ABSTRACT
The word ‘colour’ is a dangerous word because it has very different senses. We all have one intuitive understanding of this notion, and we think we know what is green, or orange. In fact, the word ‘colour’ regains two fundamentally different notions:
• on the one hand, the visual sensation that our eyes receive from the surface of an illuminated object;
• on the other hand, the spectral curve of a radiance fixing, for every wavelength, the quantity of light emitted or transmitted by an object.
