I want to detect when an area of my notebook display goes from dark to white by placing a phototransitor focused on the area tight against the screen and metering the output. When I perform the experiment I get no voltage change on the phototransitor when I go from dark to light. If I shine a flashlight on the phototransistor I get a voltage as expected. The phototransistor is a NTE3037 which is rated white light. I know that notebook screen displays are driven on and off. Even so I would expect some voltage. Any ideas?