Frede Christoffersen (1919-1987) works with a phenomenon he calls 'after-pictures'. 'After-pictures' are the optical illusions you can get when looking directly at the sun. You might also experience them when looking intensely at colours as the eyes will create the complementary colours to the colour you are looking at. Try for yourself and test the phenomenon in Frede Christoffersens light laboratory (link in Danish).
Further reading on Frede Christoffersen here (link in Danish).
At Kunst Index Danmark (link in Danish) you can search through the works at Vendsyssel Museum of Art as well as all other Danish art museums. The site is continuously being extended with information on new works. Kunst Indeks Danmark contains the central register of works of art in Danish museums and collections as well as the work of reference Weilbachs Kunstnerleksikon (Wielbach's Encyclopaedia of Artists). Other relevant links (in Danish)
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