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From: firstname.lastname@example.org (dgm)
Subject: Re: current source for array of infrared leds
Date: 6 Dec 2002 03:40:44 -0800
References: <email@example.com> <4PMG9.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 6 Dec 2002 11:40:44 GMT
Tony Williams wrote in message news:<email@example.com>...
> In article ,
> Winfield Hill wrote:
> > Tony Williams wrote...
> > > Wavelength drift can be pure hell if the IR is going
> > > through water, because water has a series of deep
> > > (steep-sided) attenuation notches.... smack bang in
> > > the 850nm to 950nm region. Been there. :-(((((
> > Tony, kindly point us to some detailed plots, if you can.
> I last saw the graph in 1982 Win, after weeks of
> struggling to find out why my water turbidity
> meter was so unstable. But yes, it was a graph
> of optical transmission through water, versus
> frquency, over the range of about 800-1200nm,
> showing a series of attenuation notches.
> I have posted something about this before, and
> Bill Sloman muttered something about the effect
> being caused by hydroxyl ions (or something).
> I'll cc this post to Bill to see if he can come
> up with an authoritive reference.
An excellent reference source for absorption data is the HITRAN
I have a previous version on CD, but the most recent version is
available for download, although the PC you use must be properly
registered with a name server. If any one has problems with this and
is still interested in the specific data for water then I can e-mail
it over in a spreadsheet format. The only point of note is that the
data in the HITRAN database is designed for spectroscopic applications
and requires a little massaging to get it into any sort of useful
A quick look at the data shows pretty low absorption from 840-880nm.
The main peaks appearing from 880-990 and in particular at 900 and
In general the low absorption windows also correspond to the
wavelengths used for optical fibre comms. Removing OH impurities was
one of the key challenges in fabricating low loss fibres. In fact many
good texts on fibre optics also include some data on OH absorption or
at least provide a reference.
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