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In a bunch of old papers, written before lasers, the authors often refer to illumination with the same characteristics as a laser: single spatial mode and temporally lockstep.

For example in: On the diffraction theory of optical images (H. H. Hopkins, 1953)

if the object is coherently and cophasally illuminated...

What was the historic method of making this kind of illumination?

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  • $\begingroup$ Can you link to or provide citations for any of these old papers? $\endgroup$
    – rob ♦
    Jun 26, 2018 at 23:17

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You use a bright, distant source, admitted through a pin hole; see van Cittert–Zernike Theorem. Sun light works, so do certain low pressure tubes. You can easily show interference patterns with sunlight.

This is what Young did for his original experiment which showed optical interference with sunlight.

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  • $\begingroup$ I understand that this kind of illumination is spatially coherent, and made more so by the use of a pinhole. I don't understand why the photons march in lock step, as opposed to what Goodman would call a "thermal" source. $\endgroup$
    – Mikhail
    Jun 27, 2018 at 1:13
  • $\begingroup$ I remember, vaguely, the hand-waving argument of my professor: the out-of-phase elements dwindle in importance as the distance increases. The proof is in the van Cittert-Zernike theorem, which is quite detailed. $\endgroup$
    – Peter Diehr
    Jun 27, 2018 at 12:35

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