Low-quality OCR (Optical Character Recognition) in "free" PDFs often mangles complex subscripts and Greek symbols, leading to critical errors in your calculations.
A unique addition to the 3rd edition is a section titled "The Photon," where Loudon cautions readers on the nuances of the term, largely responding to the "Anti-photon" skepticism of Willis Lamb. Theoretical Scope:
One of the most difficult topics in quantum optics is optical coherence. Loudon’s explanation of first-order and second-order coherence (the g(1)g raised to the open paren 1 close paren power g(2)g raised to the open paren 2 close paren power
Often available as a free, high-quality legal PDF online, it provides a more modern, slightly more mathematical take that complements Loudon’s physical intuition.