Measuring down to 300nm was RE:PMA news so far
Measuring down to 300nm was RE:PMA news so far
- Subject: Measuring down to 300nm was RE:PMA news so far
- From: "tlianza" <email@hidden>
- Date: Wed, 5 Mar 2003 06:31:01 -0500
Graeme Gill wrote:
"It's a pity none of the economical instruments measure illumination
down to 300nm, something that's needed to figure out how the
paper Fluorescent Whitener Additives will react."
There are a number of problems that make this difficult.
1. The source would have to emit a lot of energy in a non visible, non EYE
SAFE region.
2. If it is a grating instrument, there would have to include active
(moveable)filtering to filter out the second order diffraction. That's why
low cost instruments measure over less than a 2:1 wavelength range
(generally 400nm to 700 or 730nm) . Dipping to 380 nm will drag the second
order diffraction into the pseudo visible range of 760 nm.
3. Sensing 300 nm with a low cost silicon sensor (or other photoelectric
sensors) is a near impossibility. There are real quantum issues in silicon
sensors that make that region a bear to measure. The photon is very
energetic and it generally is absorbed by the cover substrate or it get's
absorbed as heat, with out kicking out the necessary electron to get sensed.
Sensing in this region is often done with a "Bulk sensor" like a thermopile.
The sensor is optically low pass filtered and the temperature rise is
monitored. The amount of energy in the band is deduced from the heat rise.
Not a task for the faint of heart....
It's kind of ironic that the very mechanism that causes flourescence makes
it difficult to measure the energy source that created it. Even if we had a
sensor, I don't think that any manufacturer would take the chance with the
eye safety issue for a field deployable instrument. It's hard on the hired
help....
Take care,
Tom Lianza
Technical Director
Sequel Imaging Inc.
25 Nashua Rd.
Londonderry, NH 03053
email@hidden
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