In 1953, Koehler settled in a studio in the tower of the historical castle of Siegen, where he would work until his untimely death in 1970. Here he developed his first paintings with sand, glue and pigments. In the early stages these so-called 'Sandbilder' or sand paintings appear as relatively flat sand-coloured surfaces incised with Koehler's freehand scratching. Over time they developed into built-up works with hollow points, reminiscent of moon craters and spatial landscapes. From a distance these convey a level of space expansion through the effects of optical illusion. During the late 1960s she sand paintings were reduced to elegant traces of sand on raw canvas.

Koehler's works are a testimony to his powerful creative mind and individual expression which can be situated within the central tenets of the German 1950s Avant-Garde, matching the artistic endeavours of artists such as Otto Piene, Heinz Mack, Emil Schumacher and Wolf Vostell, amongst others.

The work of Koehler is in the following museums: Museum Wiesbaden, Wiesbaden; Wilhelm Hack Museum, Ludwigshafen; Kunsthalle Bielefeld, Bielefeld; Kunstmuseum Bochum, Bochum; Museumlandschaft Hessen, Kassel; Goethe Institut, Munich; Städtische Galerie, Wolfsburg; Museum Moderner Kunst Stiftung Ludwig, Vienna; The European Parliament, House of Europan History, Brussels.