This measures the dead time on the counts using the standard lamp bulb.

The above image shows the about amount of counts expected to be seen during a DT. You should expect around 3 million (3,000,000) counts on the high and around 1 - 1.5 million (1,000,000 - 1,500,000) counts on the low side You should have a fairly stable dead time (indicated by the 38.x on the high and around 36 on the low).