The three basic parts of a daylighting device are a light-collecting device, a light-transmitting device and a light emitting device. That's a fancy way of saying daylighting devices use a series of lenses and mirrors (or optic fibers) to transmit sunlight into a building.
The lenses in the light collection part of a daylighting device direct light into the light transmission component. This part of the device often has a glass dome or pyramid on top of it. The glass directs light into the device's lenses. Some daylighting devices use a pair of lenses. The first lens is a convex lens, which refracts light toward a central point. Beneath the convex lens is a concave lens. Concave lenses reflect light rays outward toward the light transmission part of the device.
The transmission portion of a daylighting device is either a tube with a reflective interior or a series of optical fibers. An optical fiber is essentially a miniaturized reflective tube, so, in general, both solutions use the same approach. As light enters the tube or fiber, it hits the reflective inner surface and bounces farther down the path. It continues to do this until it hits the light emitter section of the daylighting device. The longer the transmission section of a daylighting device, the less light will make it to the emitter. In other words, running a tube down a 60-story building to light the basement may not be the best solution.
The light emitter is the part of the device that you'd see if you were inside the building. It's the equivalent to a fluorescent light panel or an incandescent bulb. Many emitters have a special lens that diffuses light. A diffuse lens has tiny surfaces within it that reflect light in different directions. The end effect is that the lens spreads the light around.
Daylighting devices have a few advantages over windows and skylights. One is that they aren't affected by the position of the sun. The amount of light that passes through a window or skylight can change dramatically as the day goes on. But as long as the light-catching section of a daylighting device receives sunlight, the emitter will continue to give off light; although the brightness may vary a little depending on the intensity of the sun. Another advantage is that you can put the light-catching part of the daylighting device in any location on a roof and use the transmission tube or fibers to run the light to its destination. You can even bend light around corners. And you don't have to sit near an exterior wall to enjoy the benefits of a daylighting device the way you do with a window.
Daylighting devices shouldn't be the sole source of lighting for a building. A daylighting device won't do you much good if you need light past sundown. While some daylighting devices can capture ambient light even on cloudy days, you'll probably notice a decrease in performance when skies are overcast. Daylighting devices work best as a supplemental light source.