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Old May 11th 2019, 01:05 PM   #1
Obi
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Light intensity over a specific distance

Hi guys, Im looking for some help if any of you would be kind enough

I am trying to calculate how many lux/lumens an LED needs to be in order to reach the end of a long tunnel then reflect off a mirror and travel up a short distance and the light be seen at the end point.

Im not even sure where to start

The tunnel is lined with a reflective material. I have the measurements of the tunnel and the distance it has to travel.

Can anyone give me a nudge in the right direction as to how to began to solve it? Any equations or formulas would be amazing.

Thanks

Obi
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Old May 11th 2019, 07:03 PM   #2
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Originally Posted by Obi View Post

The tunnel is lined with a reflective material. I have the measurements of the tunnel and the distance it has to travel.Obi
The dimensions are essential is this a real tunnel meters diameter ??? or drainpipe sized????

This is extremely complex ... an led emits light over a 120 degree (solid) angle ... some light will be hitting the wall and bouncing back and forth many times as it moves down the tunnel , each bounce looses intensity (about 15%) ... some light will go strait down ....

You say the tunnel is already lined with reflective material ... is this mirror type ?? aluminium foil .... or white paint ... white paint is more reflective but scatters the light .....

Bottom line you want to give great attention to the light source , using a lens will focus the light down the tunnel ... very important !!

A zoomable torch is your best bet , like this one...



See how tight the beam is from this torch ??? no real need to line the tunnel with reflective material ....

Need more data !! how long tunnel , what is the aim of it all ... ??? A laser might be better.
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Old May 12th 2019, 03:42 AM   #3
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The key parameter is going to be the reflectivity of the pipe.
From the geometry of the problem you should be able to estimate how many times (on average) the light will reflect off the sides before it reaches the end.
Note that light directed down the center line will not reflect at all, while light directed sideways will reflect a lot (hence average).

I would have thought that with modern reflective materials you should be able to achieve hundreds of metres fairly easily.
The clarity of the air might start to make a noticeable difference at these distances.

As Oz says, the practicalities of your specific application make a big difference.

Try googling light pipes.
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