[Halld-cal] Fwd: Re: Light uniformity measurement over the plexiglas pane with the blue LED / fwd from George

[Elton Smith]

-------- 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>



Hi all,

     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.

Bye,
George.

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