I've been looking into diffraction for AOLM and stuff like that, and these produce diffraction. Binary Spatial Light Modulators like Digital Micromirror Devices also produce diffraction. This diffraction is then focused and filtered using a pinhole. But, and here is the question: Why is always the first order of diffraction used and not the zeroth order? The first order has more power than the 2nd, 3rd or other higher orders. That's something I get. But why is the zeroth order of diffraction not used? I've been seeing on several websites that the zeroth order is not 'modulated', but why isn't it? Isn't the zeroth order just as valid to use as the 1st? I have added an image of a 4f system and you can see it discards the zeroth order. Why? EDIT: AOLM stands for Acoustic Optical Light Modulator (more commonly known as just AOM) and adds the frequency of soundwaves to light. For an example, if you want to add several MHz to light (of several THz) you can do so using an AOLM.