Camera obscura
A camera obscura is the natural phenomenon in which light passing through the small hole of a dark chamber or box will project an image of a scene outside the chamber onto the surface opposite to the hole, resulting in an inverted and reversed projection of the view outside.
Camera obscura refers to analogous constructions such as a darkened room, box or tent in which an exterior image is projected inside or onto a translucent screen viewed from outside. 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 technology 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 image of a lensless camera obscura is also referred to as a "pinhole 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 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 works much like a camera obscura, with rays of light entering an opening, getting focused through a convex lens and passing a dark chamber before forming an inverted image on a smooth surface. The analogy appeared early in the 16th century and would in the 17th century find common use to illustrate Western theological ideas about God creating the universe as a machine, with a predetermined purpose. This had a huge influence on behavioral science, especially on the study of perception and cognition. In this context, it is noteworthy that the projection of inverted images is actually a physical principle of optics that predates the emergence of life and is not characteristic of all biological vision.
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, where the scene is reproduced, inverted and reversed, but with color and perspective preserved.To produce a reasonably clear projected image, the aperture is typically smaller than 1/100 the distance to the screen.
As the pinhole is made smaller, the image gets sharper, but dimmer. With too small of a pinhole, sharpness is lost because of 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 makes a larger aperture work which achieves a usable brightness while maintaining focus.
If the image is caught on a transparent screen, it can be viewed from the back so that it is no longer reversed. Using mirrors, it is possible to project a right-side-up image. The projection can also be displayed on a horizontal surface. 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 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. The location of the bright circle can be measured to tell the time of day and year. In Middle Eastern 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 image is found in the Chinese text called Mozi, dated to the 4th century BC, traditionally ascribed to and named for Mozi, a Chinese philosopher and the founder of Mohist School of Logic. These writings explain how the image in a "collecting-point" or "treasure house" is inverted by an intersecting point that collects the light. Light coming from the foot of an illuminated person gets partly hidden below and partly forms the top of the image. Rays from the head are partly hidden above and partly form the lower part of the image.Another early account is provided by Greek philosopher Aristotle, or possibly a follower of his ideas. Similar to the later 11th-century Middle Eastern scientist Alhazen, Aristotle is also thought to have used camera obscura for observing solar eclipses. The formation of pinhole images is touched upon as a subject in the work Problems – Book XV, asking: and further on:
In an attempt to explain the phenomenon, the author described how the light formed two cones; one between the Sun and the aperture and one between the aperture and the Earth. However, the roundness of the image was attributed to the idea that parts of the rays of light are cut off at the angles in the aperture become so weak that they cannot be noticed.
Many philosophers and scientists of the Western world would ponder the contradiction between light travelling in straight lines and the formation of round spots of light behind differently shaped apertures, until it became generally accepted that the circular and crescent-shapes described in the "problem" were pinhole image projections of the sun.
In his book Optics, Euclid proposed mathematical descriptions of vision with "lines drawn directly from the eye pass through a space of great extent" and "the form of the space included in our vision is a cone, with its apex in the eye and its base at the limits of our vision." Later versions of the text, like Ignazio Danti's 1573 annotated translation, would add a description of the camera obscura principle to demonstrate Euclid's ideas.
500 to 1000: Earliest experiments, study of light
In the 6th century, the Byzantine-Greek mathematician and architect Anthemius of Tralles experimented with effects related to the camera obscura. Anthemius had a sophisticated understanding of the involved optics, as demonstrated by a light-ray diagram he constructed in 555 AD.In his optical treatise De Aspectibus, Al-Kindi wrote about pinhole images to prove that light travels in straight lines.
In the 10th century Yu Chao-Lung supposedly projected images of pagoda models through a small hole onto a screen to study directions and divergence of rays of light.
1000 to 1400: Optical and astronomical tool
Middle Eastern physicist Ibn al-Haytham extensively studied the camera obscura phenomenon in the early 11th century.In his treatise "On the shape of the eclipse" he provided the first experimental and mathematical analysis of the phenomenon.
He understood the relationship between the focal point and the pinhole.
In his Book of Optics, Ibn al-Haytham explained that rays of light travel in straight lines and are distinguished by the body that reflected the rays, writing:
Latin translations of the Book of Optics from about 1200 onward seemed very influential in Europe. Among those Ibn al-Haytham is thought to have inspired are Witelo, John Peckham, Roger Bacon, Leonardo da Vinci, René Descartes and Johannes Kepler. However, On the shape of the eclipse remained exclusively available in Arabic until the 20th century and no comparable explanation was found in Europe before Kepler addressed it. It were actually al-Kindi's work and especially the widely circulated pseudo-Euclidean De Speculis that were cited by the early scholars who were interested in pinhole images.
In his 1088 book, Dream Pool Essays, the Song dynasty Chinese scientist Shen Kuo compared the focal point of a concave burning-mirror and the "collecting" hole of camera obscura phenomena to an oar in a rowlock to explain how the images were inverted:
Shen Kuo also responded to a statement of Duan Chengshi in Miscellaneous Morsels from Youyang written in about 840 that the inverted image of a Chinese pagoda tower beside a seashore, was inverted because it was reflected by the sea: "This is nonsense. It is a normal principle that the image is inverted after passing through the small hole."
English statesman and scholastic philosopher Robert Grosseteste was one of the earliest Europeans who commented on the camera obscura.
English philosopher and Franciscan friar Roger Bacon falsely stated in his De Multiplicatione Specerium that an image projected through a square aperture was round because light would travel in spherical waves and therefore assumed its natural shape after passing through a hole. He is also credited with a manuscript that advised to study solar eclipses safely by observing the rays passing through some round hole and studying the spot of light they form on a surface.
Polish friar, theologian, physicist, mathematician and natural philosopher Vitello wrote about the camera obscura in his influential treatise Perspectiva, which was largely based on Ibn al-Haytham's work.
English archbishop and scholar John Peckham wrote about the camera obscura in his Tractatus de Perspectiva and Perspectiva communis, falsely arguing that light gradually forms the circular shape after passing through the aperture. His writings were influenced by Bacon.
French astronomer Guillaume de Saint-Cloud suggested in his 1292 work Almanach Planetarum that the eccentricity of the Sun could be determined with the camera obscura from the inverse proportion between the distances and the apparent solar diameters at apogee and perigee.
Kamāl al-Dīn al-Fārisī described in his 1309 work Kitab Tanqih al-Manazir how he experimented with a glass sphere filled with water in a camera obscura with a controlled aperture and found that the colors of the rainbow are phenomena of the decomposition of light.
French Jewish philosopher, mathematician, physicist and astronomer/astrologer Levi ben Gershon made several astronomical observations using a camera obscura with a Jacob's staff, describing methods to measure the angular diameters of the Sun, the Moon and the bright planets Venus and Jupiter. He determined the eccentricity of the Sun based on his observations of the summer and winter solstices in 1334. Levi also noted how the size of the aperture determined the size of the projected image. He wrote about his findings in Hebrew in his treatise Sefer Milhamot Ha-Shem Book V Chapters 5 and 9.