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If Timer itself is responsible for determining duration of the next event, this timer is called Active timer. Typical Shutter operation uses Timer in active mode. This Timer is responsible for scheduling specified time delay and notifying Shutter when this delay has elapsed and it is time to close. Duration of an event controlled by Active timer is explicitly specified before the event starts.
In some cases Timer itself is not responsible for determining event duration or scheduling it, but rather relies on some external element(s) to decide when it is time to terminate the event. Such timer is called Passive timer. An example of passive timer event is data acquisition by most spectral detectors, where Detector settings determines how long data are being acquired and when data are ready to be read out. In cases like this not only the duration of an event is not explicitly specified, it may not be known at all because of uncertainty in data processing and transfer overhead.
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