Ibn al-Haytham's dark room (the camera obscura experiments)

In the first book of his Kitab al-Manazir (Book of Optics), Ibn al-Haytham lets the light of the sun, the moon, the stars and a fire into a dark room through a small hole, and studies it on the opposite wall. He follows the light through dusty air, cuts its path with an opaque body, lays a ruler along it, and finds it perfectly straight ¹. Later in the same book he sets several candles before one hole into a dark place: on the wall appear as many spots of light as there are candles, each opposite its own candle along a straight line through the hole, and when one candle is covered only its spot goes out ². From this he concluded that light travels in straight lines and that lights crossing the same opening do not mix. TDV calls the dark-room experiment, devised to prove that light travels in straight lines, the basis of today's photographic camera ³. The museum records it as an improvement: Ibn al-Haytham did not discover that light passes through a hole, but he turned that familiar sight into an ordered, repeatable test.

The dark room (in Arabic al-bayt al-muzlim, in Latin camera obscura) is a closed, unlit room or place with one small opening to the light outside. Whatever bright thing stands outside, the sun, the moon, a star or a candle, sends its light through the opening, and the light falls on the wall or on a body facing the opening inside.

In the Manazir the dark room is not a toy or a picture-making device but an instrument of proof. Ibn al-Haytham uses it to show two things: that light from any luminous body travels along straight lines ⁴, and that the lights of different bodies pass through the same opening without blending, then part again on the far side ². The editor of the Arabic text describes the whole book as a study of the properties of light in its three states, travelling straight, reflected and bent, built on experiment, which Ibn al-Haytham called i'tibar, together with mathematical methods ⁵. The dark room is where that method can be watched at work.

Light coming into a room through a gap is an everyday sight, and Ibn al-Haytham says so himself: the straightness of light shows plainly to anyone who examines the light that comes into houses through holes, cracks and doors ⁶.

What earlier scholars lacked, in the editor's account, was a study of light that put experiment first. For the Greeks optics was a branch of mathematics; Ibn al-Haytham described it instead as a science compounded of the natural and the mathematical sciences ⁷. On how we see, he faced three schools: the mathematicians such as Euclid and Ptolemy, the natural philosophers following Aristotle, and the physicians following Galen. His own theory differed from all three ⁸. TDV adds that he overturned the old view that rays go out from the eye to the object ³.

Ibn al-Haytham, known in medieval Europe as Alhazen, was born around 354 AH (about 965 CE) and came from Basra ³. There, under the Buyids, he worked in a government office with the title of vizier, then left it. TDV reports that the Fatimid caliph al-Hakim heard that he claimed he could regulate the Nile's flood and invited him to Egypt. Ibn al-Haytham travelled south to the region of Aswan, studied the site, saw that the plan could not be carried out, apologised and was given a post. He only found peace after al-Hakim's death in 411/1021 ³.

He spent most of his life in Egypt. Ibn Abi Usaybi'a, quoted by the editor, says he lived in Cairo at the mosque of al-Azhar, and that each year he copied Euclid's Elements and Ptolemy's Almagest, sold the copies and lived on their price ⁹. TDV describes him as an ascetic who lived by copying books; it places him in Baghdad in 418/1027, notes that Sa'id al-Andalusi has him still in Egypt in 430, and gives 432/1040 as his probable year of death ³.

The Manazir carries no date of writing. Ibn Abi Usaybi'a preserves three lists of Ibn al-Haytham's works; the third, which runs to the end of 429 AH (1037–1038 CE), holds 92 titles and names the Manazir among them ¹⁰. The editor believes Ibn al-Haytham may have drawn up this list himself, near the end of his life ¹¹. In it the Manazir comes before his short works on shadows, on the marks on the face of the moon, on light, on the burning sphere and on the form of the eclipse, so the editor takes these to be later ¹².

Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham is named so at the head of the first book of the Manazir, which opens with the basmala ¹³. TDV counts him among the contemporaries of Qadi Abd al-Jabbar, al-Baqillani, Ibn Miskawayh, Ibn Sina and al-Biruni. Besides his works in mathematics, astronomy and optics, it lists treatises in which he answered Ibn al-Rawandi, answered Abu Bakr al-Razi's criticism of prophethood, and took issue with the Mu'tazila on the divine attributes and on the threat of punishment ³. The faith entry in his record rests on these works and on his own opening of the Manazir.

