Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!ub!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: UV light
Summary: UV radiometry...
Message-ID: <4024@kitty.UUCP>
Date: 13 Sep 90 03:02:10 GMT
References: <4092@disk.UUCP>
Distribution: sci
Organization: Recognition Research Corp., Clarence, NY
Lines: 57

In article <4092@disk.UUCP>, stevenw@disk.UUCP (Steven Weller) writes:
> I would like to make a UV light detector, but do not know
>       a) what device I should use to detect UV light, and

	I assume that you would like to make a quantitative measurement
of some sort, and not just detect the presence or absence of a UV radiation
source.

	The first question you must answer is whether you will be measuring
the intensity of *just* UV radiation, or will here be visible and infrared
radiation also present.

	In the case of, say, a spectrophotometer, the light being measured
is reasonably monochromatic and is shielded from any other radiation
source.  If you are measuring, say, the UV from a UV lamp in the presence
of room illumination you will require a filter to exclude wavelengths
longer than those of UV - unless, of course, the UV radiation is orders of
magnitude greater than ambient illumination.

	For general purpose use, various glass types of glass bandpass
filters are available from such vendors as Schott.  Be aware, however,
that some glass bandpass filters intended for UV tranmission *also*
have some transmission in the red region.  I have learned this fact
The Hard Way. :-)

	For specific wavelengths, narrow and accurate metal-dielectric
interference filters are available.

	Once you have addressed the filter issue, you can then select a
photodetector.  There are many detector methodologies available, but
the three most common are:

(1)	Vacuum photodiodes, a photoemissive two-element tube requiring
	an excitation voltage of between 50 and 150 volts.  Vacuum
	photodiodes are available with useful detection at wavelengths
	as low as 120 nm.  With this type of detector there is a direct
	relationship between current and watts of irradiated energy.

(2)	Photomultiplier tubes, for applications where sensitivity greater
	than vacuum photodiodes are required.  Utilizes an excitation
	voltage of typically 1,000 volts.

(3)	Solid-state photodiodes, generally less sensitivity than the
	above two devices, but more rugged, compact and easier to use.
	Solid-state photodiodes may be operated in either photovoltaic
	mode for maximum sensitivity, or in photoconductive mode for
	maximum linearity.
  
>       b) how to use it to achieve a meaningful intensity reading.

	If you build a device, the easiest way to calibrate it is to
borrow a UV radiometer of known accuracy, such as one manufactured by
International Light.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231   {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635                  {utzoo, uunet}!/      \aerion!larry