[Ronja] Further lcd thoughs...

[J.D. Bakker]

At 02:11 +0100 07-09-2008, ronjalist wrote:
>OK 50Hz is not enough, but you do have 1024x768 (or better) resolution
>and you could forgo the main optics, use a fresnel lens and subdivide
>the panel into 1000 segments...
>
>With a telescopic lens arrangement and an lcd webcam as the rx.
>
>Thoughts ?

It'll work, but at the expense of link range. Assuming equal transmit 
power, and comparing two systems with comparable optics (no fair 
giving a telescope to the webcam and not to the regular Ronja), each 
of those 1000 segments will have 1/1000th of the total power. 
Achievable link distance is roughly proportional to the square root 
of power, so those 1000 segments reduce the max link distance by a 
factor of ~30, to just over 40 meter for a standard Ronja config.

Now, in the real world,

- LCD panels transmit less than 50% of all incident light
- on top of that, a significant portion of the light (>60%) will be 
diffused/diffracted off-axis. This leaves only 10-20% of all 
transmitted energy to hit the receiver
- you *will* need some separation between segments, or accept that 
light from one segment will bleed into the next one. This, too, costs 
TX energy.
- alignment is going to be harder
+ on longer ranges, in-air absorption will eat more energy than the 
square law suggests (so this would work in favor of a short-range 
segmented system)

More on link budgets here:

http://www.pointless.net/pipermail/ronja/2006-August/009130.html

JD 'no free lunch' B.
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