<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<br>
<div class="moz-forward-container"><br>
<br>
-------- Original Message --------
<table class="moz-email-headers-table" border="0" cellpadding="0"
cellspacing="0">
<tbody>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Subject:
</th>
<td>Re: Light uniformity measurement over the plexiglas pane
with the blue LED</td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Date: </th>
<td>Tue, 15 Jan 2013 13:31:15 +0200</td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">From: </th>
<td>Georgios Voulgaris <a class="moz-txt-link-rfc2396E" href="mailto:gboulgaris@phys.uoa.gr"><gboulgaris@phys.uoa.gr></a></td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Reply-To:
</th>
<td><a class="moz-txt-link-abbreviated" href="mailto:gboulgaris@phys.uoa.gr">gboulgaris@phys.uoa.gr</a></td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">Organization:
</th>
<td>University of Athens</td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">To: </th>
<td>Christina Kourkoumeli <a class="moz-txt-link-rfc2396E" href="mailto:hkourkou@phys.uoa.gr"><hkourkou@phys.uoa.gr></a></td>
</tr>
<tr>
<th align="RIGHT" nowrap="nowrap" valign="BASELINE">CC: </th>
<td>Matthew Shepherd <a class="moz-txt-link-rfc2396E" href="mailto:mashephe@indiana.edu"><mashephe@indiana.edu></a>, Elton
Smith <a class="moz-txt-link-rfc2396E" href="mailto:elton@jlab.org"><elton@jlab.org></a>, Efstratios Anassontzis
<a class="moz-txt-link-rfc2396E" href="mailto:eanason@phys.uoa.gr"><eanason@phys.uoa.gr></a>, Pavlos Ioannou
<a class="moz-txt-link-rfc2396E" href="mailto:pioannou@phys.uoa.gr"><pioannou@phys.uoa.gr></a>, George Lolos
<a class="moz-txt-link-rfc2396E" href="mailto:George.Lolos@uregina.ca"><George.Lolos@uregina.ca></a>, Zisis Papandreou
<a class="moz-txt-link-rfc2396E" href="mailto:zisis@uregina.ca"><zisis@uregina.ca></a></td>
</tr>
</tbody>
</table>
<br>
<br>
<div class="moz-cite-prefix">Hi all,<br>
<br>
We have studied the effect of the edge surface (presented in
the meeting of 12-4-20121 <a moz-do-not-send="true"
href="https://halldweb1.jlab.org/wiki/images/b/b6/FCAL-Monitoring.pdf">https://halldweb1.jlab.org/wiki/images/b/b6/FCAL-Monitoring.pdf</a>
) 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. <br>
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. <br>
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.<br>
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. <br>
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.<br>
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.<br>
<br>
Bye,<br>
George. <br>
<br>
</div>
</div>
</body>
</html>