Inspired by Tony and John who contributed compact hologram and lighting systms...
Until now I have tried to shoot holograms that are best lit/replayed with track style lighting with low dispersion...
So if I understand the basic idea we want to match the reference beam with the light source used to replay the final hologram. In order to cover the full final hologram from a short distance we must have a light source with a wider angle than when using a light source at distance of 1 meter.
Now, what exactly am I trying to match. I presume I need to get a new diverging lense, but which ones? Am I trying to match the divergence rates of the reference beam and replay light source?
Any suggestions of a light and lense combinaion, sources of materials... Now I have some small dots lying around, anyone wish to share some techniques?
Mark
Tuning for LEDs
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Dinesh
Tuning for LEDs
You match both the direction and the divergence of the original reference beam. If the reference beam were near the plate and had a wide divergence, you place the light source near the plate and also with a wide divergence. If the source were far away, you place a led far away and try to match divergence. If the beam divergence were small, you may need a lens to alter the beam divergence of the led to make it smaller. Take a couple of examples.
Let's say you're shooting on a plate that's 5" across from a point source that's 24 inches away and at an angle of 60 degrees. In this case, the beam divergence of the ref is 6 degrees (assuming you're covering just the plate, if you're not, you need to recalculate for the beam spread). To reconstruct this hologram, you need a source of light that's 24 inches away, at 60 degrees to the hologram and diverging by about 6 degrees. If you used a standard led to reconstruct, then most leds have a beam divergence of about 20 to 40 degrees (it's usually on the spec). This will cause the image to bloat and curve outwards, as if it was on the surface of a ball. You could place a led further away than 24 inches and place positive lens between the led and the hologram, such that the light from the led focused and then diverged to the required 6 degrees, but you'd have to calculate the lens parameters.
Now, you have the same plate and the same angle, but the point source of the ref is only 5 in away.So, a point source at 60 degrees to the normal 5in from the plate. In this case, the beam divergence is 18 degrees. In this case, the led with a divergence of about 20 degrees placed at 60 degrees and 5in away would need no lens.
This is all assuming a single beam Denisyuk. For other geometries, the calculations are a little more complicated.
Let's say you're shooting on a plate that's 5" across from a point source that's 24 inches away and at an angle of 60 degrees. In this case, the beam divergence of the ref is 6 degrees (assuming you're covering just the plate, if you're not, you need to recalculate for the beam spread). To reconstruct this hologram, you need a source of light that's 24 inches away, at 60 degrees to the hologram and diverging by about 6 degrees. If you used a standard led to reconstruct, then most leds have a beam divergence of about 20 to 40 degrees (it's usually on the spec). This will cause the image to bloat and curve outwards, as if it was on the surface of a ball. You could place a led further away than 24 inches and place positive lens between the led and the hologram, such that the light from the led focused and then diverged to the required 6 degrees, but you'd have to calculate the lens parameters.
Now, you have the same plate and the same angle, but the point source of the ref is only 5 in away.So, a point source at 60 degrees to the normal 5in from the plate. In this case, the beam divergence is 18 degrees. In this case, the led with a divergence of about 20 degrees placed at 60 degrees and 5in away would need no lens.
This is all assuming a single beam Denisyuk. For other geometries, the calculations are a little more complicated.
Tuning for LEDs
Ah! A rookie mistake! We don't see many of those from you, Dinesh.Dinesh wrote:...
In this case, the beam divergence of the ref is 6 degrees (assuming you're covering just the plate, if you're not, you need to recalculate for the beam spread).
...
The overall beam spread doesn't matter, just the perspective from the plate's point of view. Any point source of light illuminating a 5" plate from 24" away will have a 6 degree spread at the plate. If the light source illuminates far more than just the plate, it makes no difference: 6 degree spread at the plate.
World's worst holographer
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Dinesh
Tuning for LEDs
Oops! This is what happens when I try to answer BC (Before Coffee).
Yes, you're right. Any point source under the conditions given would subtend 6 degrees at the plate. That is, the plate would subtend 6 degrees, so the from the plate's perspective it "sees" 6 degrees.The wider the source, the less light at the plate and so the dimmer the hologram. However, in terms of aberrations, it doesn't matter. I try to match the beam divergence of the led to the beam divergence of the original reference beam to get the maximum possible light on the plate, so I suppose I was trying to think how I would do it.
Yes, you're right. Any point source under the conditions given would subtend 6 degrees at the plate. That is, the plate would subtend 6 degrees, so the from the plate's perspective it "sees" 6 degrees.The wider the source, the less light at the plate and so the dimmer the hologram. However, in terms of aberrations, it doesn't matter. I try to match the beam divergence of the led to the beam divergence of the original reference beam to get the maximum possible light on the plate, so I suppose I was trying to think how I would do it.
Tuning for LEDs
I still do not fully understand you, Dinesh. Do you assume a LED to be a point light source limited by an aperture, or an area light source that limits its light spread by interference effect? In the first case, only LED distance from the hologram should matter because the wavefront curvature always depends just on this distance. The second case... well, my knowledge of mutual coherence is too weak! I even cannot imagine how a wavefront of a (incoherent) LED light looks like!
Petr
Petr
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Dinesh
Tuning for LEDs
I meant an led limited by an aperture, your first case. Yes, both jsfisher and my wife (twice!) reminded me that the wavefront curvature depends only on distance. As I explained, I try to limit the "aperture" so that the entire light from the led hits the plate, since the wider the spread, the less the beam hitting the plate and the weaker the hologram. So I work "backwards". I determine the angular spread of the led, and then tune my ref to match. For example, in this shot, I used an led of known divergence, then formed my ref accordingly. The led is in the wooden base in front, and underneath, the hologram. I must have been thinking of this, when I stated that you need a lens to limit the aperture. In fact, yes, in terms of the curvature of the wavefront, only the distance matters (I get the feeling I'm not going to live this one downlobaz wrote:I still do not fully understand you, Dinesh. Do you assume a LED to be a point light source limited by an aperture, or an area light source that limits its light spread by interference effect? In the first case, only LED distance from the hologram should matter because the wavefront curvature always depends just on this distance. The second case... well, my knowledge of mutual coherence is too weak! I even cannot imagine how a wavefront of a (incoherent) LED light looks like!
Petr
):Tuning for LEDs
I was perfectly happy to raise it just the one time; wives, on the other hand, can be a mean and vicious lot.Dinesh wrote:(I get the feeling I'm not going to live this one down
World's worst holographer
Tuning for LEDs
So, what diverging lenses should I purchase?
How do I measure the relevant divergence? Say I have an LED with a beam spread of 120 degrees. If only a portion of this is needed to cover the plate would my reference beam need to be different using the LED as is versus with an aperture. (by aperture I presume you are talking about a reflective cone or other aperatus to shape light output --- or are you taking about a lense system used to change beam spread such as I have with my techline flashlight)
Wavefront curvature? does this mean I need to match the distance LED will be placed from hologram to distance of diverging lense from plate?
Dinesh, can you share what LED you used and lenses used to make ?
Mark
How do I measure the relevant divergence? Say I have an LED with a beam spread of 120 degrees. If only a portion of this is needed to cover the plate would my reference beam need to be different using the LED as is versus with an aperture. (by aperture I presume you are talking about a reflective cone or other aperatus to shape light output --- or are you taking about a lense system used to change beam spread such as I have with my techline flashlight)
Wavefront curvature? does this mean I need to match the distance LED will be placed from hologram to distance of diverging lense from plate?
Dinesh, can you share what LED you used and lenses used to make ?
Mark
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Joe Farina
- Posts: 805
- Joined: Wed Jan 07, 2015 2:10 pm
Tuning for LEDs
Wise manDinesh wrote:I plead the fifth!
To incorporate a LED into a display system for holography, I would suggest not getting lost in the details of divergence, etc. in the beginning. I think more can be learned from experiments. Of course, it's always good to know what's causing a distortion, so it can be corrected if needed. I would just get a good LED, one that fulfills all of your requirements, and buy a decent quantity. Then, begin to tailor your system around the LED. Then design and execute the hologram, tailored around the display system.
If you take an ordinary hologram, and start playing a LED flashlight around it, it's obvious that a lot of variations are possible, yet a good image can still be obtained (i.e., from different angles and distances from the LED). The type of image will also affect how apparent any distortions are. If the viewer is very familiar with the shape of an object, and the object is large (filling the hologram) then distortions will be more apparent.
My friend once told me "Prove you have a problem, and fix the problem. Otherwise, life will pass you by." I have tried to adopt this approach, and it has always been a success.