Abstract: A light-transmittance meter (10) employs a housing (12) that forms a slot (16) in which the edge of a window pane (14) can be received. A microswitch (24) detects initial insertion of the window (14) into the slot (16) and causes a sample-and-hold circuit (32) to store a value representing the output that a photodiode (20) produces in response to light received from a light-emitting diode 18 before the window pane (14) is interposed between them. A ratio meter (36) then receives the output of the photodiode (20) when the window (14) has been interposed between the LED (18) and the photodiode (20), and it generates a display (22) of the ratio of that value to the initial, reference value determined before the window 14 was completely inserted.