light cell is covered pulp.n.paper

[Sungsang]

This is about guiding a defective technological roll (log of toilet paper) "на отбраковку" using photoelements.
Here's the sentence (no original in English):

Signal lamps are blinking if the light cell has been "covered" too long what causes a jam.
- I understand that there is no light coming to light cell, but how to say it briefly in Russian?

[Cutie]

фотоэлемент must be that

[Sungsang]

Cutie, I don't catch what you mean. I was asking a Russian translation/phrase for "Light cell is "covered".