Tuning for LEDs

[Dinesh]

Mark,
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.

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 ?

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.

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

[Ed Wesly]

Methinks that the crux of the question asked by Mark is how to tame a wildly divergent LED angular spread to the smaller angle of the reference beam.

To get a clue on how to solve this problem, go to http://physics.bu.edu/~duffy/java/Opticsa1.html and set up a scenario like this screen shot to play with the concept. (Click on Lens, make it a long focal length, click on Source, slide it closer and further from the lens to see how the lens controls divergence.)

Screen shot 2013-05-20 at 6.36.46 PM.png (21.29 KiB) Viewed 2157 times

Everyone is familiar, to some degree, with the fact that the rays of light from a source positioned one focal length away from a lens comes out parallel, the basic principle of collimation. A source positioned greater than one focal length away from the lens will have the output rays converging, which is not what we want. But if the source is placed closer than one focal length away, the output rays are diverging, but not at as big of an angle as they were input!

However, to do this in the real world, we need to first start sketching. Here is a ray trace of a 200 mm (>8”, just to make the numbers easy to work with) tall hologram lit with a reference beam along the normal from 1 meter away.

A lens that will control the divergence of the LED is introduced into the system. What might be a good candidate would be one of the condensing lenses from a 2 ¼” photographic enlarger. They are generally about 80 mm in diameter. The position of the lens from the plate can be calculated, or if one is trigonometry-shy, a scale drawing should suffice.

But the appropriate distance of the LED from the lens needs to be calculated to find its position behind the lens so that it virtually looks like it’s coming from the same 1 meter distance away from the plate, using the simple lens formula. The focal length of the lens needs to be found if scavenged from an enlarger or projector, finding the distance the lens is held from the ground to fry an ant is the time-honored tradition.

Hopefully there is a decent match between LED spread and aperture of the lens at that spot!

Getting the DIY lens set from Surplus Shed might yield some pieces to play with! Popular Optics by Sam Brown, from Edmund, is a great source for all the formulae! Have fun!