Clarifying Coherent and Incoherent Sum of Plane Waves

[ppoonamk]

Hi everyone,

I understand that in Kohler illumination, each point at the source is converted into a plane wave illuminating the object. From what I understand, the total illumination at the object is the incoherent sum of the plane waves. Suppose my source is an ideal coherent laser i.e He-Ne laser. Is the total illumination at the object just a single plane wave?

Could anyone clarify this for me? I am very confused about the coherent and incoherent sum of plane waves at the object. Thank you

 

[Andy Resnick]

Kohler illumination is a little more than what you stated, but close enough. Using a source with some spatial coherence (the laser has high temporal coherence but lower spatial coherence) will result in 'speckle' at the sample plane from interference. Spatially filtering the laser will, AFAIK, remove speckle.

 

[ppoonamk]

Kohler illumination is a little more than what you stated, but close enough. Using a source with some spatial coherence (the laser has high temporal coherence but lower spatial coherence) will result in 'speckle' at the sample plane from interference. Spatially filtering the laser will, AFAIK, remove speckle.

Hi Andy,

Thank you for your response. I understand how the Kohler illumination reduces speckle. So I am writing this program to model an imaging system. So my problem is with the modelling of the illumination on the object. With a He-Ne laser, is the illuminating beam a coherent sum of plane waves or just a single plane wave?

 

[Andy Resnick]

It depends on what you've done to the beam. If it's a 'raw' Gaussian , then you decompose the field accordingly. If the beam is expanded and clipped by the aperture stop, then your decomposition will be altered. If you've spatially filtered and expanded the beam, then you have close to a single plane wave at the aperture stop.