of cheaper optical fibers and fiber-optic concrete walls, daylight (and shadow images) can then pass directly through a solid concrete wall, making it translucent; fiber optics will lead light around bends and over tens of meters. Typically only a few percent of the light is transmitted (the percent transmittance is about half the percent of the surface that is fibers, and usually only ~5% fibers are used). Both glass and concrete conduct heat fairly well, when solid, so none of these walls insulate well. They are therefore often used outdoors, as a divider between two heated spaces (see images), or in very temperate climates. Greenhouse walls (and roofs) are made to transmit as much light and as little heat as possible. They use a variety of materials, and may be transparent or translucent. Remote distribution It is possible to provide some daylight into spaces that have low possibility of windows or skylights through remote distribution devices such as mirrors, prisms, or light tubes. This is called anidolic lighting, from anidolic (non-image-forming) optics. The non-linear response of the human eye to light means that spreading light to a broader area of a room makes the room appear brighter, and makes more of it usefully lit. Remote daylight distribution systems have losses, and the further they have to transmit the daylight and the more convoluted the path, the greater the inefficiency. The efficiency of many remote distribution systems can also vary dramatically from clear to overcast skies. Nonetheless, where there is no other possibility of providing daylight to a space, remote distribution systems can be appreciated. Light reflectors and shelves Once used extensively in office buildings, the manually adjustable light reflector is seldom in use today having been supplanted by a combination of other methods in concert with artificial illumination. The reflector had found favor where the choices of artificial light provided poor illumination compared to modern electric lighting. Light shelves are an effective way to enhance the lighting from windows on the equator-facing side of a structure, this effect being obtained by placing a white or reflective metal light shelf outside the window. Usually the window will be protected from direct summer season sun by a projecting eave. The light shelf projects beyond the shadow created by the eave and reflects sunlight upward to illuminate the ceiling. This reflected light can contain little heat content and the reflective illumination from the