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© 2000-2016   Gérard P. Michon, Ph.D.

 Arms of Christiaan Huygens 
 1629-1695  Arms of Isaac Newton 

Light and Color Vision

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Visible and Invisible Light

(2015-09-26)   A Brief History of Light
What ancient thinkers thought about light...

Because it's directly accessible to human senses,  visible light  has been an object of meditation ever since human beings became capable of meditating.

Light was once thought to propagate  from  the eyes of the observer  to  the observed object.  This  extramission theory  was due to the founder of rhetoric,  Empedocles (c. 492-432 BC)  who is also responsible for the long-held belief that Nature could be explained in terms of  four elements  (Earth, Water, Air and Fire).  He  preached  that human vision was primarily due to the Fire that the goddess Aphrodite had put into the human eye.  Other obvious components were deemed only of secondary importance...

Curiously,  Empedocles  also argued  (correctly)  that light should propagate at a finite speed, which is extremely difficult to reconcile with the wrong direction of propagation which he advocated.  (This contradiction was noted in Euclid's Optica, around 300 BC.)

Following  Aristotle (384-322 BC), most scholars thought that light was an instantaneous phenomenon.  This helped mask the inadequacy of their religious belief in the weird causation of light described above  (thus delaying its demise).

Epicurean physics  is best known from its presentation by  Lucretius  (c. 99-55 BC)  in  De Rerum Natura  (55 BC).  Its atomistic take on light anticipates modern developments but was largely ignored at the time.

In  Catoptrica  (c. AD 60)  Hero of Alexandria (c. AD 10-75)  remarked that the  law of reflection  used by Euclid  (the angle of reflection is equal to the angle of incidence)  can be derived from a  Principle of Least Length,  according to which light should travel along a  minimal  route.  The propagation of light along straight lines in free space can also be deduced from that principle, which would later be generalized by Fermat.

In  AD 1015,  Alhazen (965-1039)  finally put an end to centuries of confusion by using his  camera obscura  to dismiss the need for an hypothetical beam emanating from the eye.  He concluded that sight  must be  entirely due to light coming into the eye from outside sources.  (Alhazen's use of Occam's razor directly inspired Newton.)

 Galilei Galileo (1564-1642)  attempted to determine the speed of light experimentally, but his method was too crude to produce definite results because it involved human reaction times.

Galileo was simply sending light signals to a distant assistant, who was responding with his own lantern as fast as he could.

The finiteness of the speed of light was first established in  1676  by the Danish astronomer  Ole Rømer (1644-1710).  By observing the motion of  Io  around  Jupiter,  Roemer deduced it should take about  22 minutes  for light to travel a distance equal to the mean diameter of the orbit of the Earth  (this time is now known to be 24% less:  16 minutes and 38.01 seconds).

 de Fermat Another issue, which took a couple centuries to settle, is that light travels  slower  in a denser medium.  This was correctly proposed by Fermat in 1655 as part of his  Principle of Least Time  to provide a unified explanation for  Hero's law of reflection  and  Snell's law of refraction   Willebrord 
 Snell (which had been discovered independently by Harriot in July 1601,  by Snell in 1621 and by Descartes in 1637).  However, the issue remained controversial among scientists until 1850, when the celerities of light in water and air were actually compared directly by  Fizeau and Foucault,  using a protocol which Arago had suggested in 1838.

 Newton  Christiaan 
 Huygens The corpuscular nature of light was championed by  Isaac Newton (1643-1727)  against  Christiaan Huygens (1629-1695) who argued it was made of waves  (subject to diffraction).  Newton also established that white light is the superposition of different colors of light and that the index of refraction of light in glass or in water depends on its color  (that's called  dispersion,  it explains  rainbows).

In 1802, Thomas Young (1773-1829)  demonstrated interferences of light waves, which seemed to settle the issue in favor of Huygens... for a while.

Young thought that light-waves were mostly longitudinal but  Augustin Fresnel (1788-1827; X1804)  and  François Arago (1786-1853; X1803)  showed, in 1821, that light-waves are entirely transversal.  They consist of a superposition of two orthogonal states of polarization which do not interfere with each other and are refracted differentlty  (Fresnel equations, 1821).

 Coat-of-arms of James 
 Clerk Maxwell In 1861, James Clerk Maxwell (1831-1879)  found that a  dynamic generalization of Ampère's law  makes the speed of light appear in the laws of electromagnetism, providing a decisive clue to the electromagnetic nature of light anticipated by  Michael Faraday (1791-1867).

In 1883, the Irish physicist George FitzGerald (1851-1901) remarked that an oscillating current ought to generate electromagnetic radiation.   Heinrich 
 Hertz This was demonstrated experimentally by Heinrich Hertz (1857-1894) in 1888.  At this point, visible light could simply be thought of as an electromagnetic wave of very high frequency.

However, in 1887, Hertz himself had discovered the  photoelectric effect  which could not be fully described in those terms...

