The color at any point on a painting can be measured as a reflectance spectrum, which is a curve showing the proportion of light reflected at every wavelength through the visible spectrum (400-760 nanometers). The National Gallery in London plans to set up two complementary systems of measurement: (a. ) measuring the complete spectrum at chosen points on the paintings, and (b.) recording the reflectance of the whole painting at 6 wavelengths spaced through the spectrum. For detecting fine differences there seems to be no alternative to scanning images electronically and combining scans to obtain a difference signal.