Film Thickness

Dichromated Gelatin.
Tony

Film Thickness

Post by Tony »

Thanks Bob that makes sense

I tryed again last night in shifting my yellow holograms to blue. This time it worked much better. Maybe it need to age a bit more.

Strange stuff :?
Dinesh

Film Thickness

Post by Dinesh »

Don't forget there's also the latent image difference. Assuming you're shooting a reflection with the ref at Brewster's with p polarisation. This means that all the light from the reference beam hits the back of the emulsion - the side attached to the glass. Now, you measure, or otherwise determine that your ratio is 1;1 at the film plane, ie at the surface of the dcg. This means that you have (ideally) I unit of light on the glass side of the plate from the ref and, on average, 1 unit of light on the surface side of the dcg emulsion (this is for a single beam Denisyuk setup). Now, the light hitting the back of the emulsion propagates to the front of the emulsion and the light hitting the front of the emulsion propagates to the back. At every plane within the emulsion, there are now two light waves - one from the back propagating forward and one from the front propagating backwards; also, both are initially 1 unit. However, the dcg itself is also absorbing the light (if it didn't you wouldn't be able to record anything!). So, the light going from front to back is getting weaker as it goes to the back and so is the one from the back to front as it goes to the front. This means that any given plane within the emulsion, the two beams have a different ratio, unless by some amazing chance the rate of absorption of both beams is exactly equal, since absorption is exponential this is probably unlikely; the actual rate of dissipation/absorption of the light depends on the concentration of the dichromate. This results in a different delta n inside the emulsion for each plane - not by much, but a little different - and so a different hardness. Now you put it into the various liquids and the varying hardnesses process the various planes in different ways and also add to this the differential swelling mentioned by everyone, ie the face of the emulsion starts to swell before the back.

In a "thin" hologram, there's not too much absorption as the beams propagate through the emulsion and there's not too much differential swelling, so the Bragg planes are pretty much all of equal distance and equal strength and you get narrow band. In a "thick" hologram, the light absorption effect is more pronounced and so is the differential liquid absorption. This may result in a large variation of Bragg plane distances and so a broad band. However, as John says, "thick" and "thin" are relative. There are papers from Denisyuk and Nadya where they expose emulsion a millimetre thick!
Paulos
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Joined: Thu Jan 08, 2015 11:46 am

Film Thickness

Post by Paulos »

tony wrote: ..... The orginal question was thick film results in narrow band while thinner was BB.
Your results indicate the opposite.
I found the same thing, I thicken up my film I observed more BB deeper yellow.
Adding more water (making the film thinner) I got a greener result that also was easier to reprocess to blue.
The bandwidth is inversely proportional to the material thickness. It also strongly depends on the fringe spacing :
The following equation is from an old paper:
Δλ½=1,77*(Λ² × n × cosθ) / d
Δλ½: width at half max.
Λ: grating spacing
n: refr. index
θ: angle inside the emulsion
It also explains quite clearly why transmission are broad-, whereas reflection holograms are narrow band.
I cover my own AgX plates and I can say for sure, that thicker holograms are indeed narrower.
BobH wrote:In general, thicker material means narrower spectral bandwidth. That's true regardless of material. More thickness means more filtering. When the material is very thick, one might have the ability to chirp the fringes inside it by re-processing techniques like for DCG, but that's a different mechanism. No other recording materials offer the same manipulation.
I never made a single DCG, but I think that BobH explained very well what and why it is happening.
holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Film Thickness

Post by holomaker »

Not to cause confusion to this subject, but during this past week @ Isdh, i had a long conversation with Auggie about this very same subject and what i got out of it was in practiaity thickness dose not play a role in narrowbanding only fringe spacing, this was my original observation and now reconfermed by August Muth ..........

