The openness factor on a blind refers to how tightly the fabric is woven. The tighter the weave, the more sunlight it blocks. Conversely, a looser weave will allow more sunlight to come in through the window. The openness factor is expressed using a percentage, with lower numbers representing a higher level of opacity. For example, a blind with a 1% openness factor will provide more shade than one with a 5% openness factor.