Abstract
This year it is appropriate to celebrate an event of great significance for all illuminating engineers. This year is the 250th anniversary of the publication of major books by Pierre Bouguer and Johann H. Lambert. These books summarise the principles of photometry and of daylight science in particular. It is also an opportunity to mention the basic criteria of daylight design and evaluation still used today, that is the Daylight and Sky Factors together with the methods for the calculation of window solid angles developed by Lambert and several approximations derived later. All 20th Century daylight design tools, for example diagrams, nomograms or protractors are based on these. Although the simplified Daylight or Sky Factors are obsolete now, the solid angle for large area sources with a particular sky luminance specification is still needed for calculating the Daylight Coefficient or absolute illuminance in computer programs.
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