question about step wedge

Dichromated Gelatin.
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

question about step wedge

Post by Joe Farina »

The first photo shows a "transmission step wedge" bought from Photographer's Formulary. I'm planning to lay this on top of a Denisyuk hologram during exposure, to get a range of exposures.

The envelope for the wedge says "This guide has 21 steps in density increments of 0.15. Each step increases or decreases the exposure by 1/2 f stop and every second stop is equal to 1 f stop."

I was wondering if these densities can be related to seconds of exposure. The #1 wedge is clear, so that area will get the full exposure, say for example 20 seconds. The #2 is wedge is less transmissive, so it will be like a shorter exposure. Can it be defined in seconds of exposure? Maybe a dumb question, but I wanted to throw it out there, for the fun of it. Thanks in advance for any help.

The second photo shows a graph of exposure vs. efficiency for a MBDCG grating. I'm seeing this bell-shaped graph more and more frequently with regards to DCG and MBDCG. I want to expose enough, but not too much.
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MilanKarakas

question about step wedge

Post by MilanKarakas »

Joe Farina wrote:The first photo shows a "transmission step wedge" bought from Photographer's Formulary. I'm planning to lay this on top of a Denisyuk hologram during exposure, to get a range of exposures.
It looks very interesting, Although bit expensive (close to $30 US).
The envelope for the wedge says "This guide has 21 steps in density increments of 0.15. Each step increases or decreases the exposure by 1/2 f stop and every second stop is equal to 1 f stop."
I am not an expert, but if I remember correctly from my photographic experience, it is related either to f stop number, ISO sensitivity, or exposure time.
I was wondering if these densities can be related to seconds of exposure. The #1 wedge is clear, so that area will get the full exposure, say for example 20 seconds. The #2 is wedge is less transmissive, so it will be like a shorter exposure. Can it be defined in seconds of exposure? Maybe a dumb question, but I wanted to throw it out there, for the fun of it. Thanks in advance for any help.
Not dumb question at all. It is bit confusing, and I think only way to know is to make two tests. One with some math, and if this math fit the theory, then second test calculated backward. For example, if the 'correct' exposure is 20 seconds for #6, and if you expose film 40 seconds, number #8 should be correctly exposed, while #6 overexposed, and #10 should be underexposed.

Next test is to check to expose 'correctly' some another number. Here I have great doubt what is actually the 'correct exposure' anyway. I suppose this is for AgX emulsions, where sensitivity is pretty linear. Here is the question how DCG (or MBDCG) will respond to that. And, B/W film is one thing, but dealing with interference fringes is something bit different, where math is not so simple (if you ask me, I am completely lost in such math). Actually, this will be very interesting and VERY USEFUL information!

If you do experiment with that card, please post the result here.
The second photo shows a graph of exposure vs. efficiency for a MBDCG grating. I'm seeing this bell-shaped graph more and more frequently with regards to DCG and MBDCG. I want to expose enough, but not too much.
I saw that too, not only for exposure time, but also for some developers for AgX emulsion, where one type of developer has sharp peak, while others has more or less flat top bell shaped graph. Does this means that such flat response allow emulsion to deal with such high contrast between close (overexposed) and far (underexposed) objects? I am asking this because all what I can mix in my chem-lab here currently is only AAC developer (missing whole lot of chemicals for other type of developers).

Found some paper where are analyzed few developers for PFG-01 plates, but I hope that it can be used for other plates as well:

http://rua.ua.es/dspace/bitstream/10045 ... 000pre.pdf

(some formulations starting from page 15)

Btw, graph showed here... is that whole 1 Joule per centimeter squared?! That is a lot of light! And all that for only just bit over 10% of DE?!

Thank you for your input here, it is very interesting.

Best--
m--
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

question about step wedge

Post by Joe Farina »

Thanks for the reply and information, Milan. You're right, some tests need to be made. Probably I'm thinking about it too much.

Yes, that exposure graph for the MBDCG looks like a really lousy result. But if you noticed in the caption it said "pure MBDCG" since they were comparing it to their "improved" system which has another (better) graph. They probably weren't bothered in showing such a miserable result for the "pure" MBDCG, since it made their system look a lot better.
MilanKarakas

question about step wedge

Post by MilanKarakas »

Joe Farina wrote:Thanks for the reply and information, Milan. You're right, some tests need to be made. Probably I'm thinking about it too much.
You welcome. I found myself thinking a lot in past time, not doing practical experimenting. There are many reasons, too long list of it to fit onto the forum.
Yes, that exposure graph for the MBDCG looks like a really lousy result. But if you noticed in the caption it said "pure MBDCG" since they were comparing it to their "improved" system which has another (better) graph. They probably weren't bothered in showing such a miserable result for the "pure" MBDCG, since it made their system look a lot better.
I am exactly interested (and now I know) what is the sensitivity for pure MBDCG (lack of chemicals and informations if something else than TMG can be used instead).

Not only MBDCG, but also red sensitive AgX formulations as well. The problem is with lasers. Red lasers (those around 660 nm or so), are bit cheaper than other. I think that red diode lasers are much more easy to stabilize in order to get decent longitudinal coherence than blue lasers. I hope after all of that holidays continue to finishing all projects started, from thermostat to fringe locker.

In meanwhile tried to answer myself why nobody uses methylene blue for AgX emulsion. Tried to dissolve small amount in water and tried to keep it blue. It is very though job. Small amount of alkaline (NaOH), and it become leuco dye. Adding ascorbic acid did not helped (only when air bubbles are formed due to splash in).

