Work Plane : The plane at which work is usually done and on which the luminance is specified and measured. Unless otherwise indicated, this is assumed to be a horizontal plane 30" above the floor
Young & Company Electric Power Products and Related Engineering Services, Glossary of Electrical Terms, http://www.youngco.com/young2.asp?ID=4&Type=3, CA., USA 2001. (Electr Sfty - Amer) {514}