The editor of the Arabic text counts the Manazir the most important book on optics written between Ptolemy in the second century CE and Kepler and Descartes in the early seventeenth, and the most valuable work the Arab scientists produced in physics ⁵. Zahir al-Din al-Bayhaqi, TDV notes, called him the second Ptolemy ³.

Ibn al-Haytham sets out several versions of the test, each meant to be repeated by the reader.

Sunlight and a cutting body. When sunlight enters a dark room through a hole, and the air of the room is clouded with dust or smoke, the light can be seen stretching in a straight line from the hole to the spot where it falls on the floor or wall ⁶. Set an opaque body anywhere on that line and the light falls on the body and vanishes from the spot beyond; remove the body and the spot returns. Lay a straight ruler along the line between the hole and the spot and the light keeps to the ruler's edge: its path is perfectly straight ¹⁴. The same, he says, holds for the light of the moon ¹⁴.

Starlight and firelight. On a night without a moon, if a bright star faces a hole into a dark room, its light appears on the wall opposite the hole, on the straight line from the star through the hole, and is cut off in the same way by an opaque body ¹⁵.

The eclipse and the cone of light. During a partial eclipse of the sun, light still comes through the hole from the whole of the visible part of the sun. Light passing through a hole always spreads out in a cone, wider the farther it goes, and this is plain when the hole is fine ¹⁶.

The candles. In the sixth chapter of the first book he takes several candles (suruj) placed at different points facing a single hole that opens into a dark place, with a wall or an opaque body inside opposite the hole. On that wall appear separate lights, as many as the candles, each on the straight line that runs from its candle through the hole. Cover one candle and only the light opposite it vanishes; uncover it and the light comes back ¹⁷. He gives a practical way to do it: in a room at night, with a door of two leaves opened only slightly, set candles outside; then narrow the opening to a small hole, and the result is the same ². Since all the lights meet at the hole and then part again after it, he concludes that lights do not mix in the air or in other transparent bodies ¹⁸.

A consequence follows from the geometry: because each spot lies on the straight line from its candle through the hole, a candle standing to the right of the hole throws its spot to the left of the wall, and one standing higher throws it lower. This is the principle of every pinhole camera.

For a time the Manazir seems to have dropped out of circulation in the eastern Islamic lands. The editor tells how Qutb al-Din al-Shirazi remembered seeing it in a library in Fars when he was young, and later had a copy brought from far away; the story itself shows that the book was then hard to find ¹⁹. His student Kamal al-Din al-Farisi then wrote the Tanqih al-Manazir (The Revision of the Optics) ²⁰.

In Europe the book arrived through a Latin translation whose translator, date and place are unknown. The editor thinks it was probably made in Spain in the late twelfth or early thirteenth century; the first European author known to cite it is Jordanus de Nemore, active around 1220–1230 ²¹. About twenty Latin manuscripts survive, and an Italian translation was made from the Latin in the fourteenth century ²². One detail matters here: the Latin version lacks the first three chapters of the first book ²³. So the dark-room passage of chapter 3, with the dust, the ruler and the starlight, did not reach Latin readers, while the candles experiment of chapter 6 did.

In 1572 Friedrich Risner printed the Latin text at Basel under the title Opticae thesaurus, together with a treatise on dawn and twilight attributed to Ibn al-Haytham (in fact by Ibn Mu'adh al-Jayyani) and the ten books of Witelo's optics ^{24 3}.

• Everyday light through holes and doors. Ibn al-Haytham starts from what anyone could see in a house: light entering through holes, cracks and doors ⁶.
• Greek optics and the theories of vision. He wrote against the background of Euclid and Ptolemy, of Aristotle and of Galen, and the editor stresses that his theory of vision differed from all of them ⁷.

