At last, here’s an Andy Warhol exhibition consisting of work that is largely unknown. In 1978, at the age of 50, Warhol began putting together a monumental body of work entitled Shadows, with the assistance of his entourage at his New York studio, The Factory. Shadows was conceived as one painting in many parts, the final number of canvases being determined by the dimensions of an exhibition space. The result was 102 silk-screen-printed canvas panels. All are on view at the Guggenheim Museum Bilbao. The backgrounds of Shadows show Warhol’s typical palette of bright hues. Seven or eight different screens were used to create the series. The “shadows” of the title alternate between positive and negative imprints as they march along the wall of the gallery.   In focusing on the shadow to devise light – that is to say, sparks of colour – Warhol returns to the quintessential problem of art, perception. The exhibition runs until October 2. guggenheim.org