Real World Color Management (2nd Edition)

Every graphics professional worth his or her salt knows the importance of color management. No matter how much thought artist and client put into the color scheme for a given project, all of that work is for naught if you can't get your results to match your expectations. Enter Real World Color Management, Second Edition. In this thoroughly updated under-the-hood reference, authors Bruce Fraser, Chris Murphy, and Fred Bunting draw on their years of professional experience to show you everything you need to know about color management. Whether your final destination is print, Web, or film, Real World Color Management, Second Edition takes the mystery out of color management, covering everything from color theory and color models to understanding how devices interpret and display color. You'll find expert advice for building and fine-tuning color profiles for input and output devices (digital cameras and scanners, displays, printers, and more), selecting the right color management workflow, and managing color within and across major design applications.

Real World

Second Edition

This page lefi blank

I s Color? Reflections on Life

"It's not easy being green," sang the velvet voice of Kermit the Frog, per- haps giving us some indication of how the frog felt to be a felt frog While none of us may ever know the experience of "being green," it's worth reflecting (as we are all reflective objects) on the experience of "seeing green."

You don't have to be a color expert to use color management But if you're totally unfamiiiar with the concepts behind the technology, color management may seem l i e magic We don't expect you to become as obsessed with color as we are-indeed, if you want any hope of leading a normal lie, we advise against it-but we do recommend that you familiar- ize yourselfwith the fundamentals we lay out in this chapter

b They'll help you understand the problem that color management ad- dresses The whole business of printing or displaying images that look like realistic depictions of the things they portray depends on exploit- ing specific properties of the way humans see color Color manage- ment is just an extension of that effort

b They'll explain some of the terminology you'll encounter when using color management software terms like colorimetric, perceptual, and

saturation, for example A little color theory helps explain these terms and why we need them

3

4 Real World Color Management, Scrond Edition

b The strengths and weaknesses of color management are rooted in our ability (or inability) to quantify human color Vision accurately If you understand this, your expectations will be more realistic

b Color theory explains why a colorviewed ina complex scene such asa photograph looks "different" from the same color viewed in isolation Understanding this helps you evaluate your results

b You need to understand the insmmentsyou may use with color man- agement This chapter explains just what they measure

Butwe have to 'Fess up to anotherreason forwritingthis chapter: color

is just really darned interesting While this chapter sets the stage and lays the foundation for other chapters in this book, we hope it will also spark your curiosity about something you probably take for granted-your ability to see colors If you're intimidated by scientific concepts, don't worry-we won't bombard you with obscure equations, or insist that you pass a graduate course in rocket science It's not absolutely necessary to understand all of the issues wecoverin this chapterto use color manage- ment But a passing familiarity with these concepts and terms can often come in handy And you may well come to realize that, although you've probably done it all your life, in reality"it5 not easy seeing green."

If you want to manage color, it helps to first understand just what i t is so let's start by examining your current definition of color Depending on how much you've thought about it-if you're reading this book you've probably done so more than most-you may have gone through several definitions at various times in your life, but they've probably resembled one of the following statements:

Color i s a property of obieds This is the first and most persistent view

of color No matter how much we mav have philosophizd about color, we all still speak of "green apples," "red lights," and "blue suede shoes."

Chapter B What Is Color? 5

Color is a ptop.r(y dlight.This is the textbookcounterclaim tothe view

of color as a property of objects Authors of color books and papers love

to stress that "light is color" or "no light, no color."

Color hap- in the ob-er This concept captures our imagination when we encounter optical illusions such as afterimages, successive con- trast, and others that don't seem to originate in the objects we see Color

is something that originates in the eye or the brain of the observer

The correct answer, of course, is a blend of all three All are partially true, but you don't have to look far to find examples that show that none

of the three statements, by itself, is a complete description of the experi- ence we call color

Color is an event that occurs among three participants: a light source,

an object, and an observer The color event is a sensation evoked in the observer by the wavelengths of light produced by the light source and

modified by the object If any of these three things changes, the color event is different (see Figure I-1)-in plain English, we see a different color

We find it interesting that the three ingredients of the color event rep-

resent three of the hard sciences: physics, chemistry, and biology Un- derstanding how light affects color takes us into the physics of color;

ngure 1.1

The color event

A eolor event always has three parricipants

6 Real World Color Management, Sewid Edition

understanding how objects change light involves the chemistry of sur- faces and how their molecules and atoms absorb light energy; and un- derstanding the n a m e of the observer takes us into biology, including the neurophysiologyof the eye and brain and the threshold of the nether regions of psychology In short, color is a complex phenomenon

The next sections explore this simple model of the color event in more detail We begin with light sources, then move on to objects, and then spend a bit more time with the subject most dear to you and us, namely you and us (the obsewersl

The first participant in the color event is light The party jusl doesn't get started until this guest arrives But all light isn't created equal: the char- acteristics of the light have a profound effect on our experience of color

So let's look at the nature of light in more detail

Photons and Waves

Many a poorphysics smdenthasrelivedthedilemrnafaced by eighteenth-

century scientists as to whether Lightis best modeled as aparticle (theview held by Sir Isaac Newton) oras a wave (as argued by Christian Hupgens) Depending on the experiment you do, light behaves sometimes like a par- ticle, sometimes like a wave The two competing views were eventually reconciled by the quanhlm theorists like Max Planck and Albert Einstein into the "wavicle" concept called a photon,

You can imagine a photon as a pulsating packet of energy traveling through space Each photon is born and dies with a specific energylevel The photon's energy level does not change the speed at which the pho- ton r r a z ~ l c t h r o u g h any given medium, the speed of light is constant for all photons, regardless of energy level Instead, the energy level of the photon determines how fast it pulsates Higher-energy photons pulsate

at higher frequencies So as these photons all travel together at the same speed, the photons with higher energy travel shorter distances between pulses In other words, they have shorter wazwlengths Another way to put it is that every photon has a specific energy lwel, and thus a specific wavelength-the higher the energy level, the shorter the wavelength (see Figure 1-21,

The wavelengths of light are at the order of magnitude of nanometers,

or billionths of a meter (abbreviated nm)

The Spectrum

The spectrum refers to the full range of energy levels (wavelengths) that photons have as they travel through space and time The part of this spec- trum that tickles our eye is a small sliver from about 380 nm to about 700

nm that we call the visible spectrum, or simply, light (see Figure 1-3)

mure 1-P @f-V-deR RUIR -a-cLh (dRbce@f-V-deR

8 Real World Cdor Management, Second Edition

Our eyes respond only to this tiny sliver of the full electromagnetic spectrum, and they have varying responses to different parts of this sliv- er-the different wavelengths evoke different sensations of color So we've come to associate the different wavelengfhs with dle colors they evoke, from the reds at the low-energy end (longer wavelengths at about 700 nm) thmugh the oranges, yellows, and greens to the blues and violets at the high-energy end (shorter wavelengths at about 380 nm) Of course, there's nothing in the electromagnetic spectrum itself that prevents us bom namingmore or fewer than sixbands Newton, for example, labeled

a seventh band, indigo, between the blues and violets (Many historians believe that Newton was looking For symmetry with the seven notes of the musical octave.)

But no matter how many bands you label in the spectrum, the order- reds, oranges, yellows, greens, blues and violets 1s always the same (Fred and Bruce spent early years in a British school system, and were taught the mnemonic "Richard ofYork Gained Battles in Vain," while in the U S , Chris was introduced to the shange personage of Mr "ROY G BiV") We could reverse the order, and list them from shortest to longest wavelength (and hence from highest to lowest energy and frequenq-the lowerthe energy thelowerthe frequency, and the longer the wavelength), but green would always lie between blue and yellow, and orange would always lie behveen yellow and red

In the graphic arts, we're mainly concerned with visible light, but we sometimes have to pay attention to those parts of the spectrum that lie just outside the visible range The wavelengths that are slightly longer than red light occupy the inpared m) region (which means, literally,

"below red") IR often creates problems for digital cameras, because the CCD (charge-coupled-device) arrays used in digital cameras to detect light are also highly sensitive to infrared, somost digital cameras include

an IR filter either on the chip or on the lens

At the other end, just abwe the last visible violets, the range of high- energy (short-wavelength) photons known as the ttltrauioler (W)

region (literally, "beyond violet"] also raises some concerns For example, paper and ink manufacturers (like laundry detergent manufacturers) often add W brighteners to make an extra-white paper or exha-bright ink The brighteners absorb non-visible photons with W wavelengths

and re-emit photons in the visible spectrum-+ phenomenon known as

OHpkr 1: what b CdorI 9

fluorescence This practice creates problems for some measuring instru-

ments, because they see the paper or inkdifferently from the way our eyes

do We address these issues in Chapters 5 and 8

Spectral Curves

Other than the incredibly saturated greens and reds emitted by lasers, you'll rarely see light composed of photons of all the same wavelength

(what the scientists call monochromaticlight) Instead, almost all the light

you see consists of a blend of photons of many wavelengths The actual color you see is determined by the specific blend of wavelengths-the

spectral energy-that reaches your eye

Pure white light contains equal amounts of photons at all the visible wavelengths Light from a green object contains few short-wavelength (high-energy) photons, and fewlong-wavelength (lowenergy1 p h o t o n s

but is comprised mostly of medium-wavelength photons Light coming from apatch of magenta inkcontains photons in the short and long wave- lengths, but few in the middle of the visible spectrum

AU of these spectral energies can be represented by a diagram called the spectral curueof thelight reflected by the object (see Figure 1-41

Spectral curves

white object'

wavelength Spectral curves of three objecu

We care about several main kinds of light sources:

Blackbody radiators are light sources whose photons are purely the result of thermal energy given off by atoms Lightbulbs and stars such

as our sun are examples of near-perfect blackbodies The wavelength composition (i.e., "color") of radiation emitted by a blackbody radiator depends only on its temperature, and not what it's made of So we use color temperature as a way of describing the overall "color" of a light source (See the sidebar 'The Color ofWhite" later in this chapter.)

Dayiight is the result of our most familiar blackbody radiator, the sun, and an enormous filter we call the atmosphere It's probablythe most important of all light sources since it's the one under whichour

Chapter 1: Whal Ir Color? 11

visual system evolved The exact wavelength composition of daylight depends on the timeofday, the weather, and thelatitude (see the black curve in Figure 1-51

r Electric dischargelamps consist of an enclosed tube containinga gas (such as mercury vapor or 'enon) that's excited by an electric charge The charge raises the energy level of the gas atoms, which then re-emit the energy as photons at specific wavelengths, producing a "spikey" spectral curve Manufacturers use various techniques, such as pres- surizing the gas or coating the inside of the tube with phosphors, to add other wavelengths to the emitted light Flirorescerlt Inrrrps are the

most common form of these 1amps.The phosphors coating the inside

of the tube absorb photons emitted by the gas and re-emit them at other wavelengths

r Computer monitors are also light sources-they emit photons CRT (cathode-ray tube) monitors use phosphors on the inside of the front glas to absorb electrons and emit photons at specific wavelengths (either red, green, or blue) The red phosphor in particular is charncreristically spikey (see the red curve in Figure 1-5l.We'll describe monitors in more detail, includmgother types of monitors such as LCDs, in Chapter 6

Illurninants

The word ill~rrnirznnt refers to a light source that has been measured or specified formally in terms of spectral energy The CIE(Commission Intenm-

tionnle de I'Eclairuge, or the International Commission on Illumination) a

body of color scientists and technologistsfrom around the world that has accumulated a huge amount of knowledge about color since the 1 9 2 0 s

has specified a number of CTE Standard Illuminants

b NuminantA represents the typical spectral curve of a tungsten lamp (a standard Lightbulb).This is the green curve in Figure 1-5

b Illuminant B represents sunlight at a correlated color temperature of

4874 K This is seldom used, if ever

Illuminant C is an early daylight simulator (correlated color tempera- ture 6774 K) It has been replaced by the D illuminants, although you

occasionally still find it

19 Real World Cdor Management, kcond Wtion

b Illuminants D is a series of illuminants that Gpkseiii- various modes

of daylight The most commonly used D illuminants are D50 and D65 with correlated color temperatures of 5000 K and 6504 K, respectively The D65 spectral curve is the black curve in Figure 1-5

b IUuminant E is a theoretical "equal energy" illurninant that doesn't represent a real light source, and is mostly used for calculations

F Illuminants F is a series of "fluorescent" illuminants that represent the wavelength characteristics of various popular fluorescent lamps

These are named F2, F3, and so on, up to F12

The second participant in the color event is the object The way an object interacts with light plays a large role in determining the nature of the color event, so in this section we examine the various ways that objects interact with light, and the ways that this interaction affects our experi- ence of color

Chapter b What Is Color? 13

Many nf the light sources

we use suchaslightbulbs orsun-

light-produce light in a cbarac-

teristic way that dves us n handy

terminology to describe the color

of light: color tenrpernn~re Every

dense object radiates what5 called

fher~nnl energy Atoms re-emit

energy that they'veabsorbedfrom

some process such as combustion

(burning of fuel) or metabolism

(burning off those fries you had

for lunch) A1 low temperatures

this radiation is in the infrared

region invisible to humans, and

we call ir hear But at higher tem-

peratures the radiation is visible

and we call it light

To study this phenomenon,

physicists imagine objects where

they have eliminated all other

sources of light and areonly look-

ing at the radiation from thermal

energy They call these objects

blarkbody radiators If you're

standing in a pitch-black room,

you are a blackbody radiator

enlining energy that only an in-

frared detector,or an owl, cansee

Stars (such as our sun) are almost

perfect blackbodies as they aren't

illuminated by any other light

source and the light the!, emit

is almost entirely from the heat

t h e ~ r atoms have absorbed from the furnaces in the stars' cores

A lightbulb in a dark room is an almost-perfect blackbody radia- tor-all the light is from a heated filament of tungsten A candle is mostly a blackbody (although if

you look closely, you can see a small region of blue tight that's due to direct energy released by the chemical reaction of burning wax rather than absorption and re-emission of energy) To see a blackbody in acrion, turn on your toaster in a darkened kitchen

Figure 1-6 shows the spectral curves of a blackbody at various temperatures (Temperatures are in kelvins fk7, where a kel- vin is a degree in the physicist's temperature scale from absolute zero.) At lower temperatures, the blackbody gives off heat in the

low-energyllong-wavelength

part of the visible spectrum, and

so Is dominated by red and yel- low wavelengths At 2000 K we see the dull red we commonly call "red hot." As the temperantre gets higher, the curve shifts grad-

ually to the higher-energylshorter wavelengths At 3000 to 4000 K,

the light changes color from dull red to orange to yeUow.The tung-

sten filament of an incandescent lightbulb operates at about 2850

to 3100 K, giving itscharacteristic yellowish light At 5000 to 7000 K

the blackbody's emitted light Is relatively flat in the visible spec- trum, producing a more neutral white At higher temperatures

of 9000 K or above, short wave- lengths predominate, producing

a bluer light

This is the system weuse to de- scribe colors of "white light." We refer to their "color temppratuce"

to describe whether the light is orange, yellowish, neutral or bluish Purists will remind you

!hat the correct term is actually correlnted color ce~nperanrre as most emissive light sources- including daylight (which is filtered bythe earth'satmosphere), fluorescent lamps and coniputer monitors aren't true blackbody radiators, and so we're picking the closest blackbody tempera-

turr to the apparent color of the light source

Reflection and Transmission

An object's surface must interact with lightto affect the lighr'scolor Light strikes t h e object, travels some way into the atoms at the surface, then re-emerges During t h e light's interaction with these surface atoms t h e object absorbs some wavelengths and reflects others (see Figure 1-7), so

the spectral makeup of t h e rellected light isn't the s a m e as that of t h e

14 Real World Color Hanagcmcnt, Second Edition

Flgure 1-7 Reflection

Incoming white light containsall wavelengths \

of speculnr reflecrion may be unaffected by surface

Scattered reflection

surface absorb long

and short wavelengths

Surface of a reflective object

incoming light The degree to which an object reflects some wavelengths and absorbs othersis called its spectral reflectance Note that if you change the light source, the reflectance of the object doesn't change, even though the spectral energy that emerges is different Reflectance, then, is an invariant property of the object

A transmissive object affects wavelengths in the same way as the reflective object just described, except that the transmissive object must

be at least partially translucent so that the light can pass all the way through it However, it too alters the wavelength makeup of the light by absorbing some wavelengths and allowing others to pass through The surface of a reflective object or the substance in a transmissive object can affect the wavelengths that strike it in many specific ways But it's worth pausing to examine one phenomenon in particular that sometimes bedevils color management-the phenomenon known as jluorescence