Pigments: Energy and Color

References: Brill, Light Its Interaction with Art and Antiquities, Chapters 1 and 2

X-Rite, The Color Guide and Glossary

Daniel Smith Catalog Spring 1995: Study of Color by Sally Drew (=DS)

Electromagnetic Radiation

Light can mean the whole electromagnetic radiation spectrum or just visible light

Light can be thought of as a wave, and then c = λν

c is speed of light, 3 x 10⁸ m/s
ν is wavelength in some length units such as m, cm, nm (nanometer, 10⁹ nm/m)
ν is frequency, in cycles per second, or just 1/s, or s^-1, or hertz (Hz)
Rearranging, ν = c/λ

Light can also be thought of as a particle, and then DE = n hν

Δ means a change, final conditions - initial conditions
E is energy, e.g., calories, kilocalories (kcal), joules (J), electron volts (eV)
ΔE = E_final - E_initial
n is an integer, usually taken as 1
h is Plancks' constant, or proportionality constant and = 6.63 x 10^-34 J^. s
ν is still frequency, as above
Idea is that energy is quantized or can only occur in discrete units of size hn

XÐRite pg 4

HP: 7.5

BLB: 6.1, beginning of 6.2

The two ideas (views) can be combined and then ΔE = h ν = h c/λ

ΔE = E_final - E_initial = E_f - E_i = h (νf - νi) = h c (1/λf - 1/λi )

If E_f > E_i

Energy has been absorbed
Visible light can cause low energy valence shell electrons to be excited to higher energy states

If E_f < E_i

Energy has been given off
Visible light is emitted as electrons fall from higher energy states to lower energy states

HP: 7.7

Spectra

Continuous spectrum from white light - all wavelengths and thus all colors

Isaac Newton (1642-1725) used glass prism to disperse colors of sunlight

r, o, y, g, b, i, v

red, orange, yellow, green, blue, indigo, violet

...green, cyan blue, unltramarine blue, violet-blue

Line spectra of atoms

Hydrogen famous example

Bohr model and quantum mechanics deal with 4 emission lines in visible spectrum

Demonstrations

HP: 7.7

BLB: 6.3

UV, visible, IR - parts of spectrum most important to humans

UV - ultraviolet - 10-400 nm - Several common divisions of UV

10-180 nm - vacuum UV - must have a vacuum!
180-280 nm - far UV - Quartz and photographic gelatin transmit these wavelengths
280-300 nm - middle UV - responsible for tanning of skin
300-400 nm - near UV - ordinary glass transmits in this region
200-400 nm - enough energy to break bonds in organic molecules but also useful to art conservator in variety of ways (Brill 10)

Visible light - 400-700 nm

White light - continuous spectrum - equal amounts of all wavelengths

Williamson and Cummins Fig 2-3 gives correlation between wavelengths and colors

IR - infrared - 700-1400 nm - produce heat which speeds up chemical reactions, and causes shrinking, cracking, and drying

Molecular events that take place with different energies (wavelengths) of electromagnetic radiation

Electromagnetic spectrum

The FSU source mentioned above.

Color

Color is a combination of light source, object, and receptor

How the eye sees color

Some background on color vision and the eye

Light sources

Need to understand before you can understand how materials interact with light

For good color rendering, need to have the light source produce wavelengths of light that are characteristic of the true colors of the object

Physically, visible and UV light come from rapid oscillation of electrons in atoms. Differences come from types of atoms, geometry of the radiator, É

We are concerned with the radiant energy from the illuminating source - NOT with reflected light

Common sources are (blackbody radiators), sunlight, incandescent bulbs, sodium and mercury lights, fluorescent light, and flash bulbs

FSU sources.

Blackbody radiator - though really not a light source it IS a standard

If you heat a material it is either:

destroyed,
or it gives off light as a result of electrons in the matter being excited and then giving off energy as they stop being excited

Process called thermal radiation

Get continuous spectral distribution as a function of the material and temperature, i.e., distribution = f(material, temperature)

Blackbody thermal radiation - intensity and wavelengths = f(T) and NOT f(material)

All wavelengths are absorbed/emitted with equal efficiency

Cavity inside a closed body approximates a blackbody, E = f(T) Make a small hole to see inside!

Get a distribution - see diagrams

As T rises, principal wavelength gets shorter

Most energy is IR (heat)
300 K (Kelvin, about 25 deg C, about 78 deg F) - IR, warm
>1000 K (about 700 deg C, about 1400 deg F) starts red - weak
>2000 K - more intense red. UV starts to appear
3000 K - orange-yellow
4000 K - bright yellow
5000 K - white - uniform over visible spectrum
higher T - starts to be blue

HP: 7.6

Blackbody radiator the first several paragraphs

Blackbody radiator illumination

"Écolor and saturation (strength of color) of any light source can be matched to the color of a blackbody radiator operating at a particular temperature. The temperature in (deg) K of the blackbody that gives a color match to the light source of interest is called the color temperature of the source of interest)." Brill, pg 17.

