Al-Haytham (Alhazen), Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn
Abu Ali al-Hasan Ibn al-Haytham (Latinized name: Alhazen) made important discoveries in astronomy, mathematics, and optics. His Kitab al-Manazir (Book on Optics) made the works of Aristotle, Ptolemy, and Euclid obsolete. He merged their classical theories, combining the mathematical, physical, and physiological aspects into one unified optical theory. The Kitab al-Manazir also introduced the distinction between optics and physiology/psychology that is a cornerstone of modern optics and colorimetry.
In his work, Ibn al-Haytham attempted to show how illumination, hue, and saturation combine together into color perception. Although he was not able to establish a complete system of color attributes, his account is largely consistent. “In terms of scope, details and comprehensiveness of treatment, Ibn al-Haytham’s observations on the subject of color perception are unequalled in any single writer before him” (Vol. 2, p. 43 in Ref. ).
The Kitab al-Manazir would be the dominant text on optics for several centuries to come, both in the East and West. Latin translations appeared from 1200 to 1572 (Perspectiva, De aspectibus, Opticae Thesaurus) and in Italian around 1350 (Prospettiva). Basing themselves on what they read in this book, Kamal al-Din al-Farisi and Theodoric of Freiburg around 1300 discovered the correct explanation for the colors of the rainbow. It also made Kepler to formulate in 1604 the theory of the retinal image and Rudolph Snel van Royen to discover a few years later what is now known as Snellius’ law of refraction. In the East, Ibn al-Haytham’s work survived through its thorough treatment and further elaboration by Kamal al-Din al-Farisi (d. 1318).
Ibn al-Haytham was born around 965 AD in Basra (Iraq) where he became a famous mathematician. Caliph al-Hakim of Cairo, impressed by Ibn al-Haytham’s claim that he could regulate the flow of the Nile, persuaded him to come to Egypt. However, Ibn al-Haytham soon found out that he was not up to the task. He fell in disgrace and was placed under house arrest for many years. This proved to be beneficent for continuing his studies in mathematics, astronomy, and optics.
According to Aristotle, light was a manifestation of a change of the state of the medium from opaque to transparent. Ptolemy had considered vision to be the result of visual rays, spreading out from the eye that were reflected or refracted by surfaces of objects. Ibn al-Haytham (as well as Ibn Rushd) proposed an alternative view, that light has a much more active role in color vision. It is light which is seen, according to Ibn al-Haytham . Moreover, light is directed toward the eye instead of spreading from it. Ibn al-Haytham was the first to consider light as an entity by itself, traveling from visible objects to the eye, according to mathematical laws originally proposed by Euclid . Regarding the role of the medium for color vision, Ibn al-Haytham adopted the view from al-Kindi (d. 873) that the medium plays a passive role in color vision in the sense that it should not block vision.
In Ibn al-Haytham’s theory, color is a distinct property of material bodies. He stated that color and light are distinct but that colors behave exactly as light does in transmission, reflectance, and refraction. Therefore, Ibn al-Haytham’s theory of light is at the same time a theory of color . But apart from formulating these theories, Ibn al-Haytham went further and designed many experiments by which he verified the proposal step by step and afterward subjected the results to thorough mathematical analysis  (and also Vol. 2, p. xli in Ref. ). Nevertheless, Ibn al-Haytham also explored some more philosophical treatments of color and light .
Ibn al-Haytham’s view of the active role of light in color vision made him investigate many crucial aspects of color vision. He described how changing the type and intensity of light affects the color appearance of objects and how a stronger light intensity increased perceived color differences [6, 7]. Thus Ibn al-Haytham came close to what has been called a key notion in colorimetry that color is the product of the eye, the light, and the object. In the words of Ibn al-Haytham: “This behavior indicates that the eye observes the colors of colored objects only according to the colors that fall on them” [6, 7].
Ibn al-Haytham was one of the first to use the word saturation as describing an aspect of color. Ibn al-Haytham used this word in his Kitab al-Manazir. Generally it refers to the sensation one feels after a copious meal, indeed analogous to the modern English word saturation. In modern Arabic, the same word is still used for color saturation . About this and other words related to color that appear in the Kitab al-Manazir, see Vol. 2, p. 43 in Ref. .
In other aspects, Ibn al-Haytham followed the classical traditions. He supported the Aristotelian view that the colors of the rainbow are the result of mixing light and darkness and the Ptolemaic interpretation that color mixing in the spinning top is a visual illusion.
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