Speckle reduction I: There is no free lunch!

Is speckle noise just an "accidental" disease when lasers are involved in imaging? No! Coherent noise is ubiquitous and fundamental in optical imaging. And low noise systems are not for free. The cost can easily be estimated via the information theoretical measure of the channel capacity C, taking into account the degrees of freedom and the signal-to-noise ratio. We will show that an optical system with low coherent noise has to provide significantly more degrees of freedom than a coherent imaging system. The consequence for the technical optical system is serious: significant speckle reduction can only be achieved by an excessively large imaging aperture. We will illustrate this by the example of speckle reduction for scanning laser projectors, in theory and in experiments.