• His own treatise on the form of the eclipse. Maqala fi surat al-kusuf comes after the Manazir in the list of his works ¹². TDV notes that Kamal al-Din al-Farisi commented on it, and that Eilhard Wiedemann studied it in German under the title Über die Camera obscura bei Ibn al Haiṯam (Erlangen 1914) ³.
• Al-Farisi's Tanqih al-Manazir. Written at the suggestion of Qutb al-Din al-Shirazi, it began as an abridgement but grew into a work that explains, completes and objects; al-Farisi marks Ibn al-Haytham's words with qala (he said) and his own with aqulu (I say). It was printed at Hyderabad by the Da'irat al-Ma'arif al-Uthmaniyya in 1348 AH (1930 CE) in two volumes ²⁵.
• The Latin Manazir and Risner's edition. The anonymous Latin translation and its printed form of 1572 ²⁶.
• Optics in Europe. Witelo wrote his own optics after reading the Latin Manazir and followed it closely in subject, figures and often wording ²². TDV names Pecham, Witelo, Roger Bacon, Kepler, Snell, Fermat and Descartes among those whose work it shaped ³.

The dark room left the study and entered daily life much later, through the camera. TDV calls Ibn al-Haytham's experiment the basis of today's photographic camera ³: a camera, from the simplest pinhole box to a modern device, is a dark chamber into which light enters through a small opening and falls on a surface opposite. The candles test shows why the picture is sharp rather than a blur: the light from each point outside reaches its own point inside, along its own straight line ².

The change was one of method. Before the Manazir, that light travels in straight lines was a premise of geometrical optics; Ibn al-Haytham made it a result, shown with a hole, a dark room, a cutting body and a ruler, and repeated with sunlight, moonlight, starlight and firelight ¹. The editor sees the book's lasting value in this joining of experiment to mathematics ⁵, and TDV says Ibn al-Haytham changed optics at its roots ³. Through al-Farisi in the Islamic world and through the Latin translation and Witelo in Europe, the Manazir remained the reference work on optics for centuries ²⁷.

No one source names a first inventor of the dark room, and Ibn al-Haytham does not claim the title: he presents light through holes as a familiar sight and turns it into a test ⁶. The museum therefore records him as an improver. TDV credits him with devising the dark-room experiment to prove that light travels in straight lines ³.

A separate scholarly question concerns the man himself. TDV reports that Roshdi Rashed argued that the sources join two different men named Ibn al-Haytham, and that A. I. Sabra rejected this view in 1998 ³.

The Arabic text of the Manazir is available in a critical edition: the first three books, which contain the dark-room and candles passages, were edited by A. I. Sabra and published in Kuwait in 1983 ²⁸. TDV links the experiment to the photographic camera ³, and every camera keeps the same plan of a dark chamber with a small opening.

• The Arabic manuscripts. The editor lists the manuscripts he used; among them, Ayasofya MS 2448 in Istanbul contains all seven books and was copied in 869 AH (1464–1465 CE) ²⁹.
• The Latin manuscripts. About twenty survive, at least seven of them from the thirteenth century, one of them, at the Royal Observatory in Edinburgh, dated 1269; a fourteenth-century Italian translation survives in a single manuscript in the Vatican Library ²².
• The Tanqih. Al-Farisi's revision is in print (Hyderabad 1347–1348 AH) ³. The museum's image shows a page that Wikimedia Commons describes as from the author's own copy ³⁰.

Latin Europe knew Ibn al-Haytham as Alhazen, and under that name the Manazir circulated in manuscript and in Risner's printed Opticae thesaurus ³. In the Islamic sources he is remembered as a scholar who lived simply on the copying of books ⁹, and as "the second Ptolemy" in Zahir al-Din al-Bayhaqi's phrase ³.

About a thousand years ago a Muslim scholar named Ibn al-Haytham lived in Cairo. One night he closed the door of a room, leaving only a small gap, and set several candles outside it. On the wall inside he saw one spot of light for each candle! When he covered a candle, its spot vanished, and when he uncovered it the spot came back. That is how he showed that light travels in a straight line, and that the lights of many candles can pass through one small hole without getting mixed up. A camera works on the same idea today: it is a dark box with a small opening.

• Ibn al-Haytham, Kitab al-Manazir, Books 1–3, edited by A. I. Sabra, Kuwait, 1983 (1404 AH): editor's introduction and Book 1, scan pp. 9–78 and 171–172. ³¹
• Hüseyin Gazi Topdemir, "İBNÜ'l-HEYSEM", TDV Encyclopedia of Islam, vol. 21 (Istanbul 2000), pp. 82–87, online edition. ³
• Wikimedia Commons, "Tanqih al-Manazir.JPG", Danieliness, CC BY-SA 3.0. ³⁰