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Light and Color Measurement: Glossary A - Z
A B C
D E F
G H I
J K L M
N O P
Q R S T
U V W X Y
Z
A  top
absolute white
In theory, a material that perfectly reflects all light
energy at every visible wavelength.
In practice, a solid white with known spectral reflectance
data that is used as the "reference white" for
all measurements of absolute reflectance. When calibrating
a spectrophotometer, often a white ceramic plaque is measured
and used as the absolute white reference.
absorb/absorption
Dissipation of the energy of electromagnetic waves into
other forms (e.g., heat) as a result of its interaction
with matter; a decrease in directional transmittance of
incident radiation, resulting in a modification or conversion
of the absorbed energy.
achromatic color
A neutral color that has no hue (white, gray or black).
additive primaries
Red, green and blue light. When all three additive primaries
are combined at 100% intensity, white light is produced.
When these three are combined at varying intensities, a
gamut of different colors is produced. Combining two primaries
at 100% produces a subtractive primary, either cyan, magenta
or yellow:
- 100% red + 100% green = yellow
- 100% red + 100% blue = magenta
- 100% green + 100% blue = cyan.
See subtractive primaries
AFS – Adaptive Front Lighting Systems
AFS is already state-of-the-art in top range cars. It includes
function such as static bend lighting (switching on if the
care is approaching a traffic junction), adaptive bend lighting
(the headlamps follow the vehicle’s direction on the
road), adaptive motorway lighting (with wider intensity
distribution for a low beam headlamp) etc.
appearance
An object's or material's manifestation through visual attributes
such as size, shape, color, texture, glossiness, transparency,
opacity, etc.
ARS – Adaptive Rear Lighting Systems
As AFS a state-of- the-art solution in automotive lighting
to control flexibly rear lamps in accordance with weather
conditions, emergency braking etc.
artificial daylight
Term loosely applied to light sources, frequently equipped
with filters, that try to reproduce the color and spectral
distribution of daylight. A more specific definition of
the light source is preferred.
attribute
Distinguishing characteristic of a sensation, perception
or mode of appearance. Colors are
often described by their attributes of hue, chroma (or saturation)
and lightness.
B top
black
In theory, the complete absorption of incident light; the
absence of any reflection. In practice,
any color that is close to this ideal in a relative viewing
situation - i.e., a color of very low saturation
and very low luminance.
brightness
The dimension of color that refers to an achromatic scale,
ranging from black to white. Also called lightness, luminous
reflectance or transmittance (q.v.). Because of confusion
with saturation, the use of this term should be discouraged.
C top
c*
Abbreviation for chromaticity.
chroma/chromaticity
The intensity or saturation level of a particular hue, defined
as the distance of departure of a chromatic color from the
neutral (gray) color with the same value. In an additive
colormixing environment, imagine mixing a neutral gray and
a vivid red with the same value. Starting with the neutral
gray, add small amounts of red until the vivid red color
is achieved. The resulting scale obtained would represent
increasing chroma. The scale begins at zero for neutral
colors, but has no arbitrary end. Munsell originally established
10 as the highest chroma for a vermilion pigment and related
other pigments to it. Other pigments with higher chroma
were noted, but the original scale remained. The chroma
scale for normal reflecting materials may extend as high
as 20, and for fluorescent materials it may be as high as
30.
chromatic
Perceived as having a hue - not white, gray or black.
chromaticity coordinates (CIE)
The ratios of each of the three tristimulus values X, Y
and Z in relation to the sum of the three - designated as
x, y and z respectively. They are sometimes referred to
as the trichromatic coefficients. When written without subscripts,
they are assumed to have been calculated for illuminant
C and the 2° (1931) standard observer unless specified
otherwise. If they have been obtained for other illuminants
or observers, a subscript describing the observer or illuminant
should be used. For example, x10 and y10 are chromaticity
coordinates for the 10° observer and illuminant C.
chromaticity diagram (CIE)
A two-dimensional graph of the chromaticity coordinates
(x as the abscissa and y as the ordinate), which shows the
spectrum locus(chromaticity coordinates of monochromatic
light, 380-770nm). It has many useful properties for comparing
colors of both luminous and non-luminous materials.
