1. The rods in the retina respond to much lower light levels. They are saturated at normal light levels but at low levels they produce an achromatic response to the wavelength range below about 600 nm. Moonlit scenes are therefore lacking in colour. To become active the rods need to recover from the saturation caused by high light levels and this process of dark adaptation takes up to 30 min or so. Their activity can be preserved by using red illumination. For example, in a submarine if the red lights used were to fail suddenly the rods would still be responsive whereas white light failure would leave the crew essentially blind for several minutes
2. Artists may disagree. Artists’ primaries are blue, red and yellow. Many do recognise that the best blue for mixing with yellow to get the best greens is ‘Phthalo’ blue, a phthalocyanine pigment which is in fact the best cyan available in the paint box. Also, Permanent Rose which is a decent magenta is often recommended for a mixing red. Since it is possible to get a blue from a cyan and magenta and a red from a magenta and yellow, while it is impossible to achieve a magenta or a cyan from a mixture of blue red and yellow, the traditional artists’ case is weak if not simply wrong.
3. The letter K is used to avoid confusion with Blue.
4. In practice, two or even three layers make up each blue-, green- and red-sensitive sets of layers to improve the sensitivity/image structure (grain and sharpness) response.
5. Dispersion of very small silver particles can be highly coloured and of various hues depending on particle size. Carey-Lee silver is a suspension of very small silver particles of narrow size distribution. It provides a strong yellow colour and is removed conveniently during processing when the developed image silver is removed by bleaching and fixing.