by Dinesh » Mon Dec 27, 2010 7:15 pm
Joe Farina wrote:Thanks Tony. Hopefully others will chime in on this subject. I do remember someone saying in a book that the closer an image is to the actual plane of the emulsion in the H2, the brighter the image will be in the H2. But I don't know if that's true.
The aberrations of an image are directly dependent on the distance from the image plane. These aberrations are caused by many factors including a mismatch of the light source used for reconstruction and the angle of the reconstructing light. As the image moves away from the image plane, these aberrations cause a blurring and distorting of the image which shows up as a decrease in brightness. So, for a flat image right on the image plane, almost any reconstruction source shows a good bright image (some of you may recall the small "medical" hologram sent out by Polaroid to demonstrate their polymer in the 80s- the image was almost flat and right on the image plane making the polymer look great!). As the image veers away from the image plane, the source needs to be more and more coherent, ie more and more smaller in dimension and mono-coloured. Since the sun subtends about half a degree, it has a high spatial coherence and so shows more depth for a reflection hologram. For a laser transmission H1, the image is a good 6 to 10 inches away (usually) and so the reconstruction source needs to be highly coherent. Sometimes, if the H1 is not too far aay, an LED may have enough coherence to see an outline of the image. I sometimes use this technique when I come across an H1 and ask, "What the H*&%$" is that!" I don't need to fire up the laser, I simply use an LED to find out.
An H1/H2 is brighter because you can control the ratio. The efficiency of a hologram is given by the fringe contrast (What's known as the "visibility"). The fringe contrast is a function of the beam ratio. Ideally, if the beam ratio is 1, you get the best fringe contrast and the highest efficiency. However, most good holograms are about between 40 and 70% efficient, which means that only 40 to 70% the reconstruction source goes to reconstruct the H1, while all of the light goes to reference the H2. This usually results in a low ratio, unless you can kill the H2 ref in favour of the H1 recon.
DCG is not much better than silver for H1's because the fringes (Bragg planes) are perpendicular to the plane of the hologram, not parallel to it as in the case of a reflection hologram. This means that the bandwidth in a transmission hologram results in an angular shift of the image while bandwidth in a reflection hologram results in all of the image re-appearing in the same spot but with different reconstruction colours.
Clean holograms are better because of shadows, as Ed says, and also because of a higher image contrast. Noise in a hologram is usually represented by a halo of light surrounding the image. Such a halo reduces image contrast, making it difficult to differentiate image from not-image. If you want to see noise, invert the beam ratio. That is, make the object light brighter than the reference light. If done properly, the final image looks like some medieval painting of saints - with a halo around them.
[quote="Joe Farina"]Thanks Tony. Hopefully others will chime in on this subject. I do remember someone saying in a book that the closer an image is to the actual plane of the emulsion in the H2, the brighter the image will be in the H2. But I don't know if that's true.[/quote]
The aberrations of an image are directly dependent on the distance from the image plane. These aberrations are caused by many factors including a mismatch of the light source used for reconstruction and the angle of the reconstructing light. As the image moves away from the image plane, these aberrations cause a blurring and distorting of the image which shows up as a decrease in brightness. So, for a flat image right on the image plane, almost any reconstruction source shows a good bright image (some of you may recall the small "medical" hologram sent out by Polaroid to demonstrate their polymer in the 80s- the image was almost flat and right on the image plane making the polymer look great!). As the image veers away from the image plane, the source needs to be more and more coherent, ie more and more smaller in dimension and mono-coloured. Since the sun subtends about half a degree, it has a high spatial coherence and so shows more depth for a reflection hologram. For a laser transmission H1, the image is a good 6 to 10 inches away (usually) and so the reconstruction source needs to be highly coherent. Sometimes, if the H1 is not too far aay, an LED may have enough coherence to see an outline of the image. I sometimes use this technique when I come across an H1 and ask, "What the H*&%$" is that!" I don't need to fire up the laser, I simply use an LED to find out.
An H1/H2 is brighter because you can control the ratio. The efficiency of a hologram is given by the fringe contrast (What's known as the "visibility"). The fringe contrast is a function of the beam ratio. Ideally, if the beam ratio is 1, you get the best fringe contrast and the highest efficiency. However, most good holograms are about between 40 and 70% efficient, which means that only 40 to 70% the reconstruction source goes to reconstruct the H1, while all of the light goes to reference the H2. This usually results in a low ratio, unless you can kill the H2 ref in favour of the H1 recon.
DCG is not much better than silver for H1's because the fringes (Bragg planes) are perpendicular to the plane of the hologram, not parallel to it as in the case of a reflection hologram. This means that the bandwidth in a transmission hologram results in an angular shift of the image while bandwidth in a reflection hologram results in all of the image re-appearing in the same spot but with different reconstruction colours.
Clean holograms are better because of shadows, as Ed says, and also because of a higher image contrast. Noise in a hologram is usually represented by a halo of light surrounding the image. Such a halo reduces image contrast, making it difficult to differentiate image from not-image. If you want to see noise, invert the beam ratio. That is, make the object light brighter than the reference light. If done properly, the final image looks like some medieval painting of saints - with a halo around them.