Hi Guilherme, you mean to cover the scanner? The excessive light comes from the transparency light, not from outside. Due to the fact that the light tube is much wider than the frame you're working on, fraction of light penetrates from the sides of the film and illuminates the CCD itself while scanning. Due to that you can see errors on your scanned image, like orange casts on highlights for example. Even tiny sprocket holes on 135 film will ruin your scan if not covered.