[Laser] Laser Digest, Vol 71, Issue 6

[Chris L]

Side-emitting Luxeons, which I have used, will be no of use whatever for collimation; and a torch reflector, even a deep one of very low f/D ratio, will have nowhere near the accuracy of a cheap Fresnel lens in terms of collimation. Most modern Fresnel lenses, unlike simple glass lenses, are corrected for spherical aberration. As the source is monochromatic, chromatic aberration is of no concern. If you use the side-emitting Luxeon "bare" as a beacon, the tiny effective size of the emitter will produce very marked scintillation effects in atmospheric turbulence, if it's received at any appreciable distance from the source.

When Luxeons were first marketed, in 2002, Mike VK7MJ and I bought samples of the three types of red Luxeon then available:

(1) Side emitting (which proved to be quite unsuitable for collimation by any satandard optical system).
(2) Batwing (a type of relatively low efficiency, with a standard type of cubic LED chip).
(3) Lambertian, with the truncated inverted pyramid ("TIP") chip.

Only the Lambertian type proved to be suitable for collimation into a broad (in cross-section) but minimally divergent beam.

In comparison with cheaply available Fresnel lenses - like the 20 cm by 28 cm "sheet magnifiers" at Aust. $2 each - most glass optics are not, comparatively, large - and it's collimator aperture, rather than f/D ratio, which largely determines the amount of flux received by the distant photodetector. If you'd like the maths of this, I can provide it.

Best wishes,

Chris Long VK3AML.

=======================================

> Date: Thu, 16 Sep 2010 18:34:48 +0100
> From: b.chambers at sheffield.ac.uk
> To: laser at mailman.qth.net
> Subject: Re: [Laser] Laser Digest, Vol 71, Issue 6
> 
> Dear All,
> 
> Many thanks for all the information on the best way forward and the new 
> LEDs now becoming available. I always enjoy reading submissions to the 
> Digest even though I don't often post comments!
> 
> I already have a small stock of LuxeonIII LEDs which were bought several 
> years ago and so I'll try these out just to get me used to working with 
> LEDs rather than lasers. The new Luminus devices will have to wait until 
> next spring when the next rebuild season starts!
> 
> Has anyone played with the side emitting Luxeons and large torch/strobe 
> reflectors (I'm thinking along the lines of a radio triggered remote 
> optical beacon for testing purposes)?
> 
> 73
> 
> Barry, G8AGN
> 
> 
> 
> On 16/09/2010 17:00, laser-request at mailman.qth.net wrote:
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