Meanwhile, back at the ranch, what was being done about optical coherence? First, ideas from coherence theory and quantum statistical thermodynamics were being injected into the somewhat arcane field of radiometry, which concerns itself with the transport of energy in light-fields.  This is an important area, interesting results are coming from it.  

And then, in the mid-1970s Wolf, Mandel, Carter, and one or two others initiated a significant development in the study of coherence.  Hitherto it had been mainly - and most usefully - a theory of the correlations which developed in narrow-band (or ‘quasi-monochromatic’) light.  In the 1970s it has developed into an effective theory of polychromatic light fields.  What had to be done to achieve this sounds simple if you describe it physically, but there were some quite subtle mathematical difficulties  -  so subtle that only people with very good mathematical backgrounds would have known that they were there (there are some things it's better not to know about!).  In effect, what was done was to express the coherence functions, not as functions of the fields at P₁-at-time-t₁ and P₂-at-time t₂, but as functions of the spectral components of frequency w at P₁ and at P₂.  This doesn't sound like a big step to have taken, but it has made possible substantial advances in radiometry; and very recently indeed it has produced some very remarkable results about spectra.

Questions about spectra weren't easy to ask as long as the frequency didn't appear in descriptions of coherence.

You may think that I've lost my way here - didn't I say earlier that one of the vital steps in coherence was the elimination of the unobservable high-frequency oscillations?  Yes.  But we're not now putting them back in - the coherence functions of this new branch of the subject do have the high-frequency oscillations eliminated, but we can still assess the contribution to the coherence or incoherence of a light-beam arising from a particular narrow region of an inherently broad spectrum.  It's simply that we can put a narrow band filter in front of the source, or the detector, when we measure the degree of coherence, or we can imagine that to have been done when we do calculations on paper.

So what can possibly come out of that, that's new?