1998-05-06 Added Jeff Medkeff’s paragraph stilb PDF visual observing and the physiology of the eye. 1998-06-10 Some corrections to the relation between candela and lumen. Also added a paragraph about Lambert’s law.
2000-03-11 Added a table of horizontal illumination from sunlight during clear weather at different solar altitudes above the horizon. 2000-11-15 Added a list of the brightness of the radio sky. Also added something about conversion from radiometric from photometric units. 2001-01-18 Corrected a confusion of luminance and illuminance at a few places. Also added a table of the correspondence between radiometric and photometric units. 2001-02-02 Added Tom Polakis paragraph about big telescopes and light pollution.
Also rearranged the paragrahs at the end. 2017-03-05 Updated numerous obsolete links and removed those for which no update could be found. Corrected a few typos as well. What units are used in photometry and radiometry?