Forbidden Planet! ;)

Actually, I knew of the difference between the blue LED and the blue Laser, but they were both slow coming to fruition due to longevity issues and other problems.

PE

Hologram
06-28-2012, 02:49 AM
Yes, a spectrometer with a mirror (by using a reflection grating) does have quite a bit different layout than one that uses a transmission grating. The reflection gratings, both flat and with curvature, make spectrometers that can be more compact then transmittion gratings as the light path can fold back over itself.

I see. In that case you could optically contact (index match) a transmission grating to a mirror.

By the way, what about simply use a CD, DVD, Bluray disc as a grating, wouldn't that be an option?





One thing to think about is that LEDs can have fairly wide emission spectra - some designs like "superirradiant" LEDs are a bit more like LASER diodes and have more narrow spectra emision.

Yes. In my opinion LEDs are probably too much broadband for such an application. Lasers might be far better for "callibration". Lasers at 405, 445, 532 and 650 nm have become very affordable...

Hologram
06-28-2012, 02:51 AM
Yes, the ruling angle controls the angle of diffration, The more lines per inch or mm controls the range of wavelengths that can practically be diffrated. Replica gratings will have the same diffration angle as the ruled grating that they were copied from.

Speaking in "holographics" the number of lines per mm is called spatial resolution. The higher the resolution the larger the angle between the input and the output (the diffracted) beam.

Hologram
06-28-2012, 02:55 AM
Blu-Rays use LASER diodes at 405 nm - funny thing is that is considered a "violet" wavelength!

It is. Moreover, in contrast to what the eye sensitivity curve suggests, these lasers are highly (painfully) visible!


I see Sony unvieled a Blu-Ray prototype in Oct. 2000 - I'm not sure when the diodes were invented, but it's certainly a few years before they showed the Blu-Ray.

Meanwhile these lasers have become pretty cheap and are available with powers >1W. I assume they might be interesting for photographic applications involving the use of dichromates or ferric salts etc.

holmburgers
07-02-2012, 01:36 PM
Thanks for the answers to my questions Kirk.

So if we imagine a simple "shoe-box" spectrosensitometer, it might look like this.

Light source (let's imagine a simple tungsten flash-light bulb for now) -> far enough away or with collimating lens -> diffraction grating -> focusing lens -> film plane.

The light needs to hit the grating dead on, and then we need to focus that image sharply onto the focal plane. Any idea what kind of lenses we'd need, and approximate focal lengths? There are some acrylic lens sets (http://www.scientificsonline.com/acrylic-lens-set.html)that you can get, or we might be able to order some more appropriate lenses, both from Edmund (http://www.edmundoptics.com/optics/optical-lenses/aspheric-lenses/molded-acrylic-aspheric-lenses/2734).

It seems one of the lens kits would be great for this; they have all the different types of lenses for abotu $15.