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Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!ihnp4!kitty!larry
From: larry@kitty.UUCP
Newsgroups: sci.electronics
Subject: Re: Wireless data link
Message-ID: <1648@kitty.UUCP>
Date: Wed, 11-Mar-87 09:15:13 EST
Article-I.D.: kitty.1648
Posted: Wed Mar 11 09:15:13 1987
Date-Received: Thu, 12-Mar-87 19:25:24 EST
References: <214@pluto.UUCP> <250@pluto.UUCP> <175@tiger.Princeton.EDU>, <7760@utzoo.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 40
Keywords: high speed, short distance
Summary: Reason why IR LED's are favored over visible LED's

In article <7760@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes:
> > 	Since you are referring to semiconductor lasers, such as gallium
> > arsenide injection laser diodes, you don't have much choice as to the
> > wavelength range of available products: 820 nanometers to somewhat less than
> > 1.0 micron.  Infrared LED's are also available only in the same range.
> 
> A thought comes:  if you do end up using LEDs, you might want to consider
> visible light rather than IR.  I haven't done the numbers on things like
> efficiency and sensitivity, but ASSUMING THOSE ARE COMPARABLE, you will
> find visible-light hardware quite a bit easier to work with during debugging.
> (It's not suitable if you want your final system to be inconspicuous, of
> course.)

	In general, infrared LED's (I am NOT referring to injection laser
diodes here) are available with higher output energy than visible LED's of
comparable size.  It is sometimes difficult to compare specifications of
visible LED's versus IR LED's since many manufacturers specify the former in
luminous intensity (measured in millicandellas) and the latter in radiometric
intensity (mw/steradian).
	However, if one does fairly compare the specifications, the IR LED's
will outperform the visible light LED's in propagation.  As an example, the
Fairchild FLV104 visible LED is one of the highest intensity devices on the
market, providing a typical 150 mcd of luminous intensity.  However, its
radiometric intensity (which one need be concerned with for long distance
propagation) is typically 4.0 mw/sr.  Now, this can be compared to a
typical high-intensity IR LED, like the Fairchild FPE520, which has a
typical radiometric intensity of 50.0 mw/sr.  As you can see, we are talking
about an order of magnitude greater propagation energy which is available
through IR LED's as compared to visible light LED's.
	There is another advantage of IR over visible light for some
applications (like hand-held remote controls).  IR tends to reflect rather
well off the surface films of many items, and will do a nice job of
"bouncing" its way across a room without having a precise aim.  Try this
with a TV remote control some day: aim it at the ceiling or a wall, and
it may still control the set!

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry
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