Film Orientation

Tony · Post by **Tony** » Fri Jan 13, 2012 10:24 am

I read and heard that which way the DCG plate is placed in relationship with the beam makes a difference. Film facing towards the object or way alters I believe the bandwidth.

My question is why? And throw in a how?

Many thanks,
Tony

dannybee · Post by **dannybee** » Fri Jan 13, 2012 10:52 am

well you have the retention on the gel on glass.....as the light pass thro the glass it will effect the gel next to the glass first then the outside ....so ther will be alittle warping of the bandwith giving you a more of a white blend

Tony · Post by **Tony** » Fri Jan 13, 2012 11:41 am

Thanks Danny

Interesting. So the thickness of glass would also make a difference.

Dinesh · Post by **Dinesh** » Fri Jan 13, 2012 2:20 pm

Tony wrote:I read and heard that which way the DCG plate is placed in relationship with the beam makes a difference. Film facing towards the object or way alters I believe the bandwidth.

My question is why? And throw in a how?

Well, bandwidth has to do with differential swelling. When you first shoot the hologram, all the planes are an exact distance d apart (d = lambda/(2*n*sin(theta)) where theta is to the hologram face and n is the index of the material. If the beams are counter-propagating, theta is 90 degrees and so the planes are d = lambda/(2*n*sin(90)) = lambda/(2*n) ). This is known as the latent image.

When you dunk the hologram into water, the water hits the front surface of the gel and starts swelling it. By the time the water penetrates to the lower part of the gel (nearest to the glass) the top surface has been swelling all the time. So, the top surface of the emulsion has been swelling for longer than the lower depths of the emulsion. This separates out the planes on the top of the emulsion more than the planes lower down in the emulsion. Now you have a range of plane-separations and so a range of replay wavelengths. However, as the light penetrates into the emulsion, it also gets absorbed in the emulsion (obviously! Without absorption there would be no actinic reaction). So, less light hits the far end of the emulsion than the near end relative to the beam, depending on the ratio. This causes a loss in modulation, which also affects the water swell. Which is the "far" end of the emulsion and which the "near" end depends on how the light goes in - from the glass side or the emulsion side.