What is the CR product of the photorelay?

The CR product is one of the important indicators for treating high-frequency signals with a photorelay. "C" as in the CR product refers to the capacity across the output terminals, Coff, when the photorelay is turned off; and "R" as in the CR to the resistance across the output terminals, Ron, when the photorelay is turned on, respectively.

If Coff is high, the high-frequency signal will pass, despite the photorelay being turned off. If Ron is high, there will be the signal's pass loss and deterioration.

When you want to switch the high-frequency signal, therefore, you will need the photorelay with low Coff and Ron, that is, one with low CR product.