where the scene is reproduced, inverted (upside-down) and reversed (left to right), but with color and perspective preserved. To produce a reasonably clear projected image, the aperture is typically smaller than 1/100th the distance to the screen. As the pinhole is made smaller, the image gets sharper, but dimmer. With a too small pinhole, however, the sharpness worsens, due to diffraction. Optimum sharpness is attained with an aperture diameter approximately equal to the geometric mean of the wavelength of light and the distance to the screen. In practice, camera obscuras use a lens rather than a pinhole because it allows a larger aperture, giving a usable brightness while maintaining focus. If the image is caught on a translucent screen, it can be viewed from the back so that it is no longer reversed (but still upside-down). Using mirrors, it is possible to project a right-side-up image. The projection can also be displayed on a horizontal surface (e.g., a table). The 18th-century overhead version in tents used mirrors inside a kind of periscope on the top of the tent. The box-type camera obscura often has an angled mirror projecting an upright image onto tracing paper placed on its glass top. Although the image is viewed from the back, it is reversed by the mirror. History Prehistory to 500 BC: Possible inspiration for prehistoric art and possible use in religious ceremonies, gnomons There are theories that occurrences of camera obscura effects (through tiny holes in tents or in screens of animal hide) inspired paleolithic cave paintings. Distortions in the shapes of animals in many paleolithic cave artworks might be inspired by distortions seen when the surface on which an image was projected was not straight or not in the right angle. It is also suggested that camera obscura projections could have played a role in Neolithic structures. Perforated gnomons projecting a pinhole image of the sun were described in the Chinese Zhoubi Suanjing writings (1046 BC–256 BC with material added until circa 220 AD). The location of the bright circle can be measured to tell the time of day and year. In Arab and European cultures its invention was much later attributed to Egyptian astronomer and mathematician Ibn Yunus around 1000 AD. 500 BC to 500 AD: Earliest written observations One of the earliest known written records of a pinhole camera for camera obscura effect is found in the Chinese text called