CIE (Commission Internationale de l'Eclairage, International
Commission on Illumination)
As its name implies, the International Commission on Illumination
- abbreviated as CIE from its French title Commission Internationale
de l'Eclairage - is an organization devoted to international
cooperation and exchange of information among its member
countries on all matters relating to the science and art
of lighting. The CIE is a technical, scientific and cultural,
non-profit autonomous organization. It has grown out of
the interests of individuals working in illumination. Since
its inception 90 years ago, the CIE has become a professional
organization and has been accepted as representing the best
authority on the subject and as such is recognized by ISO
as an international standardization body.  http://www.cie.co.at/cie/
CIE 1976 L*a*b* color space
A uniform color space utilizing an Adams-Nickerson cube
root formula, adopted by the CIE in 1976 for use in the
measurement of small color differences.
CIE 1976 L*u*v* color space
A uniform color space adopted in 1976. Appropriate for use
in additive mixing of light (e.g., color TV).
CIE chromaticity coordinates
See chromaticity coordinates (CIE).
CIE chromaticity diagram
See chromaticity diagram (CIE).
CIE daylight illuminants
See daylight illuminants (CIE).
CIE luminosity function (y)
See luminosity function (CIE).
CIE Publications
- No. 54.2-2001 Norm. Retroreflection. Definition and measurement
of retroreflection, issued by the CIE in 2001
- No. 69 Norm: Methods of characterizing illuminance and
luminance meters, issued by the CIE in 1987
- No. 84 Norm: The measurement of luminous flux, issued
by the CIE in 1986
- No. 121 Norm. The photometry and goniophotometry of luminaries
issued by the CIE in 1996
- No. 127 Norm. Measurement of LEDs. issued by the CIE
CIE standard illuminants
See standard illuminants (CIE).
CIE standard observer
See standard observer (CIE).
CIE tristimulus values
See tristimulus values (CIE).
CIELAB (or CIE L*a*b*, CIE Lab)
Color space in which values DL*, Da* and Db* are plotted
using Cartesian coordinate system. Equal distances in the
space approximately represent equal color differences. Value
DL* represents lightness; value Da* represents the red/green
axis; and value Db* represents the yellow/blue axis. CIELAB
is a popular color space for use in measuring reflective
and transmissive objects.
CMC (Colour Measurement Committee of the Society of
Dyes and Colourists of Great Britain)
Organization that developed and published in 1988 a more
logical, ellipse-based equation based on
L*C*h° color space for computing DE (see delta E*) values
as an alternative to the rectangular coordinates of the
CIELAB color space.
color
One aspect of appearance; a stimulus based on visual response
to light, consisting of the three dimensions of hue, saturation
and lightness.
color attribute
A three-dimensional characteristic of the appearance of
an object. One dimension
usually defines the lightness, the other two together define
the chromaticity.
color difference
The magnitude and character of the difference between two
colors under specified
conditions.
colorimetry
Colorimetry is based on the fact that observers can match
colors with additive mixtures of three reference stimuli
in amounts known as tristimulus values. Using reference
stimuli at specified wavelengths, CIE has defined a standard
set of tristimulus values to match each different wavelength
of the spectrum. These data constitute the CIE 1931 standard
colorimetric observer. The reference-color stimuli are radiations
of wavelength 700 nm for the red stimulus (R), 546.1 nm
for the green stimulus (G) and 435.8 nm for the blue stimulus
(B).
color-matching functions
Relative amounts of three additive primaries required to
match each wavelength of light. The term is generally used
to refer to the CIE standard observer color-matching functions.
color measurement
Physical measurement of light radiated, transmitted or reflected
by a specimen under specified condition and mathematically
transformed into standardized colorimetric terms. These
terms can be correlated with visual evaluations of colors
relative to one another.
color model
A color-measurement scale or system that numerically specifies
the perceived attributes of color. Used in computer graphics
applications and by color measurement instruments.
color order systems
Systems used to describe an orderly threedimensional arrangement
of colors.
Three bases can be used for ordering colors: 1) an appearance
basis (i.e., a psychological basis) in
terms of hue, saturation and lightness; an example is the
Munsell System; 2) an orderly additive color
mixture basis (i.e., a psychophysical basis); examples are
the CIE System and the Ostwald System; and 3) an orderly
subtractive color mixture basis; an example is the Plochere
Color System based on an orderly mixture of inks.
color space
Three-dimensional solid enclosing all possible colors. The
dimensions may be described in various geometries, giving
rise to various spacings within the solid.
color specification
Tristimulus values, chromaticity coordinates and luminance
value, or other color-scale values, used to designate a
color numerically in a specified color system.
color temperature
A measurement of the color of light radiated by a black
body while it is being heated. This measurement is expressed
in terms of absolute scale, or degrees Kelvin. Lower Kelvin
temperatures such as 2400K are red; higher temperatures
such as 9300K are blue. Neutral temperature is white, at
6504K.
