Mozi correctly asserted that the camera obscura image is inverted because light travels in straight lines. The oldest known record of this principle is a description by Han Chinese philosopher Mozi (ca. Camera obscura ( Latin for "dark room") is the natural optical phenomenon that occurs when an image of a scene at the other side of a screen (or for instance a wall) is projected through a small hole in that screen to form an inverted image (left to right and upside down) on a surface opposite to the opening. Projectors share a common history with cameras in the camera obscura. Main articles: Camera obscura and Pinhole camera Joseph Needham sums up some possible projection examples from China in his 1962 book series Science and Civilization in China Prehistory to 1100 Shadow play Projections were often presented or perceived as magic or even as religious experiences, with most projectionists unwilling to share their secrets. Many did not understand the nature of what they had seen and few had ever seen other comparable media. Spectators did not always provide the details needed to differentiate between for instance a shadow play and a lantern projection. There probably existed quite a few other types of projectors than the examples described below, but evidence is scarce and reports are often unclear about their nature. Revolving lanterns (see below for details). Mini portable home theatres projector.Enlarger (not for direct viewing, but for the production of photographic prints).Steganographic mirror (see below for details).The magic lantern is best known for the projection of still images, but was capable of projecting moving images from mechanical slides since its invention and was probably at its peak of popularity when used in phantasmagoria shows to project moving images of ghosts. For instance: video projectors were basically developed for the projection of prerecorded moving images, but are regularly used for still images in PowerPoint presentations and can easily be connected to a video camera for real-time input. Some of the listed projectors were capable of projecting several types of input. Projectors can be roughly divided into three categories, based on the type of input. Movie theaters used a type of projector called a movie projector, nowadays mostly replaced with digital cinema video projectors. The newest types of projectors are handheld projectors that use lasers or LEDs to project images. These earlier types of projectors were mostly replaced with digital video projectors throughout the 1990s and early 2000s, but old analog projectors are still used at some places. Video projectors are digital replacements for earlier types of projectors such as slide projectors and overhead projectors. The most common type of projector used today is called a video projector. A virtual retinal display, or retinal projector, is a projector that projects an image directly on the retina instead of using an external projection screen. Most projectors create an image by shining a light through a small transparent lens, but some newer types of projectors can project the image directly, by using lasers.
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