Plastic

Steven · Post by **Steven** » Thu Apr 17, 2014 11:32 am

Tony DCG wrote:Thanks Steven, that was extremely helpful.

You are welcome.

Steven · Post by **Steven** » Thu Apr 17, 2014 12:08 pm

142laser wrote:Wow I really learned something reading this. I had NO idea it took any real time for an object to "settle". I suppose is depends heavily on the kind of object and the environmantal conditions. Amazing. Makes some sense though.

My two hour settling time was to allow the DCG glass plate to settle and normalize to the surrounding temperature.
I keep my fresh DCG plates in a light safe (just a wooden box pained black on the inside) in the same room that I shoot the holograms, but I still give them a two hours settling time.

When I'm working on a new composition, I will leave it set up for several hours or a day before attempting an exposure.
Handling the subjects using cotton gloves/tweezers also helps to reduce the transfer of body heat.
My heating in the room is also turned off on shooting days.

Obviously, most of the above precautions can be relaxed considerably if you are just doing an eight second exposure on silver halide.
A twenty minute exposure is 1200 seconds. If there is a slight change in ambient temperature, the object will have plenty of time to respond to the temperature change and move, change size/shape.

DCG is very deaf, even using a laser that pumps out 100mW

Edit to add:

An object that is metallic will tend to take longer to respond (move) to a small change in air temperature due to it's specific heat and thermal conductivity.
Plastic generally, is not a good thermal conductor, so the outside surface of a plastic object can respond (move) more readily to a change in ambient air temperature.

Tony DCG · Post by **Tony DCG** » Fri Apr 18, 2014 8:42 am

Thanks Steven,

Can we discuss the results of various types of movement. Although I've been doing this for years I still get stretches were I get odd results. My biggest fear is that my laser is mode hopping but with the equipment that Phil and Sam let me borrowit seem unlikely be the case but it's still on the table.

I've seen 4 types of issues

One, is just plain dark areas. The object is not seen at any angle
Two, the object has light and dark bands
Three, there is a multiple band of rings from the center of the plate emanating outward as the plate is tilted
Four, there is another type of narrower dark bands that effect one object but not the others

So one and two and four I think are object movement
Three is either mode hopping or glass bending or glass movement

Can anyone verify this?
How do you know if the object moved or the plate?
Temperature vs actual movement?
With mode hoping this is not exactly what I'm seeing the ring are broader. Is this the only way mode hopping can be seen.
http://www.youtube.com/watch?v=hf07F9tpsWw
I can go into more detail about what I see with possible mode hopping if needed.

I did try oreinting the plate and object 90 degrees. I placed a box around everything. I allowed 1 hour settling time. I even glued the film glass to the post.
A bit frustrated
Thanks all

Steven · Post by **Steven** » Sat Apr 19, 2014 2:13 am

Hi Tony,

I have not touched holography as a hobby since about 1985, until starting again last October with DCG, so I'm no expert if you measure it in laser running hours.
Maybe others can pipe in to help?

A photo of your hologram would be a great help for us to identify what's happening, also a photo of your set up.
Are you shooting single beam Denisyuk?

A photo of my larger adjustable (200mm x 200mm) fixture attached.
Are you shooting with your plate facing down on the scene? This is one of the more stable set ups, see my photo.
Notice, that none of the objects are glued down
I have not found it necessary to glue my glass plates to the fixtures I'm using, I just lay them on top and use gravity.

It looks like you are doing the right things, boxing in etc.

My $0.03 from a none expert that's been away from holography for far too long:

"One, is just plain dark areas. The object is not seen at any angle"
This could be object movement or, object placed at a distance from the plate that is more than half the coherent length of the laser, if shooting a reflection hologram.

"Two, the object has light and dark bands"
Maybe slight movement, but the other could be a second source of illumination causing an interference pattern on the object.
The latter would be viewable on the object during exposure. Replace your holographic plate with plain glass and illuminate with your laser. do you see it?
Are you shooting at Brewster's angle?

"Three, there is a multiple band of rings from the center of the plate emanating outward as the plate is tilted"
Funny, I shot a hologram that had a couple of rings emanating from it's centre, I put it down to the glass bending or insufficient drying time after sensitizing.
I only had it occur once though, so it may be the latter, or a drift in laser wavelength. I'm only guessing. Do you have a photo?

