Really, the simplest thing is to place the led at the same position as the origin of the reference beam. The problem I see is that the light coming off the led may be wider than the angle of the ref so the light spills over the edges of the hologram and so wastes light. In this case, get a narrower led. I agree with Joe that getting a bunch of leds and simply trying them is probably the best approach at first. Be sure that the colour of the led is close to the colour of the hologram. A hologram shot on silver has a bandwidth from 20 to 40 nm and a led has a bandwidth of about 50nm. So, it's quite possible that you have a hologram that looks green and an led that looks green, but their peaks might miss. Ten years ago, when we were first experimenting with led illumination for holograms, the choices were not that great; the colours were limited and the brightness was nothing to write home about. These days, they're much brighter with a much larger range of colours, and you don;t have to mess around with resistor values and drivers.
If you want a compact small box, like the one above, then you need to have the source of the ref also very close up, since the led must be in the same position as the ref source. This means you need to diverge the beam pretty fast on the table. As for lenses, I'd think that a lens on the table for the ref would be better, since you have to widen the beam to the plate quite a ways very fast. It's easier to find the source point of the ref and place the led there, rather than messing around with lenses in the display unit. If you do use a lens on the table, be sure to estimate/measure where the source point for the ref is. If you're using a wide angle objective, such as a 60X, then the source point is at the lens itself, but if you're using a negative lens, the source point will be behind the lens, not at the lens. If you're using a positive lens on the table, then use a card or something to find where the light focuses before diverging out. This is the source point for the ref. By the way, all of this holds true even if you're not diverging very fast. If you're ref is collimated, then you need to use a lens to collimate the output of the led. Having said that, it may not be necessary if you have room to place the led a long way away.holomark wrote: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 ?
The big advantages of a led is that it produces a very clean image (less glare) and it shows more depth because of the greater coherence of the led. So, if you're making deep imagery, then an led may be the recosntruction