In 1933 Gabor fled from Nazi Germany, where he was considered Jewish, and was invited to Britain to work at the development department of the British Thomson-Houston company in Rugby, Warwickshire. During his time in Rugby, he met Marjorie Louise Butler, and they married in 1936. He became a British citizen in 1946, and it was while working at British Thomson-Houston in 1947 that he invented holography, based on an electron microscope, and thus electrons instead of visible light. He experimented with a heavily filtered mercury arc light source. The earliest visual hologram was only realised in 1964 following the 1960 invention of the laser, the first coherent light source. After this, holography became commercially available.