Hi All, I have read articles and watched videos demonstrating and explaining the basics of Schlieren photography but one point that I am not at all clear on is why a point light source is needed. Can anyone offer a general explanation or point me to a basic illustration that might explain this detail? I am thinking it is in part because the goal is to focus the image back onto a single point after the light reflects from a mirror but I get cloudy thinking about how a single point light would show more than just a point of the subject. For instance, if I am using a laser to visualize air flow around a soda bottle which is being opened, and the laser is slightly behind the soda bottle, pointed toward the mirror then the beam of light passes through just one small point around the soda bottle on its way to the mirror. Does the mirror itself "widen" the beam of light (if the general idea is right, please correct my terminology). I would like to understand why any light source can not be used - that is, what would happen if a flashlight were used instead of a point light source. I am hoping to eventually take some Schlieren photos at home so maybe this will help with the understanding but that will be a while from now and any info you can give me would be appreciated. Thanks!