Coloured shadows—Why they can be photographed

Coloured shadows have been observed, studied, and described for centuries, raising heated debates on whether they were objective or subjective, real or imaginary, psychological or physiological. We can demonstrate this phenomenon by projecting coloured light from one and white light from the other projector on a screen, where an object casts two shadows: one (illuminated by coloured light) expectedly coloured and the other (illuminated by white light) unexpectedly coloured. One of the seemingly most solid arguments for the objectivity of the unexpected colour was the fact that it could be photographed. The aim of this work was to photograph the shadows both with conventional (colour slide film) and digital cameras and explain why the unexpected colour of the shadows appears in these photographs. With spectroradiometric measurements of the shadows themselves and also those of the photographs, it may be proven that this is simply a spectacular case of adaptation and simultaneous contrast. Digital rendering of the coloured shadow situation also supports this conclusion. Coloured shadows are excellent tools for the demonstration of adaptation and simultaneous contrast.