Optical coherence

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Optical "coherence"

A question on the nature of coherence that came up on Wikipedia...

If we pass the light from a laser through a stack of frosted plates, is the result still "coherent light?" For most optical applications requiring coherent light, this type of heavily diffused laser light is fairly useless. A spatial filter thinks it's incoherent: pass it through a lens with a pinhole at the focus nearly all the light will be blocked. Yet if we compare the phase of this light at various 3D locations throught the wave, we'll find that the EM wave still varies coherently.

Somehow the textbook definition of coherence doesn't seem correct when it declares diffuse laser light to be coherent light. So what does "coherent light" really mean?


Similar question: should we say that a "white paint laser" emits spatially INcoherent light? See http://tinyurl.com/aybjn Laser Action in Very White Paint
 

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