by Din » Tue Mar 26, 2019 8:49 am
Nana wrote: ↑Mon Mar 25, 2019 10:14 pm
1) what is the difference in setup between diffraction gratings and transmission holograms?
A diffraction grating is simply two beams, no object. You illuminate the plate with two beams and you have a grating. If the two beams are from the same side, you have a transmission grating, also called a 'surface grating'. If the beams are from opposite sides, you have a reflection grating, also called a 'volume grating'. So, they're both 'transmission holograms', but in one case you're scattering light off an object and the recording is the light from the object, and in the other case, the recording is just two beams.
Nana wrote: ↑Mon Mar 25, 2019 10:14 pm
Regarding playback of transmission holograms, I hear that you should be able to play them back with a laser even before drying. (Integraf pfg-03m processed with jd-4.) Is this entirely true
Yes, but at a different angle. The light refracts through the wet plate, so you have to change the replay angle.
Nana wrote: ↑Mon Mar 25, 2019 10:14 pm
If the frequency of the laser pointer is slightly different than the original, will that also make it impossible/harder to find the real image?
No. For a transmission hologram any light source will show an image. If the source is reasonably coherent, like a laser, then any laser will show an image, if there is one.
Nana wrote: ↑Mon Mar 25, 2019 10:14 pm
Regarding diffraction gratings: first of all, is "diffraction grating" synonymous with "HOE"?
Not synonymous. A "HOE" is a Holographic Optical Element, any simulation of any optical element using holography is a HOE. A transmission grating is an optical element using holography, so a transmission grating is a HOE. But, it's one possible HOE, there are many others. For example, you can reecord a reflection grating with power, so that the hologram acts like a focusing mirror; this also is a HOE (in fact, these kinds of HOEs are used in AR/VR goggles).
One problem in your setup may be that the plate is 'seeing' very little object light. According to your drawing, most of the light reflected from the object is reflecting away from the plate. It should be reflecting to the plate, not away from it. I would suggest carding off the actual reference, ie put a card in front of the beam until you block the reference and only the object is illuminated. Then, look at the object through the plate. If it looks dim, then most of the object light is not hitting the plate. The ratio of object light to reference should be about 3:1 or 4:1, ie the reference should be about 4 times, or 3 times, as bright as the object. If the object is dim, the ratio is too low.
[quote=Nana post_id=70673 time=1553570046 user_id=5525]
1) what is the difference in setup between diffraction gratings and transmission holograms?
[/quote]
A diffraction grating is simply two beams, no object. You illuminate the plate with two beams and you have a grating. If the two beams are from the same side, you have a transmission grating, also called a 'surface grating'. If the beams are from opposite sides, you have a reflection grating, also called a 'volume grating'. So, they're both 'transmission holograms', but in one case you're scattering light off an object and the recording is the light from the object, and in the other case, the recording is just two beams.
[quote=Nana post_id=70673 time=1553570046 user_id=5525]
Regarding playback of transmission holograms, I hear that you should be able to play them back with a laser even before drying. (Integraf pfg-03m processed with jd-4.) Is this entirely true
[/quote]
Yes, but at a different angle. The light refracts through the wet plate, so you have to change the replay angle.[quote=Nana post_id=70673 time=1553570046 user_id=5525]
If the frequency of the laser pointer is slightly different than the original, will that also make it impossible/harder to find the real image?
[/quote]
No. For a transmission hologram any light source will show an image. If the source is reasonably coherent, like a laser, then any laser will show an image, if there is one.
[quote=Nana post_id=70673 time=1553570046 user_id=5525]
Regarding diffraction gratings: first of all, is "diffraction grating" synonymous with "HOE"?
[/quote]
Not synonymous. A "HOE" is a Holographic Optical Element, any simulation of any optical element using holography is a HOE. A transmission grating is an optical element using holography, so a transmission grating is a HOE. But, it's one possible HOE, there are many others. For example, you can reecord a reflection grating with power, so that the hologram acts like a focusing mirror; this also is a HOE (in fact, these kinds of HOEs are used in AR/VR goggles).
One problem in your setup may be that the plate is 'seeing' very little object light. According to your drawing, most of the light reflected from the object is reflecting away from the plate. It should be reflecting to the plate, not away from it. I would suggest carding off the actual reference, ie put a card in front of the beam until you block the reference and only the object is illuminated. Then, look at the object through the plate. If it looks dim, then most of the object light is not hitting the plate. The ratio of object light to reference should be about 3:1 or 4:1, ie the reference should be about 4 times, or 3 times, as bright as the object. If the object is dim, the ratio is too low.