color wheel
The visible spectrum's continuum of colors arranged in a
circle, where complementary colors such as red and green
are located directly across from each other.
colorants
Materials used to createcolors - dyes, pigments, toners,
waxes, phosphors.
colorimeter
An optical measurement instrument that responds to color
in a manner similar to the
human eye - by filtering reflected light into its dominant
regions of red, green and blue.
colorimetric
Of, or relating to, values giving the amounts of three colored
lights or receptors - red,
green and blue.
colorist
A person skilled in the art of color matching (colorant
formulation) and knowledgeable concerning the behavior of
colorants in a particular material; a tinter (q.v.) (in
the American usage) or a shader. The word "colorist"
is of European origin.
complements
Two colors that create neutral gray when combined. On a
color wheel, complements are directly opposite from each
other: blue/yellow, red/green and so on.
contrast
The level of variation between light and dark areas in an
image.
D top
D65
The CIE standard illuminant that represents a color temperature
of 6504K. This is the color temperature most widely used
in graphic arts industry viewing booths. See Kelvin (K).
daylight illuminants (CIE)
Series of illuminant spectral power distribution curves
based on measurements of natural daylight and recommended
by the CIE in 1965. Values are defined for the wavelength
region 300 to 830nm. They are described in terms of the
correlated color temperature. The most important is D65
because of the closeness of its correlated color temperature
to that
of illuminant C, 6774K. D75 bluer than D65 and D55 yellower
than D65 are also used.
delta (D or Δ)
A symbol used to indicate deviation or difference.
delta E*, delta e*
The total color difference computed with a color difference
equation (ΔEab or ΔEcmc). In color tolerancing,
the symbol DE is often used to express Delta Error.
DIN Deutsches Institut für Normung (German
Standardization Institution). www.din.de
DIN Publications
- DIN 5031, section 1 Norm: Photometry - Part 1: Methods
of measurement, issued 1999
- DIN 5031, section 4 Norm: Photometry - Part 4: Measurement
of luminaries, issued 1999
- DIN 5032, section 7 Norm: Methods of characterizing illuminance
and luminance meters, classifying 4 accuracy classed for
illuminance and luminance meters ranging from C (low accuracy),
B (medium accuracy), up to A (high accuracy) and L (highest
accuracy). Issued in 1985. DIN 5032-7 became widely respected
in the lighting world and greatly influenced the CIE publication
No. 69.
dye
A soluble colorant - as opposed to pigment, which is insoluble.
dynamic range
An instrument's range of measurable values, from the lowest
amount it can detect to the highest amount it can handle.
E top
electromagnetic spectrum
The massive band of electromagnetic waves that pass through
the air in different sizes, as measured by wavelength. Different
wavelengths have different properties, but most are invisible
- and some completely undetectable - to human beings. Only
wavelengths that are between 380 and 720 nanometers are
visible, producing light.Waves outside thevis ible spectrum
include gamma rays, x-rays, microwaves and radio waves.
emissive object
An object that emits light. Emission is usually caused by
a chemical reaction, such
as the burning gasses of the sun or the heated filament
of a light bulb.
F top
fluorescent lamp
A glass tube filled with mercury gas and coated on its inner
surface with phosphors. When the gas is charged with an
electrical current, radiation is produced. This, in turn,
energizes the phosphors, causing them to glow.
Foot-Candle
One footcandle is equal to 10.76 lux, although in the lighting
industry, typically this is approximated as 1 footcandle
being equal to 10 lux. In the lighting industry, footcandles
are a common unit of measurement used to calculate adequate
lighting levels of workspaces in buildings or outdoor spaces.
Foot-lambert
A foot-lambert is a unit of measurement of luminance in
U.S. customary units. A foot-lambert equals π - 1 candela
per square foot. The luminance of a perfect lambertian diffuse
reflecting surface in footlamberts is equal to the incident
illuminance in foot-candles. For real diffuse reflectors,
the ratio of luminance to illuminance in these units is
roughly equal to the reflectance of the surface. This unit
is obsolete, and is rarely used in practice by electrical
and lighting engineers, in favor of the clearer candela
per square foot.
G top
gloss
An additional parameter to consider when determining a color
standard, along with hue, value,
chroma, the texture of a material and whether the material
has metallic or pearlescent qualities. Gloss is an additional
tolerance that may be specified in the Munsell Color Tolerance
Set. The general rule for evaluating the gloss of a color
sample is the higher the gloss unit, the darker the color
sample will
appear. Conversely, the lower the gloss unit, the lighter
a sample will appear. Gloss is measured in gloss units,
which use the angle of measurement and the gloss value (e.g.
60° gloss = 29.8). A 60° geometry is recommended
by the American Society for Testing and Materials (ASTM)
D523 standard for the general evaluation of gloss.
Goniometer
The word gonio (γονιο) derives from the Greek meaning angle.
A goniometer is an instrument that measures angles most
accurately.
Goniophotometer
Goniometer + Photometer = Goniophotometer
The word gonio (γονιο) derives from the Greek meaning angle.
A goniometer is an instrument that measures angles most
accurately. Photometer derives from the Greek photon (φοτον)
= light and is an instrument that measures light. A goniophotometer
therefore performs the measurement of the spatial distribution
of a radiation source and displays the photometric properties
of the light visible to the human eye in relation to a defined
angular position. The automotive and general lighting industries
use goniophotometers for lighting research and as a control
measure in their manufacturing workflow.
grayscale
An achromatic scale ranging from black through a series
of successively lighter grays to white. Such a series may
be made up of steps that appear to be equally distant from
one another (such as the Munsell Value Scale), or it may
be arranged according to some other criteria such as a geometric
progression based on lightness. Such scales may be used
to describe the relative amount of difference between two
similar colors.
GTB - Groupe de Travailles de Bruxelles
The GTB is a working group of vehicle lighting specialists
working on the UNECE regulations on automotive lighting.
The working group photometry of GTB issued also the Photometry
Accuracy Guidelines applicable for photometry equipment
used to measure automotive lamps and retroreflectors.
H top
HID
High intensity discharge lamps (also called Xenon bulbs)
widely used today for automotive applications. HID has a
a larger total luminous flux and a slightly more blue luminous
color closer to daylight in comparison to traditional incandescent
and halogen bulbs which are closer to standard illuminant
A.
hue
1) The first element in the color-order system, defined
as the attribute by which we distinguish red from green,
blue from yellow, etc. Munsell defined five principal hues
(red, yellow, green, blue and purple) and five intermediate
hues (yellowred, green-yellow, blue-green, purple-blue and
red-purple. These 10 hues (represented by their corresponding
initials R, YR, Y, GY, G, BG, B, PB, P and RP) are equally
spaced around a circle divided into 100 equal visual steps,
with the zero point located at the beginning of the red
sector. Adjacent colors in this circle may be mixed to obtain
continuous variation from one hue to another. Colors defined
around the hue circle are known as chromatic colors.
2) The attribute of color by means of which a color is perceived
to be red, yellow, green, blue, purple, etc. White, black
and gray possess no hue.
I, J top
Illuminance
Illuminance is a quantity derived from intensity which denotes
luminous flux density. It has a special name, lux, and is
lumens per square meter, or lm/m². The symbol is
Ev. Most light meters measure this quantity,
as it is of great importance in illumination engineering.
Some examples for typical illuminances range from 100,000
lx for direct sunlight, or 500 lx on a working desk in office
to 20-50 lx for hospital corridors at night and 1 lx for
emergency lighting. E = Φ/A
illuminant
Mathematical description of the relative spectral power
distribution of a real or imaginary
light source - i.e., the relative energy emitted by a source
at each wavelength in its emission spectrum. Often used
synonymously with "light source" or "lamp,"
though such usage is not recommended.
illuminant A (CIE)
Incandescent illumination, yellow-orange in color, with
a correlated color temperature of 2856K. It is defined in
the wavelength range of 380 to 770nm.
illuminant C (CIE)
Tungsten illumination that simulates average daylight, bluish
in color, with a correlated color temperature of 6774K.
illuminants D (CIE)
Daylight illuminants, defined from 300 to 830nm (the UV
portion 300 to 380nm being necessary to correctly describe
colors that contain fluorescent dyes or pigments). They
are designated as D, with a subscript to describe the correlated
color temperature; D65 is the most commonly used, having
a correlated color temperature of 6504K, close to that of
illuminant C.
They are based on actual measurements of the spectral distribution
of daylight.
integrating sphere
Based upon the principle of multiple diffuse reflection
(resulting from the Lambertian coating), the integrating
sphere is used to spatially integrate radiant flux, either
from an external or an internal source of radiation. The
efficiency of an integrating sphere is determined by a number
of factors, including the size and number of ports, the
size and location of baffles or screens, the number of inclusions
in the sphere, and most importantly, the reflectance and
diffuse nature of the sphere coating that has to be “Lambertian”
(ideally diffuse reflecting). Depending on port position,
baffling, and to an extent, sphere size, integrating spheres
can be configured for a number of applications such as lamp
measurement systems, uniform light sources, transmittance
and reflectance measurement. The classical application is
the measurement of luminous flux: In contrast to the absolute
measuring method using a goniophotometer that scans the
complete intensity distribution of the lamp and integrates
the flux, the integrating sphere is the relative measuring
method requiring calibration of the sphere with a calibrated
flux (or spectral distribution) bulb that should be traceable
to a national standard (PTB or NIST).
