[Laser] Coherent Source - back on my soap box

[Tim Toast]

I dont claim to understand the underlying math completely, 
but the degree of coherence of a source is directly related 
to the source's spectral bandwidth mathematically. In the 
simplest form the formula is the reciprocal of bandwidth. 
(coherence = 1/bandwidth) Other formulas include the 
refractive index of the medium (air), center frequency and C. 

Technically, requiring coherence without specifying a number, 
means that you cannot use a source with an infinite bandwidth 
or zero coherence length. So technically any real light source 
qualifies as "coherent" because none have infinite bandwidth.
 
At the other extreme, a hypothetical laser producing a single 
frequency which does not drift (ever) would have perfect 
temporal coherence and infinite coherence length. 

These two imaginary extremes are the only two that can be 
described fully with "coherent and not". In the spirit of the 
rule though, i would say they imply a small bandwidth and a 
resulting degree of coherence from a real light source. 

Besides LEDs having usually less than 50nm bandwidth, i 
thought a 50nm spacing would divide up the visible spectrum 
nicely into 6 bands (Blue, Cyan, Green, Yellow, Orange and 
Red) easily estimated by eye, and with a few UV and IR bands 
beyond each end.