In this section of the guide to Layers of Fear 2, you can find the description of the third projector puzzle that has to be solved during the second act of the game. It concerns unlocking access to the ventilation shaft.
This puzzle will have to be solved in the final part of the second act of the game, namely after completing a new, long scene of escaping the monster. You will get to a new room with a projector.
Do not choose the extreme frames, i.e. those with large distortions of the face of the child (example in the picture). If you leave these frames on for too long, you will die.
Find the frame with the visible rectangle (image above).
Approach the rectangle to "materialize" the entrance to the ventilation shaft.