While haveing a thicker DCG film mean it may be better/brighter @ reflecting a narrow band, but this may also bring up the noise level as well ? :roll:
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Film Thickness

Post by Joe Farina »

holomaker wrote:While haveing a thicker DCG film mean it may be better/brighter @ reflecting a narrow band, but this may also bring up the noise level as well ? :roll:
Muth's paper (SPIE 5290, p.148) has the following:

"By increasing the emulsion thickness the shelf life is increased and the bandwidth and noise of the resulting hologram is decreased."

Don't know why the noise would be reduced however.
Dinesh

Film Thickness

Post by Dinesh »

holomaker wrote:Not to cause confusion to this subject, but during this past week @ Isdh, i had a long conversation with Auggie about this very same subject and what i got out of it was in practiaity thickness dose not play a role in narrowbanding only fringe spacing, this was my original observation and now reconfermed by August Muth ...
Actually it is fairly confusing because there's no single beast in dcg as there is in silver. Dcg has such a vast dynamic range and so many parameters that you can pretty well place whatever characteristic on it, regardless of any other characteristic! You have the relative concentration of the dichromate/gelatin, you have the type of gelatin, you have the drying time, the drying humidity (what was the humidity while the plates were drying?), the dark reaction rate (dependant on, amongst other things, the temperature), the pH etc etc etc. This is before you've actually shot anything. Once the film has been exposed the variables mount: the exposure, the wavelength, the delta n/delta rho factor (rho being the density), the temperature and humidity of the lab while the exposure is happenning etc etc. Then you process: now there's the amount of pre-hardening, the actual pre-hardening agent, the temps and humidity of the processing room etc etc. So, the actual thickness of the film is one amongst many different parameters, depending on what you intend to do. In our case, for example, our emulsions are pretty standard at about 12 microns. However, for display we tune them pretty broad band and they turn out a bright orange (the broader the bandwidth, the brighter the hologram for obvious reasons) but we've tuned them to an off-white colour. However, for tech work, I've got them as narrow as 5nm and as broad as 30nm all across the spectrum - all with the same thickness. In other words, we've got a range of about 10nm bandwidth to a 30 nm bandwidth at every wavelength range from 780,to 450. We've even got multiple reflection gratings at various parts of the spectrum at various bandwidths; say the customer wants the following: three bandwidths of 10nm centered at 630, 20nm centered at 530 and 25 nm centered at 480, all reflecting at different, but given, angles. We can do this on the same 12nm thick emulsion (While these wavelengths and bandwidths are not what we actually did - can't give specifics of a customers spec for obvious reasons - these are pretty close to what we already did). The point here is that, as I mentioed, the thickness is but one of several parameters and, for dcg, you may need to manipulate all those parameters.

Augie only does mainly broadband display stuff. Insofar as that is concerned, he's right, thickness doesn't matter.
Joe Farina wrote:Don't know why the noise would be reduced however.
Because noise is spurious light at random directions. As the noise propagates through the emulsion, it impinges on the various Bragg planes and, since noise is usually off-Bragg, it scatters within the emulsion. What does not scatter within the emulsion exits the emulsion and is seen as noise. If the emulsion is thick enough, none of the random, spurious light makes it out of the emulsion. The Bragg planes act as a sort of low frequency filter.
Joe Farina
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Joined: Wed Jan 07, 2015 2:10 pm

Film Thickness

Post by Joe Farina »

Dinesh wrote:
Joe Farina wrote:Don't know why the noise would be reduced however.
Because noise is spurious light at random directions. As the noise propagates through the emulsion, it impinges on the various Bragg planes and, since noise is usually off-Bragg, it scatters within the emulsion. What does not scatter within the emulsion exits the emulsion and is seen as noise. If the emulsion is thick enough, none of the random, spurious light makes it out of the emulsion. The Bragg planes act as a sort of low frequency filter.
That's very interesting, thanks Dinesh.
Tony

Film Thickness

Post by Tony »

My coffee taste much better today.
Thanks Dinesh, nicely explained.
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