Abstract: A system and method of monitoring light emitted from a light emitting diode (LED) display device, which typically has a plurality of selectively energizable display segments which are grouped to form desired shapes and/or alphanumeric characters, is provided, particularly for use in an Aspect Display Unit (ADU) that requires vitality when utilized in the railroad industry as a component of an Automatic Train Protection system utilized in the railroad industry. In a preferred embodiment, each particular segment of the LED display device is independently monitored by an accompanying independent dual photo-transistor circuit when a non-vital output periodically drives each segment to a known electrical state and then back to the original state. A first and a second photo-transistor is used for each segment of the LED display.