To test how well perlite blocks light we set up a window apparatus in our dark box. We filled this window with varying thicknesses of perlite and observed count rates of a P.M.T. in the box to determine how much light was getting through the window. The P.M.T. used was very unsteady in its counting, as seen here, and because of this our data is very sketchy. However, it is accurate enough to determine the depth of 100% light blocking to the nearest cm. The numbers in the chart should not be taken too seriously. For instance, numbers in one row should not be compared to numbers in another row because of the time dependence of the count rates for this P.M.T. Our conclusion from this data is that 5 cm is the minimum thickness to use for light tightness.

Note: "overloaded" means the P.M.T. stopped sending out any signal at all. This is caused by too much light getting through.