Later I did something and got it blue (don't know what exactly, but I think citric acid does it), but tried apply it to test paper stripes - not a trace of sensitivity for red laser. Silver chloride, for example, is sensitive to some extent to 532 nm without adding any dye. Discovered that paper is not a good media for experimenting, and that AgNO3 reacts with some chemicals and become false 'illuminated', or in other words; when touched AAC developer, no matter was there light or not, it become black or at least dark brown.

Then disposed whole content of MB with lot of additives into chrome plated sink, and got got blue paint over it. :D

I think MB require some chromium in order to be red sensitive?

Best--

m--
Ed Wesly
Posts: 513
Joined: Wed Jan 07, 2015 2:16 pm

question about step wedge

Post by Ed Wesly »

The photographic stop concept is an interesting number system. A full stop brighter doubles the intensity of light; a full stop dimmer halves the amount of light. This can be accomplished by opening up or closing a lens diaphragm one full marked number, if it’s marked with the typical scale of 1, 1.4, 2, 2.8, 4, 5.6, 8, … That number series is based on the square root of two to an integer exponent.

The density number series, the .15 and .3 mentioned above, is based on log base ten. A density difference of .3 changes the amount of light transmitted by a full stop. This is because log base 10 of 2 = .3 and some change.

So with your step wedge, if you assume the clear part, Step 1, acts like a 20 second exposure, then Step 3, .3 more density, cuts down the light by 1/2., so that material exposed under that gets like a 10” exposure. Step 5, density of .6, cuts down the light by a factor of 4, quartering it for an equivalent expo of 5”.

The .15 densities cut down the light by a half stop, which doesn’t translate to ½ the intensity but rather to the square root of two divided by two =.7 to one decimal place. So if step 1 is equivalent to 20”, step 2 is 14”, step 3 = 10”, step 4 = 7”, step 5 = 5”, step 6 = 3.5, step 7 = 2.5”, …

These crazy sets of numbers are rooted in the logarithmic response of the human eye and tone reproduction, and take a while to get used to, but are the jargon of literate photographers and holographers.

As far as your concerns about the step wedge during exposures, there are of course the reflections from the interfaces of the step and the hologram, for more nasty woodgrain. A lot of these wedges are made photographically, and the developed grains can cause nasty scatter. You can check how good your step wedge is before even using it by looking at a spatially filtered beam through it. If it still looks like a spot of light, you’re good; but if there is a haze, or halo, or other indistinctness, you may be losing your spatial coherence and screwing up the cleanliness of the fringes.

Good luck on your endeavor, I really would love to see how well DCG reacts over a very wide range of exposures, also! But 1 mJ/cm^2 is not excessive in the world of photopolymers and photo resists and dichromated colloids!
"We're the flowers in the dustbin" Sex Pistols
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

question about step wedge

Post by Joe Farina »

Thanks for providing such a clear answer, Ed, I was hoping you saw my question.
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

question about step wedge

Post by Joe Farina »

Just to update. Using a step-wedge to "vary" exposures in that manner isn't worth a damn.

However, I found a good way. I took a thin piece of plastic, painted black, and suspended it from the ceiling. This serves as a shutter (placed just after the spatial filter), and measures about 5 inches square. This piece of plastic has two holes punched into it (using a paper punch) in the two upper corners. Into the holes are tied two pieces of nylon thread. Then the threads go up to the ceiling, where two threaded eyelets are attached. After being strung through the eyelets, they go to a convenient place where they can be moved, to raise the shutter up and down. If done with reasonable care, there will be no vibrtation or disturbance during exposure.

This works very well. I've found that exposure time is quite important for MBDCG. I used to think that because it was so slow (a typical exposure with HeNe for a 4 X 5 holo would be ~20 minutes), a couple minutes here or there wouldn't matter. But I used the shutter described above, to do a series of exposures varied by two minutes. And it turns out that two minutes over-exposure decreases the diffraction efficiency noticeably.
Tony DCG
Posts: 147
Joined: Thu Jan 08, 2015 1:47 pm

question about step wedge

Post by Tony DCG »

Joe Farina wrote:This works very well. I've found that exposure time is quite important for MBDCG. I used to think that because it was so slow (a typical exposure with HeNe for a 4 X 5 holo would be ~20 minutes), a couple minutes here or there wouldn't matter. But I used the shutter described above, to do a series of exposures varied by two minutes. And it turns out that two minutes over-exposure decreases the diffraction efficiency noticeably.
Hey Joe,
Drifting away off topic, have you tryed making a master using MBDCG?
Can you make red narrow band holograms?
Jeffrey Weil

question about step wedge

Post by Jeffrey Weil »

You beat me to it Joe, I was going to point out that step wedge won't work well. It's going to cause all kinds of multiple reflection problems, it's going to move and it's probably birefringent. It might even be milky which would record as a single beam transmission, very strong.

What I used to do for that kind of thing was just have a foam core or cardboard thing with a rectangular hole in it. I would just move it across the plate changing the exposure along the way.

Using your string method, the longest exposures are not multi exposures are they? If so that's not good. Each exposure has to be a single one.
Joe Farina
Posts: 804
Joined: Wed Jan 07, 2015 2:10 pm

question about step wedge

Post by Joe Farina »

Tony, I haven't done much of anything yet with mbdcg. The one i did last weekend is posted in the gallery section. Jeff, no, there are no multiple exposures with that shutter. I just move it up (alternately it could go down) like a stage curtain. For example, it moves up 1 inch for the bottom 1/5 of the plate for two minutes, then up another 1/5 (the bottom 2/5ths are now being exposed), for another 2 minutes, and so on. The final "exposure" for the whole plate (including the top 1/5) is longer, say 18 minutes in my case. These would result in "single" exposures (going from top to bottom) of 18, 20, 22, 24, and 26 minutes. Hope i managed to be clear.
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