These Dutch-language lecture notes accompanied my yearly course during 1985 - 2007 for second-year students at Sterrewacht Sonnenborgh and the Sterrekundig Instituut Utrecht. They treat the basics of astrophysical radiative transfer with emphasis on spectral line formation in optically thick stellar atmospheres, with the solar spectrum as principal example. They cover the material in the first chapter of Radiative Processes in Astrophysics by Rybicki & Lightman, but much more extensively. Instead, the rest of their book is summarized in chapter 6 of these lecture notes.
I wrote them in 1988 and added content until 1991. The scanned copy linked above has penciled-in corrections.
There is also a computer-aided translation into English.
In 1993 I summarized this course, inserting new figures, for NSO summer students in an English-language Introduction to Solar Spectrum Formation. Later I also summarized this course in the second chapter of my English-language masters-level Radiative Transfer in Stellar Atmospheres.
The scanned copy linked above uncompresses to hundreds of megabytes postscript in a printer (black & white suffices). Printing them takes ages, better do so in parts with the from-to print option. And then decipher my handwriting...