[Ronja] Further lcd thoughs...

[ronjalist]

In reply to self :

Most webcams are at best 640x480 at 30fps.
And as we would running monochrome (sans ir filter) there is simply not 
enough bandwidth to make it sensible.

ronjalist wrote:
> Cluestick gratefully received :-)
> 
> What if the optics on the rx were focused to fill the ccd of a webcam 
> (which has had it's ir filter removed),  Or is it that optics for a 100 
> meter zoom shot are too bulky, need or too high a precision or are 
> simply too costly ?
> 
> If the optical zoom is high enough surely the output power only need to 
> be above ambient to illuminate ?
> 
> I just thought that as we are now looking for an image - not a light 
> level we could get funky with the rx lenses ?
> 
> 
> J.D. Bakker wrote:
>> 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.
>