A camera obscura (; ) is a darkened room with a small hole or lens at one side through which an image is projected onto a wall or table opposite the hole. Camera obscura can also refer to analogous constructions such as a box or tent in which an exterior image is projected inside. Camera obscuras with a lens in the opening have been used since the second half of the 16th century and became popular as aids for drawing and painting. The concept was developed further into the photographic camera in the first half of the 19th century, when camera obscura boxes were used to expose light-sensitive materials to the projected image. The camera obscura was used to study eclipses without the risk of damaging the eyes by looking directly into the sun. As a drawing aid, it allowed tracing the projected image to produce a highly accurate representation, and was especially appreciated as an easy way to achieve proper graphical perspective. Before the term camera obscura was first used in 1604, other terms were used to refer to the devices: cubiculum obscurum, cubiculum tenebricosum, conclave obscurum, and locus obscurus. A camera obscura without a lens but with a very small hole is sometimes referred to as a pinhole camera, although this more often refers to simple (homemade) lensless cameras where photographic film or photographic paper is used. Physical explanation Rays of light travel in straight lines and change when they are reflected and partly absorbed by an object, retaining information about the color and brightness of the surface of that object. Lighted objects reflect rays of light in all directions. A small enough opening in a barrier admits only the rays that travel directly from different points in the scene on the other side, and these rays form an image of that scene where they reach a surface opposite from the opening. The human eye (and those of animals such as birds, fish, reptiles etc.) works much like a camera obscura with an opening (pupil), a convex lens, and a surface where the image is formed (retina). Some cameras obscura use a concave mirror for a focusing effect similar to a convex lens. Technology A camera obscura consists of a box, tent, or room with a small hole in one side or the top. Light from an external scene passes through the hole and strikes a surface inside,