Yes we call this retro-reflective sensor and the sensing 
distance with normal red LED emitter can be 10m to 15m. 
With laser it could be much higher. The triple-reflector 
should be quite cheap (about US$1-2 per pc at higher quantity). 
We use short pulse for sending the pulse light. Depending
on your emitter and receiver used, 1us to 10us pulse width 
can be used with 100us or longer period. Photo-transsitor
require longer pulse width. Photo-diode is better.

The major suppliers of Opto sensors are SICK, OMRON, 
Pepperl+Fuchs, Banner Engineering, SUNX and some others.
You should be able to buy them easily from the vendors
and the price may be cheaper. Pepperl+Fuchs has
distributors around the world to serve you. In
North Americal, P+F took over Honeywell Microswitch's
two divisions in year 2000, including the optic
sensor division.

Regards,
Xiaofan

----------------------------------------------
Xiaofan Chen
R&D Engineer, Photoelectric Sensor Development
Pepperl+Fuchs Singapore
http://www.pepperl-fuchs.com
Signals for the world of automation
--------------------------------------------

-----Original Message-----
From: Bob Blick 
Sent: Thursday, September 29, 2005 10:31 AM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] simplest long range people sensor

On 28 Sep 2005 at 22:17, Damon Hamm wrote:
>  Reflecting
> the beams back from a mirror seams too hard to calibrate, 
but I'm open to ideas.

Retroreflective sheeting works well. You can get high quality stuff 
with sticky backing in red/white stripes meant for truck trailers.

You will need to modulate your beam so you can have something to 
distinguish it from ambient light and reflections.

Cheers,

Bob
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