In an unplanted or newly planted tank, direct sunlight can cause green water and algae, and should be shielded. In a balanced planted tank, direct sunlight has only positive effect. My two planted tanks receive window sunlight in mid morning during winter and I see more lush growth in plants hit by the sunlight. Before I had plants, I struggled with bba and gda in the areas hit by direct sunlight, and had to lower the window curtain. Once with plants and balanced out, the algae never return and I open up the curtain to welcome more sunlight in. I am curious whether the algae disappearance is due to more algae eaters I introduced or plant competition.
I was curious on the PAR level between direct sunlight and artificial light. Based on my measurement with a lumen meter, I estimated that PAR at direct hit area jumped from 50 to 400 even though the window is 10 ft away and my eye perception of light intensity is about the same.