Important to understand that the light source of interest is not necessarily heated to the temperature that is used in assignment of a color temperature.

Color temperature specifies the color and saturation which can be obtained. There are other ways, i.e., other spectral distributions, that would give the same color temperature

Color temperatures are most useful when spectral energy distributions of several sources are similar -like several different fluorescent lighst, but not so useful for different kinds of sources - like incandescent and fluorescent

 

Brill figures II-1, II-2

Williamson and Cummins figure 7.2

Sunlight

Exact energy distribution depends on latitude, time of day, season of year, altitude, and local atmosphere

Of sun's energy reaching the earth:

50% is visible
3% is UV
47% is IR (-> heat)

For most chemical and visual effects (and thus in this course) the range of wavelengths of interest is 300-800 nm

Distribution of energies at sea level of visible light is fairly uniform

Good color rendering because of uniform spectral energy

Color temperature is about 5000 K, though north sky is bluer and 'hotter; at about 7500 K

Brill figures II-3 and II-4

Incandescent bulbs

Normal light bulb - Edison

"Incandescence is the shining of hot bodies because they are hot."

Pass a current through a high resistance wire - temperature rises, wire (filament) glows and gives off light

Tungsten (W) used today

Shows distribution - like Blackbody radiator

Color temperature is 2700-3200 K

Lamp efficiency = amount of visible light produced divided by the total amount of energy radiated. Incandescent bulb about 25% efficient and varies some with wattage.

Emphasis on longer wavelengths - orange and red and less blue.

This makes it hard to distinguish blue shades

(Luminous) Efficacy = amount of emitted light/energy supplied to lamp. About 12-25 lm/W (lumens per watt)

Brill figure II-5, Brill II-6

"Discharge lamps" - sodium and mercury

Volatile metal is excited by electrons via high voltage

Higher efficacy, e.g., older sodium lamps about 60-90 lm/W (street lights)

Sharp, intense lines emitted, but eye combines these wavelengths into one 'impression' of color

Rectangular columns in the figures represent large number of intense lines over a narrow wavelength range

Hg - silver-green

Na - yellow-orange

Color rendering not so good since discontinuous spectrum

Brill has interesting paragraph comparing moonlight to Hg

Brill figures II-7 and II-9

Fluorescent lights

Combination of a discharge source, usually Hg, and a phosphor coating on the tube to give appearance of a continuous spectrum

Idea of fluorescence is that the fluors or phosphors absorb energy of one wavelength and then radiate energy of a different wavelength - usually with the second wavelength being longer (lower energy)

Brill gives spectral distributions of a large range of fluorescent coatings: cool white, warm white, daylight

Also deluxe and standard

Standard lacks deep red. That combined with strong yellow distort reds to greenish

Strong blue-violet and lack of pure blue and green-blue makes blues into violet.

Deluxe gives better colors, but 30% lower efficacy, (normal for fluorescent is 30-80 lm/W)

Warm, cool

Warm - more red and more IR and thus more heat
Cool - less red, less IR Warm - lower color temperature and thus emit radiation of relatively lower energy

Mixing of colored lights - additive colors

Primary colors are red, green, blue - colors to which cones of eye respond

"More colors can be matched additively with blue, green, and red than with any other three colors." Brill 71.

Think of dividing visible spectrum into three equal chunks: 400-500 nm (blue), 500-600 nm (green), 600-700 nm (red).

With r,b,g or equal intensity, get b-g (cyan), r-g (yellow), r-b (magenta)

If vary intensities get others colors in overlap regions

Overlap of all three -> white, and thus the idea of additive

Also get white from a two color overlap and the complement, e.g., cyan and red or yellow and blue

Disadvantages

Can only be used with transparent media: colored lights, mixing of dyes, mixing of colorants in transparent glass and glazes

Total intensity diminished because of use of filters

Color TV uses this idea - different manufacturers use differnt arrangements, exact colors, patterns, etc. to give their particular color rendering

Also reflected light from two closely spaced but different colored surfaces - Impressionist painters - pointillism.

X-Rite pg 8-9

Brill chapter V
Brill figure V-8

Gerritsen - Evolution of Color and others

Additive lights

FSU Primary colors

FSU site good. Java script neat, but too slow on my computer. I had to interrupt the application to regain control.

Absorption or scattering of light from a colored surface - subtractive colors

Light hits surface and some colors absorbed, the rest is transmitted to viewer

Keeping same idea of dividing the visible spectrum into three areas, the three subtractive primaries are the colors when 1/3 of the spectrum is absorbed and the other 2/3 is transmitted .

Names are cyan (red absorbed), magenta (green absorbed), and yellow (blue absorbed)

Overlap of any two of the subtractive primaries gives an additive primary; overlap of all three subtractive primaries gives black because all the visible light has been absorbed.

Subtractive colors used in photographic film, opaque printing media, paints

Mixing different amounts of the subtractive primaries gives a wide range of transmitted colors allowing something like 95% of all colors to be obtained.

Brill figure V-9, Figure V-11particularly useful

X-Rite: pg 10-11

CMYK system (scroll down)

Color description/measurement systems

Life is not so simple. What about tan, gray, brown, etc.?

Can't really measure color since color is subjective

Rather can describe color in ideal and standard terms

All systems that allow for a 'complete' color description involve 3 coordinates

Hue, saturation, brightness

•Hue is what one would normally call color. It is the physical wavelength of light
•Brightness (sometimes called value) is the lightness or darkness relative to a series of grays from white to black
•Saturation (sometimes called chroma) is the difference between the color and a gray having the same brightness
•Any one of the three can vary - giving entirely different 'colors'

X-Rite pg 11-12

Two approaches to color measurement:

Visual comparison and

Use an instrument to determine relative amount of each wavelength

Most famous visual comparison is Munsell system

It is widely used in artists' colors - arose from Munsell's desire to be able to specify a color and have everyone else know exactly what he meant

Hue

100 increments on a circle, with 5 principal hues (R,Y,G,B,P) and 5 intermediates (YR, GY, etc).
5R, 5Y, 5G, etc is the middle or main hue

Chroma

The distance out from the central axis, /chroma

Value

The equivalent lightness along the vertical, achromatic axis, i.e., black ->white ('a' means without)

A main feature of Munsell is the steps/increments of chroma and value are equal perceptual intervals For different values the choice of pigments available varies -> irregular shape of the 3-D model

Chamberlin & Chamberlin figure 3.7

DS: pg 29

Instruments

Spectrophotometers

Colorimeters

Spectrophotometer

Does wavelength by wavelength measurement of intensity of radiation transmitted or reflected by a sample

Does not tell color, but gives data from which one can deduce the color appearance from agreed conventions

Can be done in lab

Detectors are photodectors - photomultiplier tubes

Colorimeters

They also compare, as people can, by comparing a mixture of colored lights to the light reflected from the sample

Photoelectric colorimeters - like our colorimeter

They isolate selected wavelength bands using colored filters

Compare reflected/transmitted light against the incident light

Need to be calibrated

Lots of filters good, but 3 ok if they are matched to be the same as the CIE standard observer - see below

The color we sense

(As mentioned wa) above) is a product of the illuminant, the object, and the detector,

i.e., the light that leaves the source is influenced/modified by the object and then further modified by the detector/visual response

If the detector is the human eye, has to have normal color vision!

Common light sources

Several were mentioned way above

In addition there are several standards:

A - Tungsten lamp, 2856K color T
B - is direct sunlight, color temperature 4870 K
C - average daylight light from an overcast sky , color temperature 6770 K.
D - is a 'non-real' standard - mathematical construct. 6500 K

CIE system

An objective system developed in 1931 by Commission International de L'Eclariage (CIE)

An international system - WIDELY used and referred to.

Hue -> wavelength
Brightness -> luminance
Saturation -> purity

CIE starts with Maxwell color triangle

Any edge is a mixture of two of the 3 primaries in all proportions.

The third primary is 0!

The hues merge imperceptibly along the axis.

Edge shows maximum saturation.

Move to center - add the third primary - dilute the color - white in center

Locate a point and then move to an edge - as you move the hue is constant and saturation is increased

Color defined by matching three colored lights to the sample (can get all hues but not all saturations) and pinpointing position on the CIE diagram

The three primaries X, Y, Z are a supersaturated red (700 nm), supersaturated green (546.1 nm), and unattainable blue (435.8 nm), in that order

Y is adjusted to also give the luminosity

Some confusion about exactly what tristimulus values are, but usually the X, Y, Z. These values changed to x, y, z, via x = X/(X+Y+Z) , etc.

Figure 4.8 shows resulting x vs y Chromaticity plot. z not shown, since x + y + z = 1.

Problems with CIE

Nonuniformity of visual response over the area

2-D - no brightness - therefore center is achromatic and is either black or white!

Eye not equally sensitive to all colors - different wavelengths can distinguish small to large differences in color

Result is the L*a*b* system.

CIELAB color space.

L is lightness
+a is read, -a is green
+b is yellow, -b is blue

Uniform scale

Hue is circle
Saturation is distance out
Lightness is vertical axis.

LAB coordinates and color swatches

RGB coordinates and color swatches