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Posted: Fri Apr 12, 2019 6:02 pm
In this post I display a photograph of an Australian black opal with strong red play-of-color sitting on a black acrylic plate. The elliptical opal is about 1 cm along its major axis. In the photograph, the opal is illuminated by a white light LED from below, tilted toward the opal at an angle of about 30 degrees from the plate upon which the opal rests. The reason for posting the picture of the opal is for comparison with a reflection hologram of the opal displayed in the next post.
Posted: Fri Apr 12, 2019 6:15 pm
So here I display a photograph of a reflection hologram of the Australian black opal with strong red play-of-color. The hologram is illuminated by a white LED light almost exactly as the opal itself was. So... seems like no big deal, a reflection hologram made using a red laser. But take a closer look... notice the white light reflection along the lower left edge of the opal. How was that reproduced?
Of course, the opal's play-of-color arises from light wave interference. So a hologram of an opal is an interferogram of an interference effect.
The cool optics about opals is, of course, their play-of-color... colors flashing and changing as you change the orientation of the light source or the orientation of your eye. You can't reproduce this effect with a still photo. I don't have a nice movie to demonstrate, so you'll just have to believe me: as long as the light source is illuminating from somewhere below the hologram, the hologram does reproduce the opal's play-of-color.
Posted: Thu Feb 06, 2020 10:40 pm
Nice work, would love to see it with green laser as well!
Posted: Fri Feb 07, 2020 9:22 am
You have the same white light speck in the photograph of the opal, but the white light on reconstruction is more pronounced. There are also specks of white on the acrylic in the reconstruction. How was the opal illuminated during recording? If it was a shadowgram, the white light could be a 'hole' in the opal where white light passes straight through.
By the way, an opal gets it's colours because of tiny voids in the interior. These voids act as diffractive structures, so, it works much like a dcg hologram. In a dcg hologram, the image appears in only one bandwidth, and disappears when you turn the hologram. When you turn the opal in white light, you get flashes of different colours. This is because the voids in a dcg hologram lie along the Bragg planes caused by the recording angles, while in the opal, the voids are randomly scattered.
Posted: Fri Feb 07, 2020 12:35 pm
I'm afraid I've made a grievous error, for which I will forebear drinking Merlot for at least, oh about an hour. As the wife pointed out, opal does not have voids, it has solid spheres. These spheres act as diffractive structures, so it's the opposite of dcg, which has voids as diffractive structures
Here's an SEM of the opal structure: