
              Frequently asked questions about Colour Physics
                                      
                                version 1.0
                                      
  About this FAQ
  
   This FAQ concerns the measurement and control of coloured surfaces
   such as plastics, textiles, surface coatings etc. It is intended for
   practitioners rather than theoreticians. Those requiring a more
   theoretical introduction to colour science or information about
   digital colour image reproduction should, in my opinion, consult the
   Poynton Colour FAQ.
   
  Light and Matter
  
   What is the colour spectrum?
   What happens when light strikes the surface of a material?
   How is light absorbed?
   How is light scattered?
   Why are some substances coloured?
   
  Colour Vision
  
   What is colour?
   How does the eye work?
   What are scotopic and photopic vision?
   What is chromatic aberration?
   What is trichromacy?
   What is the opponent theory of colour vision?
   What are brightness, hue, and colourfulness?
   
  CIE Colour Specification
  
   What is additive colour matching?
   What are the additive primaries?
   What does CIE stand for?
   What is the CIE 1931 system?
   What is the CIE standard observer?
   What are the tristimulus values?
   Why are the CIE primaries often called imaginary primaries?
   How can tristimulus values be calculated?
   What colour measurement devices are available?
   How does a reflectance spectrophotometer work?
   What is the optical geometry of a spectrophotometer?
   How does a colorimeter work?
   What is the specular component of reflectance?
   What is the difference between a light source and an illuminant?
   What is $ D65?
   What is TL84?
   What is CIE 1931 colour space?
   Why is the 1931 standard observer called a 2 degree observer?
   What is the 10 degree observer?
   What are chromaticity coordinates?
   What is the CIE L* a* b* colour space?
   Should I use L* a* b* or L* C* H*ab specification?
   
  Colour Difference Evaluation
  
   What are CIELAB colour differences?
   How do I get descriptive colour differences?
   What does Delta E stand for?
   Which colour difference equation should I use?
   What is the CMC equation?
   What is the BFD equation?
   What is the CIE TC 1.29 equation?
   What are the M&S equations?
   How do I set the pass/fail value?
   
  Miscellaneous Topics
  
   What is colour constancy?
   What is metamerism?
   How do I measure whiteness?
   How do I measure yellowness?
   What can I do if my sample is not uniform?
   What is device-independent colour space?
   
  What happens when light strikes the surface of a material?
  
   When light strikes a surface there are two things that can happen: (i)
   the change in refractive index can cause light to be reflected by the
   surface and this surface-reflected light is called specular
   reflection; (ii) light that is not reflected at the surface can
   penetrate the body of the material although as it passes through the
   surface the change in refractive index will cause the light to be
   refracted.
   
   Light may pass completely through a material, in which case we say
   that it has been transmitted. Alternatively the light may be absorbed
   by the material or it may be scattered. Light that is scattered or
   reflected may eventually pass out of the front, back, or side of the
   material.
   
  How is light absorbed?
  
   Light can be absorbed by materials according to a number of mechanisms
   that include atomic vibrations and rotations, ligand-field effects,
   molecular orbitals, and charge transfer. It is very often the case
   that specific quantities of light (energy) are absorbed by a specific
   material and thus the light absorbtion properties of materials are
   usually wavelength selective.
   
   The energy that is absorbed by molecules can be dissipated as kinetic
   and heat energy, but sometimes the energy can be re-emitted.
   Fluourescence and phosphorescence are phenomena that result from the
   re-emission of absorbed light energy: in both cases the re-emitted
   energy is at a longer wavelength than the light originally absorbed.
   
  How is light scattered?
  
   When light strikes particles it may be scattered. When the scattering
   particles are extremely small (to the order of 1000 nm) the light is
   scattered according to a simple law proposed by Rayleigh: short
   wavelengths are scattered more than long wavelengths. For larger
   particles (to the order of 4000 nm and larger) the amount of
   scattering is according to Fresnel's equations: the amount of
   scattering depends upon the difference between the refractive index of
   the particle and of the medium in which it is dispersed and this
   difference is wavelength dependent.
   
   If light is scattered evenly in all directions this is called
   isotropic scattering but it is rarely the case. The absorption and
   scattering properties of partices are complex and a number of theories
   exist to describe them including the Kubelka-Munk theory.
   
  Why are some substances coloured?
  
   There are many reasons why substances appear coloured but for most
   physical materials it is because the absorption and scattering
   properties of the material are different for different wavelengths of
   light. Thus a substance that appears yellow may do so because it
   absorbs most strongly in the blue part of the spectrum and scatters
   most strongly in the red and green parts of the spectrum. It is often
   the case that a pigment scatters light most efficiently in one region
   of the spectrum whilst having its main absorption band in another.
   This explains why translucent and transparent coloured films can have
   different hues when viewed by reflected as opposed to transmitted
   light.
   
   It is commonly stated that colour vision is the result of the nature
   of the physical world, the physiological response of the eye (more
   strictly the retina) to light, and the neural processing of the
   retinal response by the brain. The identification of three separate
   processes in this way is probably artificial, and does little justice
   to the complex nature of colour perception, but the idea is useful and
   appealing since we shall see later that the number ``three" has an
   almost magical association with colour vision.
   
  How does the eye work?
  
   Almost the whole of the interior of the spherically shaped eyeball is
   lined with a layer of photosensitive cells known collectively as the
   retina and it is this structure that is the sense organ of vision. The
   eyeball, though no mean feat of engineering itself, is simply a
   structure to house the retina and to supply it with sharp images of
   the outside world. Light enters the eye through the cornea and the
   iris and then passes through the lens before striking the retina. The
   retina receives a small inverted image of the outside world that is
   focussed jointly by the cornea and the lens. The lens changes shape to
   achieve focus but hardens with age so that we gradually lose our
   accommodation. The eye is able to partially adapt to different levels
   of illumination since the iris can change shape to provide a central
   hole with a diameter between 2 mm (for bright light) and 8 mm (for dim
   light).
   
   The retina translates light into nerve signals and consists of three
   layers of nerve-cell bodies. Surprisingly the photosensitive cells,
   known as rods and cones, form the layer of cells at the back of the
   retina. Thus, light must pass though the other two layers of cells to
   stimulate the rods and cones. The reasons for this backward-design of
   the retina are not fully understood but one possibility is that the
   position of the light-sensitive cells at the back of the retina allows
   any stray unabsorbed light to be taken care of by cells immediately
   behind the retina that contain a black pigment known as melanin. The
   melanin-containing cells also help to chemically restore the
   light-sensitive visual pigment in the rods and cones after it has been
   bleached by light.
   
   The middle layer of the retina contains three types of nerve cells:
   bipolar cells, horizontal cells, and amacrine cells. The connectivity
   of the rods and cones to these three sets of cells is complex but
   signals eventually pass to the front of the retina and to the third
   layer of cells known as retinal ganglion cells. The axons from retinal
   ganglion cells collect in a bundle and leave the eye to form the optic
   nerve. The backward-design of the retina means that the optic nerve
   must pass through the retina in order to leave the eye and this
   results in the so-called blindspot.
   
   The rods and cones contain visual pigments. Visual pigments are much
   like any other pigments in that they absorb light with absorption
   sensitivities that are wavelength dependent. The visual pigments have
   a special property, however, in that when a visual pigment absorbs a
   photon of light it changes molecular shape and at the same time
   releases energy. The pigment in this changed molecular form absorbs
   light less well than before and thus is often said to have been
   bleached. The release of energy by the pigment and the change in shape
   of the molecule together cause the cell to fire, that is to release an
   electrical signal, by a mechanism that is still not completely
   understood.
   
  What are scotopic and photopic vision?
  
   Rods are sensitive to very low levels of illumination and are
   responsible for our ability to see in dim light (scotopic vision).
   They contain a pigment with a maximum sensitivity at about 510 nm, in
   the green part of the spectrum. The rod pigment is often called visual
   purple since when it is extracted by chemists in sufficient quantities
   the pigment has a purple appearance. Scotopic vision is completely
   lacking in colour; a single spectral sensitivity function is
   colour-blind and thus scotopic vision is monochromatic.
   
   Colour vision is provided by the cones, of which there are three
   distinct classes each containing a different photosensitive pigment.
   The three pigments have maximum absorptions at about 430, 530, and 560
   nm and the cones are often called ''blue", ''green", and ''red". The
   cones are not named after the appearance of the cone pigments but are
   named after the colour of light to which the cones are optimally
   sensitive. This terminology is unfortunate since monochromatic lights
   at 430, 530, and 560 nm are not blue, green, and red respectively but
   violet, blue-green, and yellow-green. The use of short-, medium-, and
   long-wavelength cones is a more logical nomenclature.
   
   The existence of three spectral sensitivity functions provides a basis
   for colour vision since light of each wavelength will give rise to a
   unique ratio of short-, medium-, and long-wavelength cone responses.
   The cones therefore provide us with colour vision (photopic vision)
   that can distinguish remarkably fine wavelength changes.
   
  What is chromatic aberration?
  
   The eye cannot simultaneously focus on the three regions of the
   spectrum where the cone-pigment absorptions peak since refraction at
   the cornea and lens is greater for short wavelengths than it is for
   long wavelength. Thus, it is said that the eye is not corrected for
   chromatic aberration. The medium- and long-wavelength peaks are quite
   close together and therefore the lens optimally focusses light of
   about 560 nm on the retina. Since the short-wavelength cones receive a
   slightly blurred image it is not necessary to provide the same spatial
   resolution that is provided by the other two sets of cones. The retina
   contains approximately 40 long-wavelength cones and 20
   medium-wavelength cones for every single short-wavelength cone.
   
   The rods and cones are not evenly distributed on the retina. The
   central part of the retina, the fovea, contains only cones whereas at
   greater eccentricities there is a greater preponderance of rods. In
   the fovea the cones are densely packed and it is this part of the
   retina that provides the greatest spatial resolution under normal
   viewing conditions.
   
  What is trichromacy?
  
   Since the retina contains four different types of receptor it might be
   thought that the neural pathways would carry four different signals to
   the brain, and more precisely to the primary visual cortex which is at
   the back and rear portion of the brain. It is generally believed,
   however, that colour information is coded by the retinal and
   post-retinal neural structures as just three types of signals that are
   often called ''channels".
   
   The idea of ''channels" in the brain is central to the way in which
   the operation of the brain can be viewed as an information- or
   signal-processing task. A channel is a conceptual processing route and
   thus for the visual system we can say that the information from the
   cones is processed in three separate channels. Remembering that colour
   perception is only one function of the visual system, there are other
   channels that are responsible for providing other information about
   the outside world that enables the perception of form, motion, and
   distance for example. The existence of channels for the processing of
   colour information helps explain the two contradictory theories of
   colour vision that were prevalent during the 19th century: the
   trichromatic theory and the opponent-colours theory.
   
   The trichromatic theory was postulated by Young and later by Helmholtz
   and was based upon colour matching experiments carried out by Maxwell.
   Maxwell's experiments demonstrated that most colours can be matched by
   superimposing three separate light sources known as primaries; a
   process known as additive mixing. Although any light sources could be
   used as primaries it will be seen later that the use of monochromatic
   sources of radiation enables the widest gamut of colours to be
   obtained by additive mixing. The Young-Helmholtz theory of colour
   vision was built around the assumption of there being three classes of
   receptors although direct proof for this was not obtained until 1964
   when microspectrophotopic recordings of single cone cells were
   obtained. The roots of trichromacy are firmly understood to be in the
   receptoral stage of colour vision. It is important to realize that a
   yellow stimulus produced by the additive mixture of appropriate red
   and green lights does not simply match monochromatic yellow light but
   is indistinguishable from it. Thus, the trichromatic nature of vision
   is essential for the operation of many colour reproducing processes
   such as television, photography, and three-colour printing.
   
  What is the opponent theory of colour vision?
  
   The opponent-colours theory of colour vision, proposed by Hering,
   seemingly contradicts the Young-Helmholtz trichromatic theory. It was
   advanced to explain various phenomena that could not be adequately
   accounted for by trichromacy. Examples of such phenomena are the
   after-image effect (if the eye is adapted to a yellow stimulus the
   removal of the stimulus leaves a blue sensation or after-effect) and
   the non-intuitive fact that an additive mixture of red and green light
   gives yellow and not a reddish-green. Hering proposed that yellow-blue
   and red-green represent opponent signals; this also went some way
   towards explaining why there were four psychophysical colour primaries
   red, green, yellow, and blue and not just three. Hering also proposed
   a white-black opponency but this third opponent channel has been
   abandoned in most modern versions of the theory. It is now accepted
   that both the trichromatic theory and the opponent colours theory
   describe essential features of our colour vision with the latter
   theory describing the perceptual qualities of colour vision that
   derive from the neural processing of the receptor signals in two
   opponent channels and a single achromatic channel.
   
  What are brightness, hue, and colourfulness?
  
   The perceptual attributes brightness, hue, and colourfulness have been
   defined by Professor R.W.G. Hunt as follows:
   
   Brightness: attribute of a visual sensation according to which an area
   appears to exhibit more or less light.
   
   Hue: attribute of a visual sensation according to which an area
   appears to be similar to one, or proportions of two, of the perceived
   colours red, yellow, green, and blue.
   
   Colourfulness: attribute of a visual sensation according to which an
   area appears to exhibit more or less of its hue.
   
  What are CIELAB colour differences?
  
   CIE 1976 (L* a* b*) colour space provides a three-dimensional
   representation for the perception of colour stimuli. If two points in
   space, representing two stimuli, are coincident then the colour
   difference between the two stimuli is zero. As the distance in space
   between two points (L*1, a*1, b*1) increases it is reasonable to
   assume that the perceived colour difference between the stimuli that
   the two points represent increases accordingly. One measure of the
   difference in colour between two stimuli is therefore the Euclidean
   distance Delta E* between the two points in the three-dimensional
   space.
   
  How good are CIELAB colour differences?
  
   Unfortunately several evaluations of CIELAB have shown that Delta E*
   is not a particularly good measure of the magnitude of the perceptual
   colour difference between two stimuli. The relatively poor ability of
   Delta E* to predict the magnitude of perceptual colour differences has
   led to more complicated ways of computing a colour difference from the
   CIELAB coordinates of two samples and some of these measures have been
   shown to be more reliable than Delta E*.
   
  How do I get descriptive colour differences?
  
   The L* C*ab H*ab representation is useful if qualitative colour
   differences are required. Differences can be calculated thus:
   
   Delta L* = L*btx - L*std,
   Delta C*= C*btx - C*std
   Delta H*= {(Delta a*)2 + (Delta b*)2 - (Delta C*)2}1/2
   
   where the subscripts std and btx refer to standard and batch
   respectively.
   
   If Delta L* is positive the batch is lighter than the standard, but if
   Delta L* is negative the batch is darker than the standard.
   If Delta C* is positive the batch is stronger than the standard, but
   if Delta C* is negative the batch is weaker than the standard.
   
   The hue descriptor is more difficult to determine: the radial
   direction in hue from the standard to the batch is used to give two
   hue descriptors (eg: redder/yellower): the descriptors are derived
   from the first two axes that are crossed in the a*-b* plane of colour
   space when moving from the standard to the batch in the direction of
   hue.
   
  What does Delta E stand for?
  
   The term Delta E is derived from the german word for sensation
   Emfunding. Delta E therefore literally means difference in sensation.
   The superscript asterisk is sometimes used to denote a CIELAB
   difference thus, Delta E*.
   
  Which colour difference equation should I use?
  
   It is established that the CIELAB colour difference equation is
   inadequate for many purposes - equal sizes of Delta E* correspond to
   different perceptual differences in colour. There is strong evidence
   to show that most of the modern optimized equations (such as CMC, M&S,
   BFD, and CIE 94) are more uniform than CIELAB. It is not clear,
   however, whether any one of these new equations is significantly
   better than the others. The CMC equation is a British Standard (BS
   6923) and is being considered as an ISO standard.
   
  What is the CMC equation?
  
   The CMC colour difference formula allows calculation of tolerance
   ellipsoids around the target standard where the dimensions of the
   ellipsoid is a function of the position in colour space of the target.
   The design of this formula allows for two user-definable coefficients
   l and c and the formula is thus normally specified as CMC(l:c). The
   values of l and c modify the relative importance that is given to
   differences in lightness and chroma respectively. The CMC(2:1) version
   of the formula has been shown to be useful for the estimation of the
   acceptability of colour difference evaluations.
   
   The CMC(2:1) equation is a British Standard (BS:6923) for the
   assessment of small colour differences and is currently being
   considerd as an ISO standard.
   
  What is the BFD equation?
  
   A refinement of the CMC formula led to the introduction of the BFD
   formula. Recent research suggest that the BFD equation performs
   marginally better than the CMC equation.
   
  What is the CIE TC1.29 equation?
  
   A simplication of the CMC(l:c) equation has recently been considered
   by the CIE. It is too early to state whether this new equation,
   sometimes refered to as the CIE 94 colour difference equation, is
   significantly better than its predecessor.
   
  What are the M&S equations?
  
   In the 1980s Marks & Spencer, in conjunction with Instrumental Colour
   Systems, developed their own in-house equations that are used in the
   textile industry. Research shows that there is little to choose
   between the CMC and M&S equations in terms of overall performance. The
   fact that the M&S equations have never been published has restricted
   their use.
   
  How do I set the pass/fail value?
  
   The pass/fail limit depends upon the equation that is used, but more
   importantly it also depends upon the application. The correct
   pass/fail value can only be determined from experience -
   pragmatically, the correct pass/fail limit is that such that all pairs
   of samples with a colour difference less than this limit will be
   accepted by the customer.
   
   Westland. If you have any comment regarding this document, or if you
   have any questions that you feel should be included in this document,
   I can be contacted at coa23@cc.keele.ac.uk.
