Today's projectors are based on one of four imaging technologies: digital light processing (DLP), liquid-crystal display (LCD), liquid crystal on silicon (LCOS), and laser raster. (Don't confuse laser raster projectors, which actually draw the images using lasers, with the much more common models that simply use lasers as a light source for another imaging technology, like DLP or LCD chips.) Most DLP projectors and some LCOS-based pico (aka pocket-size) projectors—including both data and video models—use only one display chip, and project their primary colors sequentially rather than all at once. This can lead to rainbow artifacts, in which bright areas on the screen break up into little red-green-blue flashes for some people when they shift their gaze or when something moves onscreen. Those who are sensitive to this effect can find it annoying, particularly for long viewing sessions. Three-chip LCD projectors are free from rainbow artifacts, because the three chips allow projection of all three primary colors at once. However, they tend to be bigger and heavier than comparable DLP models. Standard-size LCOS projectors, also rainbow-free, offer the best-quality images, but they tend to be bigger and heavier than either DLP or LCD projectors, as well as far more expensive. There aren't many laser raster projectors, so it's hard to make general statements about them. But the one clear advantage of using a laser is that the image doesn't require focusing.