Saturday, 4 April 2015

thermodynamics - Is there any optical component that uniformizes the incoming light?


Is there any optical component in existence that uniformizes randomly pointing rays?


The component (greenish-yellow) takes in random light and uniformizes it. Light is traveling from left to right



Answer



To add to Carl Witthoft's answer: your proposed device would violate Conservation of Optical Extent aka Optical Étendue unless it were an active device (i.e. one needing a work input to "uniformise" a given quantity of light).


The law that optical extent can only be held constant or increased by a passive optical system is equivalent to the second law of thermodynamics for light, because the optical extent of a light source is its volume in phase space.


The optical extent $\Sigma$ for the light radiated from a surface $S$ is:


$$\Sigma = \int_S \int_\Omega I(x) \cos(\theta(x, \Omega)) \,{\rm d} \Omega\, {\rm d} S$$



where we integrate the intensity $I$ at each point $x\in S$ over all solid angles $\Omega$ taking account of the angle $\theta$ each component of the radiation from point $x$ makes with the surface's unit normal. Then we integrate this quantity over all points on the surface $S$.


So, the $\Sigma$ for your output would be nought, whilst it would be large for your input, so no passive imaging device can do what you ask.


So, another way of putting Carl's answer would be that the proposed device would have to "forget" the state encoded in the input light's wavefront direction at each point. Thus your proposed device, if at all possible, would needfully be an active device, needing work input of $k_B\,T\,\log 2$ joules for each bit of light state forgotten in accordance with the Landauer Principle form of the second law of thermodynamics. I say more about this in my answer here.


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