Added example of Muon IDFs valid between different periods. Here for the EMU instrument. re #1803.

Code/Mantid/Instrument/EMU_Definition_32detectors.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<!-- For help on the notation used to specify an Instrument Definition File 
+     see http://www.mantidproject.org/IDF -->
+<instrument name="EMU" valid-from   ="1900-01-01 23:59:59"
+                       valid-to     ="2009-12-31 23:59:59"
+		       last-modified="2011-01-07 00:00:00">
+  <defaults>
+    <length unit="meter"/>
+    <angle unit="degree"/>
+    <parameter eq="" type="double" extract-single-value-as="mean" />       
+    <reference-frame>
+      <!-- The z-axis is set parallel to and in the direction of the beam. the 
+           y-axis points up and the coordinate system is right handed. -->
+      <along-beam axis="z"/>
+      <pointing-up axis="y"/>
+      <handedness val="right"/>
+    </reference-frame>
+  </defaults>
+
+  
+  <!-- BRIEF DESCRIPTION OF EMU INSTRUMENT: 
+  
+      Data provided by Stephen Cottrell for the EMU instrument
+  -->
+  
+  
+  <!-- LIST OF PHYSICAL COMPONENTS (which the instrument consists of) -->
+  
+  <!-- dummy components added for visualisation purposes. 
+       To easy spot what geometry the instrument is in -->
+       
+  <component type="along z">
+    <location />
+  </component> 
+       
+  <type name="along z">
+  <cylinder id="A">
+    <centre-of-bottom-base x="0.0" y="0.0" z="0.0" />
+    <axis x="0.0" y="0" z="1" /> 
+    <radius val="0.002" />
+    <height val="0.10" />
+  </cylinder>
+  </type> 
+  
+  <component type="along x">
+    <location />
+  </component> 
+       
+  <type name="along x">
+  <cylinder id="B">
+    <centre-of-bottom-base x="0.0" y="0.0" z="0.0" />
+    <axis x="1.0" y="0" z="0" /> 
+    <radius val="0.002" />
+    <height val="0.10" />
+  </cylinder>
+  </type> 
+  
+  <component type="along y">
+    <location />
+  </component> 
+       
+  <type name="along y">
+  <cylinder id="C">
+    <centre-of-bottom-base x="0.0" y="0.0" z="0.0" />
+    <axis x="0.0" y="1" z="0" /> 
+    <radius val="0.002" />
+    <height val="0.10" />
+  </cylinder>
+  </type> 
+  
+  <!-- source and sample-position components -->
+
+  <component type="source">
+    <location z="-10.0" />
+  </component>
+  
+  <type name="source" is="Source" />
+  
+  <component type="some-sample-holder">
+    <location />
+  </component>
+  
+  <type name="some-sample-holder" is="SamplePos" />
+ 
+  <!-- detector components --> 
+  
+  <component type="all-rings"  idlist="all">
+    <location />
+  </component>  
+  
+  <type name="all-rings">
+     <component type="rings-front" >
+       <location />
+     </component>
+     <component type="rings-back" >
+       <location />     
+     </component>    
+  </type>
+  
+  <type name="rings-front" >
+     <component type="front-inner">
+        <location z="-0.0765" name="ring-front-inner" />	<!-- Tube nearest magnet body, scintillator furthest from beam -->
+     </component>
+     <component type="front-outer">     
+        <location z="-0.182" name="ring-front-outer" />
+     </component>     
+  </type>
+  
+  <type name="rings-back" >
+     <component type="back-inner">
+        <location z="0.0765" name="ring-back-inner" />
+     </component>
+     <component type="back-outer">     
+        <location z="0.182" name="ring-back-outer" />
+     </component>    
+  </type>
+  
+  <type name="front-inner">
+    <component type="inner-pixel">
+       <location r="0.0918" t="90" p="292.500000" rot="292.500000" name="det1"/>
+       <location r="0.0918" t="90" p="337.500000" rot="337.500000" name="det3"/>
+       <location r="0.0918" t="90" p="22.500000" rot="22.500000" name="det5"/>
+       <location r="0.0918" t="90" p="67.500000" rot="67.500000" name="det7"/>
+       <location r="0.0918" t="90" p="112.500000" rot="112.500000" name="det9"/>
+       <location r="0.0918" t="90" p="157.500000" rot="157.500000" name="det11"/>
+       <location r="0.0918" t="90" p="202.500000" rot="202.500000" name="det13"/>
