ana.words, colors and computers

ana.words, colors and computers
9. November 2004 mahal
In Allgemein
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Colour & Computers
  by Charles Maurer

  A cynic might be tempted to say that there are two categories
  of photographer, those who admit they have problems matching
  colour, and liars. Matching colour ought to be simple, according
  to the ads, yet it rarely seems to be.

  The problem is not you, the problem is that colour is
  astonishingly complex. Controlling colour is a minefield.
  You need to know where you can walk, where you cannot, and
  where the path is uncertain. In this article I shall map the
  minefield and suggest one safe route through it.

**Colour Basics** -- We learned in school that all colours are
  formed from combinations of red, green, and blue. Unfortunately,
  this explanation is a distortion of reality and is so overly
  simple as to be wrong.

  Colours do not exist in nature, colours exist solely within
  an observer's head. Colours are perceptions. Light striking
  the eye triggers a chain of neurochemical reactions that end in
  perceptions of colour. Light has no colour itself, it is merely
  electromagnetic radiation. Different wavelengths of light induce
  different perceptions of colour but the relationship between
  wavelength and colour is neither simple nor straightforward.

* Any number of different wavelengths can induce the same colour.

* The same wavelength can induce different colours in different

* Two people viewing the same wavelength may see different

  All of the eye's colour receptors respond to a broad range of
  wavelengths but they each respond to some wavelengths more
  readily than to others. The receptors fall into three groups
  with different ranges of sensitivity. If you look at the light
  that each group is most sensitive to, you will perceive red,
  green, and blue: that is why red light, green light, and blue
  light can induce any colour. However, although there are three
  primary colours of light, there are actually _four_ primary
  colours. Red, green, yellow, and blue are perceptual primaries:
  all other colours can be identified as variants of them, even
  in cultures that do not distinguish any colours by name.

  In short, three sets of wavelengths will induce three colours of
  light and mixtures of those wavelengths will induce variants of
  four colours. Moreover, the colours induced by any particular
  combination of wavelengths may differ from one circumstance to
  the next or from one person to the next.

  This is the reason that everyone has problems matching and
  balancing colour. The transformation is not a straightforward
  mathematical function nor even a constant one. Balancing colour
  is like cooking fudge that will be sweet enough for all and
  overly sweet for none.

  Engineers can deal with wavelengths but they cannot deal with
  mathematical functions that change in shape like an amoeba. To
  get around this, to relate wavelengths to colours, a group of
  scientists and engineers have got together to define the shape
  of those amoebas. This group is the Commission International de
  l'Eclairage (CIE). The CIE has defined several amoebas suitable
  for several purposes. Standard Observers they are called. All of
  the CIE's standards are based on them, as are most instruments
  that measure light, including exposure meters and

  These standards are designed to facilitate repeatable and precise
  measurements of mechanical and electronic devices, of sensors,
  dyes, pigments and the like, and to standardize information. Thus,
  engineers create an image sensor that corresponds as closely as
  possible to the latest CIE standard amoeba in terms of how it
  matches wavelengths of light to specific colors. They measure
  how their sensor deviates from the standard, and they note those
  deviations in a profile. Other engineers create a printer, trying
  to make its output correspond as closely as possible to the
  standard amoeba. They also note the deviations between their
  printer's output and the standard in a profile. Then, when a
  computer sends the image file captured by the sensor to the
  printer, it adjusts the image according to the two profiles. The
  resulting picture comes out of the printer using colours that more
  or less match the relationships between wavelength and colour
  defined by that standard amoeba.

  This approach to matching colour would be all you'd need if you
  invited the Standard Observer to dinner and wanted to impress him
  with your photos. However, if he came, he would not deign to look
  at them hanging on your living room wall without repainting the
  wall a particular shade of grey. He would also insist on drawing
  the curtains to block out the sun and installing a special lamp.
  Moreover, this would not be a normal social experience. He would
  not view them before dinner when he was hungry, or during dinner
  when he is distracted, or after dinner when he is relaxed.

  Put bluntly, it is never practical to match colours based solely
  and precisely on CIE specs and the Standard Observer. Realistic
  calibration is imprecise at best, and more an art than a science.
  The specifications (ICC.1:2003-09) for International Color
  Consortium (ICC) profiles - the profiles used by Apple's ColorSync
  technology - make this clear:

  "Clearly, there is considerable art involved in shaping the tone-
  reproduction and color-reproduction characteristics of different
  media and much of this art is based on subjective, aesthetic
  judgments. As a result, the substrate and the colorants used in a
  medium will be exploited to impart a particular personality to the
  reproduction that is characteristic of the medium. In reproducing
  an image on various types of media, it may be desirable to adjust
  the colorimetry to accommodate the differing characteristics of
  those media. In any case, it is necessary to accommodate the gamut
  differences. Such considerations go beyond the simplistic matching
  of color stimuli or even of color appearance. These adjustments
  need to be incorporated in the color transforms of the device


  The reality is that matching colour is a chimera. The colour of
  a person's face as you perceive it may change with the background
  or a hat. When different pigments and dyes are involved, matching
  colour becomes a game spread across two ballparks. Except by
  accident, flesh tones on a monitor will never look just like
  the flesh tones in a print when the two are compared directly.

  On the other hand, it does not matter if the monitor and
  print do not match. These two versions of the picture will
  never be compared under normal circumstances. What matters is
  that the flesh tones (or whatever) always look appropriate
  _within_their_context_. The flesh tones on the monitor need
  to look natural within the photo. The flesh tones on the print
  need to look natural within the print and next to the other
  portraits on the wall.

  Your task is to calibrate your monitor and printer not so that
  their images match, but so that when a picture looks good on the
  monitor, it also looks good printed out. How the two compare side
  by side is immaterial.

**Colour Profiles** -- It is possible to spend a lot of time and
  money calibrating equipment to absurd levels of precision. Since
  fudge is a basic ingredient of profiles and colour-matching, ICC
  profiles from different sources will give different results, and
  there is no way to tell whether you will like a profile without
  buying it and trying it. Fortunately, most people don't need to
  profile their printer at all and can get by fine with the default
  settings. Long ago Microsoft and HP proposed, and the computer
  industry adopted as a formal standard, a colour-matching
  technology that's simpler than the full ICC standard while
  still being sufficient for most people outside the graphics-arts
  industry. All devices are assumed to be able to produce a range
  of colours that will fit within a range or "colour space" called
  sRGB. A standard set of numbers defines every colour within this
  space. All devices are supposed to interpret those numbers
  sensibly. It is the norm for photos on the Web, and most
  commercial printing services use it, so I've set my Mac to use
  sRGB by default (ColorSync Utility > Preferences pane > Default
  Profiles tab > RGB Default pop-up menu).

  Most inks on most papers are limited to the range of sRGB,
  although some do exceed sRGB's range. From some inks a larger
  colour space defined by Adobe, called "Adobe RGB," allows more
  vivid colours. The difference is likely to matter in print
  competitions and some corners of the graphic arts trade, but it
  not clear to me that it would matter elsewhere. Using a larger
  colour space incurs a cost: it is likely to require 16-bit colour,
  which requires more storage and processing time. A good
  description of those and other colour spaces is at:


  The sRGB standard ought to make colour-matching simple and
  invisible but Microsoft is not known for support of standards,
  even its own. When I was exploring some of Photoshop's
  preferences, I noticed some curious results. The pure green on
  the Macbeth ColorChecker showed up with these different amounts
  of red using different programs and different versions of the
  sRGB profile:

                                       sRGB     sRGB IEC61966-2.1
 Photoshop using ColorSync             0%       11.0%
 Photoshop using Adobe Color Engine    0%        5.9%
 Preview and GraphicConverter          0%        5.5%

  The first profile was supplied by Apple as part of Mac OS X.
  The second was supplied by Microsoft, was installed as part of
  Photoshop, and is built into some other applications as well.
  I don't know which of these is correct but it appears to be
  the profile that Apple supplied. In any case, that profile
  is consistent, which is more important.

**Calibrating Your Equipment** -- Before you can use a monitor to
  balance the colour of photographs, you need to calibrate it under
  its normal ambient lighting. I work with my PowerBook sometimes
  under incandescent lighting and sometimes under fluorescent
  lighting, so I have calibrations for both and switch between them.
  Apple's calibrator, accessible from the Display preference pane,
  is adequate to set up a computer for ordinary purposes but it is
  not good enough for editing photos. I suggest instead the $20
  shareware package SuperCal. However, do not use it with the photo
  built into the program. Instead use an electronic version of the
  Macbeth ColorChecker (free from the second link below). If you
  are taking the sRGB route and using only a single printer, then
  it would be reasonable just to compare your monitor directly
  to a printout of that file, but if you are using a different
  colour space or want to use multiple printers, compare it to
  a real Macbeth card. In any case, be sure to set the gamma to
  2.2. That is the de facto standard for working with colour.
  The Mac's standard of 1.8 was intended to make a grey-scale
  monitor look like a printed page.


  Your goal in calibrating the monitor should be to make the two
  images of the ColorChecker match as closely as possible overall
  and to fudge the inevitable differences so that none of the
  colours is further off than any other. From such different
  technologies any kind of real match is impossible; you are
  after the best approximation.

  To compare the target photo to the monitor, and to assess the
  colour of prints, you don't need fancy instruments - you are
  pleasing your eye, not the Standard Observer - but you do need
  a suitable lamp. Ideally this will be the same kind of lamp you
  always view your pictures with, but since most pictures are viewed
  under a variety of conditions, you really need an average lamp.
  Although there is no such thing as an average lamp, there is a
  graphic-arts standard for judging colour. It is an arbitrary
  standard that has proven to be functional. Ordinary light bulbs
  are redder than this and most fluorescent tubes are too green.
  A reasonable compromise is a desk lamp that combines a 60-watt
  incandescent bulb with a circular fluorescent tube. If you need
  to buy something, you might find a desk lamp that uses a compact
  fluorescent tube and replace the tube with an Ott-Light TrueColor.


  If you want professional-looking 8" x 10" prints that are tough
  and durable, and if you want to keep things simple, then you might
  take the sRGB approach with a $500 Olympus P-440 dye-sublimation
  printer. That's what I do. If you use the P-440 from a Macintosh,
  all you do is switch it on. It needs no special set-up and, to my
  eye, its native colour management works better than any ColorSync
  profile I have found for the machine, including a couple from the
  QImage folks and the one supplied by Olympus themselves. The
  printer requires no cleaning and uses no liquid ink that can dry
  up. The running cost is $2 per 8" x 10" print, which is less than
  good ink-jet paper and ink work out to be if a print head ever
  dries out. Its range of colours is not the broadest - it lies
  completely within sRGB space - and when I compare prints of the
  Macbeth ColorChecker made from it and from my darkroom, the tones
  from the Olympus look a bit more restricted. However, this is not
  a comparison people normally make and the P-440's prints on their
  own can be stunning. The prints I get from it do not look digital
  even under a loupe. (Do note, however, that I use special-purpose
  scaling software, I do not merely send the printer a file and
  have it fill the paper. Pictures scaled by PhotoZoom Pro are
  sharper than pictures scaled in the usual ways, even with
  PhotoZoom's sharpening switched off. See "Editing Photos for the
  Perfectionist" in TidBITS-748_ for more details on PhotoZoom Pro.)


  [Editor's note: I can vouch for the quality of these prints.
  Charles passed through Ithaca recently while on a trip and brought
  me pictures he'd taken while on a trip through the Himalayas in
  July. He used a Sigma SD-10 camera and the Olympus P-440 printer,
  and the prints were utterly gorgeous. -Adam]

  I have found only two glitches with the P-440. The first is
  trivial: the printer driver will not accept images at the
  advertised spec of 3200 pixels, it requires them to be three
  pixels shorter. The second is more serious: if the line voltage
  in your house fluctuates too much, then the printer will produce
  random blotches and lines. Fluctuating voltage is not common
  in cities but it can happen in rural areas. I need to plug the
  printer into a voltage regulator. (The computer's UPS would have
  done but I happened to have an old voltage regulator in the

  Last-minute addition: Kodak just announced the release of a
  comparably priced dye-sublimation printer that prints pictures
  up to 8" x 12". Kodak is aiming this printer at professional
  photographers, which is an important market for them and one that
  it has always served well. This printer looks very interesting.


**Balancing Colours in Photos** -- Once you have a system that
  is calibrated, you need to get to work balancing the colour of
  individual photos. The easiest way I know to do this is to relate
  everything in the picture to some spot that is some shade of
  neutral, colourless grey. There is one such spot in just about
  every photo. It may be merely the reflection of a lamp on a
  metallic surface, it may be only a few pixels across, maybe just
  the glint off a ring, but it is almost certain to be there
  somewhere. Once you've found that neutral spot, adjust the overall
  colour balance so that the red, blue, and green values for that
  spot measure the same using DigitalColor Meter or the equivalent
  function in Photoshop or your raw converter. (The raw converter is
  the best place to do this. In Photoshop you can just click on the
  spot with the eyedropper.)

  Now every colour of a similar brightness ought to be properly
  balanced. Find a colour that is particularly sensitive to colour
  casts - most likely skin tones - and look for differences in that
  one hue between highlights and shadows. Those differences indicate
  colour casts that vary with brightness. In Asiva Shift+Gain
  (again, see the previous article in TidBITS-748_ for details) it
  is easy to select the range of brightness that is off and correct
  it by nudging the hue slightly warmer or softer. If that one tone
  is corrected so that it looks the same at all levels, then other
  tones ought to be similar as well, although there may be anomalies
  if the picture was taken in mixed or fluorescent or high-intensity
  lighting. If some specific tones show an anomaly, select just
  those tones in Asiva Shift+Gain and fix them.


**The Eyes Have It** -- The approach to controlling colour that
  I have outlined here is the simplest and cheapest I know of.
  It would be a sensible approach to start with and would be a
  good one to retreat to if you are having problems doing it another
  way. However, this is certainly not the only way to control colour
  nor can it achieve the highest quality possible. Enough time and
  enough money can buy better results. If you decide to aim higher,
  though, do always keep in mind that colour is a variable
  perception; it is not a stable, objective phenomenon. There
  is rarely an intelligent way to answer the question, "Does this
  profile (or printer) give better colour?"

  Perception also ought to be kept in mind when buying a digital
  camera. The specifications of digital cameras are not what they
  seem to be if you consider them from the perspective of the eye.
  My next article will examine cameras from this angle. Among other
  things it will calculate a finite and surprisingly low answer to
  the question, "How many pixels are enough?"

   PayBITS: If Charles's recommendations for matching colours
   helped, he asks that you make a donation to Doctors Without
   Borders: <>
   Read more about PayBITS: <>

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