From a fail safe point of view, Safety Sensors/Safety Light Curtains are required to be designed so that a guarded machine will not perform a dangerous operation when any malfunction or failure occurs.

In the Light-ON output mode, the operation part of the machine is through the contact point of the Relay.
Therefore, even when current does not flow due to the malfunction or failure, the contact point A goes in the open state and the machine does not operate, or the machine performs a safe operation.

On the other hand, in the Dark-ON output mode, the operation part of the machine is through the contact point B of the Relay. Therefore, when current does not flow, the contact point B goes into the close state and the machine operates, or the machine may perform a dangerous operation.

Therefore, Safety Sensors/Safety Light Curtains adopt the Light-ON output mode in consideration for safe use.