question about color changes

[Joe Farina]

I've noticed that on some narrowband MBDCG holograms, the reconstructed color can change a lot with angle of illumination/viewing (for example from red to green). On others, the color can stay fairly constant. I'm assuming the fringe structure is getting distorted at some point during processing, and that a relatively undistorted fringe structure would play back a single color over a fairly wide range of viewing angles. The long final baking (say at 60C) causes some slow shrinkage, and I was wondering if the fringes are also getting distorted (which seems likely), and if this is the predominant reason for shifting colors during reconstruction. My layers are around 25 microns thick.

[Din]

It depends on whether you're shooting H1/H2 or single beam Denisyuks. Even so, there may be many reasons for this colour change

Generally, the colour of a reflection hologram is based on the plane separation, angle of recon wrt the reference angle and recon wavelength. There's a formula in Kogelnik connecting plane separation with colour of reconstruction with angle of recon light. But, without any further information, I'm assuming you're shooting a Denisyuk and, as you vary the angle, the angle of the recon beam is changing. As this changes, the recon beam "see" a different plane separation and so, since you're reconstructing with white light, the new plane separation alters the recon colour. Effectively, as you change angle, the planes "pick out" a different component of the white light.

If the colour stays consistent, it's either an H1/H2 setup, or it's a fairly broad band hologram.

I was wondering if the fringes are also getting distorted (which seems likely),

It's possible, but if the planes are getting distorted, then the output wave (the image) will be aberrated. In this case, depending on the amount of distortion, the image will show noise.

I'm assuming the fringe structure is getting distorted at some point during processing,

The processing alters the plane separation, but it doesn't necessarily distort the planes. Remember, in a reflection display hologram, all the planes are fairly twisted to begin with, ie they're already distorted. The nice, neat little parallel lines you see in papers and books is an idealisation. Real planes are never parallel lines all at the same angle. This is why writers who state the infamous tanh squared formula, or any formula, as the "efficiency of the hologram" don't really understand the Bragg structure. The tanh squared function is an idealisation for a non-slanted set of planes, which cannot model a display hologram.

[Joe Farina]

Thank you Dinesh, that helps. These are simple Denisyuk holograms. Some of them keep the same color over a wide angle, but some don't.