A screen is provided on a housing in such a manner that the screen protrudes toward its nearer side, and is inclined so that the nearer side becomes lower. The feed may optionally also include shorter paraffins, aromatics or cycloparaffins. For some reflector geometries it is essential to use an additional cylindrical lens for focusing, and for other reflector geometries the use of such a lens may be advantageous. For example, a program pulse is applied to a control gate for the first non-volatile storage element.