How do we test LEDs in a circuit that sets them to blink(turn on and off) at periods exceeding 400ms, eg 500ms on, 500 ms off?

The longest integration time for the Keysight LED sensor is 400ms between the ‘trigger’ and ‘learn/measure’ commands. In order to capture one reading during "LED ON" stage, you may want to insert a “wait” time in between “trigger” and “measure” (or “learn”) command.

The “Peak” mode setting allows you to measure LEDs during "LEDs ON" stage.