For a dark photocell, high resistance, the vohage across the diac rises rapidly with the line voltage due to the current through C 1, triggering the diac early in the cycle.
Photoelectric-ac-power-switch - schematic

When the photocell resistance is less than about 2000 n, the drop across it is limited to less than the diac triggering voltage, and the load power is shut off.

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