I've always wondered how materals like Photopolymer or even silver plastic film holograms can can survive bending.
Fringes are like 5000 lines per mm so like .2um spacing. I find it hard to visulaize that the bending a plastic hologram can still preserve the image. Obviously it does since I see occuring.
Can anyone put this in perspective for me?
Thanks
Photopolymer Film Question
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Dinesh
Photopolymer Film Question
I'm not sure why you think bending the plastic will destroy the image. Because of the stretch of the fringes, or a crease in the fringe structure?
First of all, you have to take the two conditions of the fringe structure: surface and volume. In the case of a surface hologram, the data is encoded as a series of hills and valleys on the surface of the emulsion. These hills and valleys are typically about a micron or so (most pure surface materials can't take much above 1000 - 1500 l/mm). In this case, so long as the bend radius was greater than a micron or so, bending the film will have no effect (here I'm assuming that distortion occurs when one fringe subtends approximately a radian at the centre of the roll). If the bend radius was smaller than a micron or so, then you may distort or even damage the hills and valleys. If the material stretched, then stretching the hills and valleys would effectively decrease the spatial frequency and so change the reference angle and cause aberrations in the image. The image would not stretch, it would just distort - a focused point image would spread into something that looked like a bloated tadpole. Of course, the plastic itself may not be capable of bending into a radius of a micron because the thickness of the plastic is over a thousand microns, so there's no real way you can damage the image data, ie the hills and valleys that encode the image.
In the case of a volume hologram, the fringes are within the emulsion itself. Whatever happens to the gelatin (or polymer) base also happens to the fringes. But this time the fringe separation is about 0.3 microns. So, using the same criterion as above for distortion occurring at about a radian, you'd have to bend the plastic with a bend radius of less than 1/3 micron to affect the fringes. Again, the plastic itself is thousands of microns thick and so cannot be bent to such a small radius.
If you folded the plastic over and creased it, you'd damage the plastic along the crease and, maybe, affect a few of the fringes, but overall both halves of the fold will still have the fringe structure undamaged. On a separate note, you cannot cut a hologram in half and see the same image on both halves!
How small can you bend a plastic substrate and how will that affect the fringes? Well, assume that you can roll the plastic into something the size of a cigarette. In this case, the bend radius is about 3 mm. One fringe with a thickness of a micron (a surface holograms) will subtend an angle of 1/3000 radians (about 0.018 degrees) at the centre of the bend and a fringe with a thickness of 0.3 microns will subtend 1/10,000 radians (about 0.005 inches). I would say that this is too small to affect the fringe.
First of all, you have to take the two conditions of the fringe structure: surface and volume. In the case of a surface hologram, the data is encoded as a series of hills and valleys on the surface of the emulsion. These hills and valleys are typically about a micron or so (most pure surface materials can't take much above 1000 - 1500 l/mm). In this case, so long as the bend radius was greater than a micron or so, bending the film will have no effect (here I'm assuming that distortion occurs when one fringe subtends approximately a radian at the centre of the roll). If the bend radius was smaller than a micron or so, then you may distort or even damage the hills and valleys. If the material stretched, then stretching the hills and valleys would effectively decrease the spatial frequency and so change the reference angle and cause aberrations in the image. The image would not stretch, it would just distort - a focused point image would spread into something that looked like a bloated tadpole. Of course, the plastic itself may not be capable of bending into a radius of a micron because the thickness of the plastic is over a thousand microns, so there's no real way you can damage the image data, ie the hills and valleys that encode the image.
In the case of a volume hologram, the fringes are within the emulsion itself. Whatever happens to the gelatin (or polymer) base also happens to the fringes. But this time the fringe separation is about 0.3 microns. So, using the same criterion as above for distortion occurring at about a radian, you'd have to bend the plastic with a bend radius of less than 1/3 micron to affect the fringes. Again, the plastic itself is thousands of microns thick and so cannot be bent to such a small radius.
If you folded the plastic over and creased it, you'd damage the plastic along the crease and, maybe, affect a few of the fringes, but overall both halves of the fold will still have the fringe structure undamaged. On a separate note, you cannot cut a hologram in half and see the same image on both halves!
How small can you bend a plastic substrate and how will that affect the fringes? Well, assume that you can roll the plastic into something the size of a cigarette. In this case, the bend radius is about 3 mm. One fringe with a thickness of a micron (a surface holograms) will subtend an angle of 1/3000 radians (about 0.018 degrees) at the centre of the bend and a fringe with a thickness of 0.3 microns will subtend 1/10,000 radians (about 0.005 inches). I would say that this is too small to affect the fringe.