+       <location r="0.0918" t="90" p="247.500000" rot="247.500000" name="det15"/>
+    </component>    
+  </type>  
+  
+  <type name="back-inner">
+    <component type="inner-pixel">
+       <location r="0.0918" t="90" p="247.500000" rot="247.500000" name="det17"/>
+       <location r="0.0918" t="90" p="202.500000" rot="202.500000" name="det19"/>
+       <location r="0.0918" t="90" p="157.500000" rot="157.500000" name="det21"/>
+       <location r="0.0918" t="90" p="112.500000" rot="112.500000" name="det23"/>
+       <location r="0.0918" t="90" p="67.500000" rot="67.500000" name="det25"/>
+       <location r="0.0918" t="90" p="22.500000" rot="22.500000" name="det27"/>
+       <location r="0.0918" t="90" p="337.500000" rot="337.500000" name="det29"/>
+       <location r="0.0918" t="90" p="292.500000" rot="292.500000" name="det31"/>
+    </component>    
+  </type>  
+    
+  <type name="front-outer">
+    <component type="outer-pixel">
+       <location r="0.0806" t="90" p="292.500000" rot="292.500000" name="det2"/>
+       <location r="0.0806" t="90" p="337.500000" rot="337.500000" name="det4"/>
+       <location r="0.0806" t="90" p="22.500000" rot="22.500000" name="det6"/>
+       <location r="0.0806" t="90" p="67.500000" rot="67.500000" name="det8"/>
+       <location r="0.0806" t="90" p="112.500000" rot="112.500000" name="det10"/>
+       <location r="0.0806" t="90" p="157.500000" rot="157.500000" name="det12"/>
+       <location r="0.0806" t="90" p="202.500000" rot="202.500000" name="det14"/>
+       <location r="0.0806" t="90" p="247.500000" rot="247.500000" name="det16"/>
+    </component>    
+  </type>  
+  
+  <type name="back-outer">
+    <component type="outer-pixel">
+       <location r="0.0806" t="90" p="247.500000" rot="247.500000" name="det18"/>
+       <location r="0.0806" t="90" p="202.500000" rot="202.500000" name="det20"/>
+       <location r="0.0806" t="90" p="157.500000" rot="157.500000" name="det22"/>
+       <location r="0.0806" t="90" p="112.500000" rot="112.500000" name="det24"/>
+       <location r="0.0806" t="90" p="67.500000" rot="67.500000" name="det26"/>
+       <location r="0.0806" t="90" p="22.500000" rot="22.500000" name="det28"/>
+       <location r="0.0806" t="90" p="337.500000" rot="337.500000" name="det30"/>
+       <location r="0.0806" t="90" p="292.500000" rot="292.500000" name="det32"/>
+    </component>    
+  </type> 
+    
+  <type name="inner-pixel" is="detector">  
+    <!-- It is implicitely assumed here that the front y-z plane (looking down
+         the x-axis) is the surface that see the neutron beam.
+         This surface is 36mm along y and 30mm along z and the dept along x is 5mm.  -->
+    <cuboid id="inner-shape">
+      <left-front-bottom-point x="0.0" y="-0.0365" z="-0.0245"  />
+      <left-front-top-point  x="0.0" y="0.0365" z="-0.0245"  />
+      <left-back-bottom-point  x="0.010" y="-0.0365" z="-0.0245"  />
+      <right-front-bottom-point  x="0.0" y="-0.0365" z="0.0245"  />
+    </cuboid>
+    <algebra val="inner-shape" />     
+  </type>
+  
+  <type name="outer-pixel" is="detector">  
+    <!-- It is implicitely assumed here that the front y-z plane (looking down
+         the x-axis) is the surface that see the neutron beam.
+         This surface is 30mm along y and 171mm along z and the dept along x is 5mm.  -->
+    <cuboid id="outer-shape">
+      <left-front-bottom-point x="0.0" y="-0.032" z="-0.093"  />
+      <left-front-top-point  x="0.0" y="0.032" z="-0.093"  />
+      <left-back-bottom-point  x="0.010" y="-0.032" z="-0.093"  />
+      <right-front-bottom-point  x="0.0" y="-0.032" z="0.093"  />
+    </cuboid>
+    <algebra val="outer-shape" />     
+  </type>        
+    
+  <!-- DETECTOR ID LISTS -->
+  
+  <idlist idname="all">
+    <id start="1" step="2" end="15"/>
+    <id start="2" step="2" end="16"/>     
+    <id start="17" step="2" end="31"/>       
+    <id start="18" step="2" end="32"/>                  
+  </idlist>
+
+
+  
+</instrument>