Photodiode for eventcode 9 and 18

Hello,
I have been trying to align the photodiode changes to event codes, to get very precise timing when stimulus is presented on the screen. However, I ran into some problems as there is no stable photodiode change for event code 9 and 18.
In some trials, photodiode change to event code 18, and some trials there is no change. This makes it difficult to align the photodiode signal to which event code it is related.
right now the photodiode is triggered at the beginning of a scene, the defualt way.
Is there any way to trigger photodiode change in event code 9 and 18, or make sure there is no photodiode change at these times?
Or can we get rid of these two default event codes?

Best,
Mary

Photodiode is supposed to change when a new scene begins, not when an eventcode is issued. So, it does not change in Events 9 & 18. It goes back to black in a new trial, if it was white at the end of the previous trial. idle() is also regarded as one scene.