"Four, there is another type of narrower dark bands that effect one object but not the others"
Are the "others" closer to the plate than the one with the narrow dark bands?

Sorry but I seem have more questions than answers.

What laser are you using?
What is it's coherent length?
Have you checked it on the table with an interferometer?
Have you observed mode hopping?
Have you observed the fringes drifting?
Are you shooting at Brewster's angle?
What is the depth of the scene you are shooting?

Start again and try and eliminate anything that could go wrong, keep it simple until you have identified the variable that is causing your problems.

Shoot a simple Denisyuk face down using a mat white (or brushed aluminium) surface as your object, supported at three points.
Three nuts can be used as support. Do you get a good hologram, no banding?

Increase the distance between the holo plate and the mat surface to the maximum depth of the scene that you are trying to shoot.
Again, just using three points of contact for support, do you get any banding?

Don't forget to use black tape on the leading edge of the glass, I have forgotten this myself at times.

Without more info, it's difficult to point the finger at the laser, but it is very very tempting.

I'm using a Coherent Compass 315M-100 here and I have identified an issue with it, they are supposed to be the muts nuts.

Steven.

holomaker · Post by **holomaker** » Sat Apr 19, 2014 6:09 am

Movement can be so difficult to chase down, especially when very slight! Shading can happen when your object shifts ever so slightly, as I've stated prior, place some type of box or corral oarround your entire denisyuk setup, allow plenty of settle time (overnight) and see what happens, if issues are still present try to shorten your exposure by narrowing your spot , many times I had to renforce the object by hiding black plastinene clay behind the object and allowing PLENTY of time to settle. I doubt mode hopping is a issue as theses would most likely be very fine banding (as seen in your beautiful white dcg bust) .

BobH · Post by **BobH** » Sat Apr 19, 2014 9:10 am

I'd recommend using a baffle card properly placed to cast a shadow in the reference beam onto the edge of the plate, instead of using black tape attached to the plate. Black tape absorbs energy and expands, possibly affecting the recording. It also eliminates a step to do for each plate loaded.

Steven · Post by **Steven** » Sat Apr 19, 2014 1:14 pm

I suspect that the characteristics of the banding would be determined by the size of the mode hop, if the laser was mode hopping.
Smaller change in wavelength = wider banding???

Giving it a bit more thought, banding on the object can only be caused by some sort of double exposure.
Either:
(1)The the object/plate moved, so that it was recorded in at least two different fixed positions. I don't think slow movement would cause this as the banding would be moving during exposure. (2)There was more than one source illuminating the object creating the interference pattern on the object.
(3)A mode hop, effectively giving the object two exposures, but at different wavelengths.

Maybe another possibility if the laser is not actually mode hopping in the usual sense of the term, is a slow frequency drift in the laser between two stable operating areas, not an actual quick jump. Is enough time being given for the laser to warm up/stabilize?

My finger is still tending to point at the laser, but without more info, or a photo, I'm not willing to put money on it.

Steven.

Tony DCG · Post by **Tony DCG** » Sun Apr 20, 2014 5:11 pm

Here I hope is is a photo of the ring
Sorry if the photo is of poor quality

The rings start in the almost center and are about an half inch apart
The Laser is a Melos Griot 457nm 200mw elipitcal beam

Tony DCG · Post by **Tony DCG** » Sun Apr 20, 2014 5:19 pm

: move.jpg (32.71 KiB) Viewed 4219 times

Here is a photo that shows my material test. I stuck on various plastic pieces on a metel base and as you can see there are dark areas.
The thinner plastic is problematic. The upper right hand corner is a girls face, note the darkness around her eyes.
I hate photos.....

Today I shot and most of the shots were good except as I tilted the plate there was very very slight dimming.

Thanks all

holomaker · Post by **holomaker** » Sun Apr 20, 2014 6:17 pm

Thanks for the pix Tony, my vote will go to plate warpage or plate movement? Just for kicks coat a 1/4" thick piece of glass and see what comes, and again try to place a box over it all w the hole in it.....