How To Fake Sunlight In a Dark Room

Realistically, there are a lot of different ways someone could approach this. In this tutorial, I'll outline the way I would approach this technique.

The first thing I would do is duplicate the layer, so that I'd have two copies of the image. One the 2nd copy (the highest layer in order) I would goto Filter > Render > Lighting Effects, and then use these settings: Light Type: Spot Light, Negative: 60, Focus: 50. I would also set the light source as the center of the window, and the area effected as the entire image.

The end result will look a bit to bright and doesn't convey the sense that the sunlight is coming directly through the window. This is where our underlying layer comes into play. Using the Polygonal Lasso Tool on our top layer, select the area you would like to have the sun shine through. Once selected, goto Select > Inverse, and then hit the Delete key.

Now we're left with a very bad looking image. Ha ha. We'll fix that by using the Filter > Blur > Motion Blur filter on the top most layer (make sure you've unselected everything first) Change the angle to -43 degrees and the pixels to 98.

Next, we're going to change some of the blending options on this layer. We'll reduce the Opacity to 28%, and add a white Outer Glow.

This gives our final image: