It is this unit that is moved back and forth inside the lamp to focus the light. This is done by a slider on the bottom of the lantern, or by a worm track with a crank in the back of the unit. The lamps are almost always mounted 'base down', i.e. with the bulb standing upwards. Burning these lamps upside down will shorten lamp life significantly. Fresnels are not very efficient. The reflector cannot be larger than the lens aperture, and thus all the radiated light that is neither redirected forward by the spherical reflector behind the bulb nor emitted directly through the lens is absorbed by the casing as waste heat. The degree to which the lamp may be focused is limited by the length of the housing. To reduce the width of the beam, the lamp and reflector are moved further back from the lens (spot focus). However, the farther back in the housing the lamp is placed, the more light is wasted in the housing. Use In the theater and dance world, Fresnels are most often used for top or backlighting, at medium throw distances. In small venues, they are occasionally used for front light, although the relative lack of control, when compared to an ellipsoidal, is a major disadvantage. The distinctive soft beam of a fresnel can make it very useful as a special in certain situations. While the focus can alter the size of the beam, the distinctive scatter of light that the Fresnel lens emits also requires a way of controlling its shape. Since Fresnels cannot use internal shutters, such as those found in an ellipsoidal spotlight, they are often fitted with distinctive barn doors to control the spill and shape the beam of light. These are large metal flaps that may be mounted just beyond the color slot at the front of the lantern. They are colloquially known as "Harris Flaps" in the United Kingdom theatre industry. References Stage lighting instruments