[Halld-cal] Fwd: Re: Light uniformity measurement over the plexiglas pane with the blue LED / fwd from George
elton at jlab.org
Tue Jan 15 07:32:11 EST 2013
-------- Original Message --------
Subject: Re: Light uniformity measurement over the plexiglas pane with
the blue LED
Date: Tue, 15 Jan 2013 13:31:15 +0200
From: Georgios Voulgaris <gboulgaris at phys.uoa.gr>
Reply-To: gboulgaris at phys.uoa.gr
Organization: University of Athens
To: Christina Kourkoumeli <hkourkou at phys.uoa.gr>
CC: Matthew Shepherd <mashephe at indiana.edu>, Elton Smith
<elton at jlab.org>, Efstratios Anassontzis <eanason at phys.uoa.gr>, Pavlos
Ioannou <pioannou at phys.uoa.gr>, George Lolos <George.Lolos at uregina.ca>,
Zisis Papandreou <zisis at uregina.ca>
We have studied the effect of the edge surface (presented in the
meeting of 12-4-20121
https://halldweb1.jlab.org/wiki/images/b/b6/FCAL-Monitoring.pdf ) and
according to this, the best results were achieved with transparent edge
or black tape on the edge. Addition of black tape could reduce the
diffused or reflected light.
The other question is the amount of light that comes out of the
pane. In order to study uniformity we use a bare PMT that with a window
made of plastic 2 mm thick and we get a strong signal with blue and
violet LEDs. When we use the PMT with the lead glass block the amount of
light is reduced to a level that is not useful for calibration.
The interpretation is simple: The light is propagating inside the
pane with multiple reflections. The light comes out of the pane either
by scattering on surface anomalies or from light that is scattered into
angles smaller than the critical angle.
So the light is coming out at small angles from the surface. This
light can reach the bare PMT but not the PMT mounted on the lead glass
block. In order to propagate in the block, light has to emerge with
angle greater than 45 deg from the surface of the pane.
Our next idea was to rub the surface with abrasive paper so that it
becomes partially opaque. We performed a preliminary test and the
results were encouraging. We had 6-8 times more light. The disadvantage
is that we don't know of a method to control abrasion and apply it
uniform over the surface. To my opinion its important to investigate
the possible options in this direction.
About the 525 LED. The given LED has very wide pulse (37 ns Fwhm)
so the light is there but with a wider pulse. Most LEDs that we use have
fwhm less than 10 ns. This was the highest intensity I could find, but
its possible that more models will appear in the market.
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