 Planck In 1900, Max Planck (1858-1947)  explained the experimental shape of the  blackbody spectrum  by postulating that matter could only exchange energy with the electromagnetic field in discrete lumps, called quanta, whose energy was  proportional to  the frequency of radiation.  In 1905, Albert Einstein (1879-1955)  showed how the features of the  photoelectric effect  (Hertz, 1887)  imply that light  consists  of those quanta of energy  (which we now call photons).

 de Broglie Photons are the particles of light envisioned by Newton.  Light is thus  both  wavelike and corpuscular.  In 1923, Louis de Broglie (1892-1987)  suggested that  all  corpuscules, massless or not, are wavelike.  This motivated the development of modern  Quantum Theory.

Light through the Ages

(2015-06-02)   Luminous Units of Measurement
Standard 540 THz light has a luminous efficacy of 683 lumen per watt.

All luminous quantities correspond to a physical  radiant  measurement, involving net exchanges of energy at all frequencies.  However, luminous quantities are weighted according to the spectral response of the normal human eye.  (Radiant quantities aren't dependent on human perception.)

The basic calibration between radiant and luminous units is determined for a specific monochromatic light.  In principle, any frequency could have been used for that purpose, but  540 THz  was chosen as good approximation to the peak sensitivity of the human retina to bright light.  The conversion factor between the luminous and radiant units of power  (the  lumen  and the  watt, respectively)  at that frequency was defined to be  683 lm/W  to match historical definitions of luminous quantities  (based on standard candles).

The translation of a radiant quantity into a luminous one, or vice-versa, is ultimately based on subjective determinations of what constitutes the same brightness for light sources of different colors.  This was standardized scientifically so that most of the population will be in rough agreement over the result.  Universal agreement is not possible because of genetic differences between individuals:

About  5%  of the population  (mostly males)  suffer from some kind of color blindness of genetic origin.  A tiny fraction  (exclusively females)  are genetically endowed with the capacity to perceive an  extra dimension  of color, like birds do.  Standard luminous unis are based on the average spectral response of the retina in the majority of human beings, not affected by any of the above.

Because the different classes of receptors in the human eye behave differently, the human eye has a different spectral reponse in bright light  (photopic conditions)  and low light  (scotopic conditions)  or anything inbetween  (mesotopic conditions, twilight).

For standardization purposes, the average photopic spectral sensitivity of the human eye was determined in 1931 by the International Commission on Illumination  (Commission Internationale de l'Eclairage,  abbreviated  CIE,  now based in Vienna, Austria).

In 1951, the  CIE  adopted the curve corresponding to  scotopic  conditions  (which is of lesser importance for color vision, since low-light is almost entirely monochromatic.

 Come back later, we're
 still working on this one...

Luminosity function   |   International Commission on Illumination  (CIE)   |   CIE 1931 color space

(2015-06-09)   RGB Coding and Luminance
Luminance   =   0.30 R  +  0.59 G  +  0.11 B.

 Come back later, we're
 still working on this one...

Scotopic vision

(2015-06-05)   Monochromacy
Under scotopic  (low-light)  conditions, humans see in black-and-white.

 Come back later, we're
 still working on this one...

Scotopic vision

(2015-06-05)   Dichromacy
8% of men and 0.64% of women suffer from red-green color blindness.

 Come back later, we're
 still working on this one...

Photopigments   |   Color blindness

(2015-06-04)   Trichromacy
How humans perceive color under photopic  (bright-light)  conditions.

 Come back later, we're
 still working on this one...

 Spectral response of rods and cone cells in human retina

Mesopic vision

(2015-06-04)   Tetrachromacy and the Fourth Primary Color
Bird's eye.  Twighlight colors.  Ancestral vision of mammals.

Having four different kinds of receptors, the normal human retina can perceive color in a four-dimensional space which is active in the  mesotopic  domain, at the border between  photopic vision  (bright-light trichromacy, based on the three classes of cone cells)  and  scotopic vision  (low-light monochromacy, based on rod cells only).

Most birds have four classes of cone cells.  So do  some  humans.

 Come back later, we're
 still working on this one...

Tetrachromacy   |   Concetta Antico   |   Maureen Seaberg (2013-10-14)

(2016-01-28)   Aphakia:  Human eyes without lenses.
By itself, the human retina is sensitive to UV light.

Normally, the lens in the human eye acts as a good UV filter.  People who are missing a lens lose their ability to focus but they gain sensitivity to short optical wavelengths, beyond violet.

The famous French painter  Claude Monet  suffered from this condition.

Aphakia   |   Claude Monet (1840-1926)

(2016-09-25)   The Two  Transverse Polarizations  of Light
Polarizing sunglasses have made it easy to demonstrate light polarization.

The cheap sheet polarizers used in polarizing sunglasses were invented by Edwin Land in 1929.

Fresnel formulas (1821)   |   Herapathite (1852)   |   (William Bird Herapath (1820-1868)
US Patent 1918848 (1929, 1933)   |   Polaroid filters   |   Edwin H. Land (1909-1991)

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