An optical low pass filter is supposed to blur the image using birefringence. I took out an optical low pass filter from a digital camera where it is used for anti-aliasing purposes. The filter looks just like a piece of glass. I pass a diode laser beam through it and look at the beam spot on the wall. I was expecting it to be a bit larger than the laser spot without the filter. However I can't see any difference. How can I tell it apart from a piece of plain glass?