K top
Kelvin (K)
Unit of measurement for color temperature. The Kelvin scale
starts from absolute zero,
which is -273° Celsius.
L top
Lambertian Coating
Lambert’s Cosine Law (also known as Lambert’s
cosine emission law) is the mathematical statement that
a radiance of certain idealized optical sources is directly
proportional to the cosine of the angle – with respect
to the direction of maximum radiance – from which the
source is viewed. Lamberts cosine law also applies to certain
idealized diffuse reflectors or coatings.
light
1) Electromagnetic radiation of which a human observer is
aware through the visual sensations that arise from the
stimulation of the retina of the eye. This portion of the
spectrum includes wavelengths from about 380 to 770nm. Thus,
to speak of ultraviolet light is incorrect because the human
observer cannot see radiant energy in the ultraviolet region.
2) Adjective meaning high reflectance, transmittance or
level of illumination as contrasted to dark, or low level
of intensity.
light source
An object that emits light or radiant energy to which the
human eye is sensitive. The emission of a light source can
be described by the relative amount of energy emitted at
each wavelength in the visible spectrum, thus defining the
source as an illuminant. The emission also may be described
in terms of its correlated color temperature.
lightness
Perception by which white objects are distinguished from
gray, and light-colored objects from dark-colored.
LiTG – Deutsche Lichttechnische Gesellschaft
The LiTG is the German Association of Lighting Technology
Luminance
Luminance is analogous to radiance, differentiating the
lumen with respect to both area and direction, and is measured
in cd/m². The symbol is Lv. It is
most often used to characterize the "brightness"
of flat emitting or reflecting surfaces. Luminance is the
only photometric quantity that can be visually seen by human
beings (except starlight). Lighting as well as illuminated
surfaces (depending on their reflectance) have a certain
luminance. The formula is L = I/A.
luminosity function (y) (CIE)
A plot of the relative magnitude of the visual response
as a function of wavelength from about 380 to 780nm, adopted
by CIE in 1924.
Luminous Flux
The lumen is a derived unit for luminous flux. Its abbreviation
is lm and its symbol is φv. The lumen is
derived from the candela and is the luminous flux emitted
into unit solid angle (1 sr) by an isotropic point source
having a luminous intensity of 1 candela. The lumen is the
product of luminous intensity and solid angle, cd-sr. It
is analogous to the unit of radiant flux (Watt), differing
only in the eye response weighting. The formula is Φ
= I x Ω.
Luminous Intensity
The candela is the basic unit in photometry. All other luminous
quantities can principally be derived from it. The candela
is the luminous intensity, in a given direction, of a source
that emits monochromatic radiation of frequency 540×1012
Hertz and that has a radiant intensity in that direction
of 1/683 Watt per Steradian. The formula is: I = Φ/Ω
M top
metamerism
A phenomenon exhibited by a pair of colors that match under
one or more sets of illuminants (be they real or calculated),
but not under all illuminants.
Munsell Color System
The color identification of a specimen by its Munsell hue,
value and chroma as visually estimated by comparison with
the Munsell Book of Color.
N top
nanometer (nm)
Unit of length equal to 10-9 meter (a.k.a. one billionth
of a meter, or a milli-micron).
O top
observer
The human viewer who receives a stimulus and experiencesa
sensate ion from it. In vision, the stimulus is a visual
one and the sensation is an appearance.
observer, standard
See standard observer.
P, Q top
Photometer
Photometer derives from the Greek photon (φοτον) = light
and is an instrument that measures light (illuminance, intensity,
luminous flux, luminance).
Photometric limit distance
Photometric measurements are conducted in a distance where
the light source can be regarded as point light source.
The photometric limit distance is no absolute quantity but
depends on the accuracy demands for measuring, the size
of the light source being measured and the light sensitive
surface of the photopic detector. Certain recommendations
are given (GTB photometry accuracy guidelines), e.g. for
automotive headlamps at 25 m, signal lamps at minimum 3
m.
Photometry
Photometry is the measurement of light, which is defined
as electromagnetic radiation detectable by the human eye
(daylight). It is thus restricted to the wavelength range
from about 380 to 780 nanometers (1000 nm = 1 μm). Photometry
is just like radiometry except that everything is weighted
by the spectral response of the eye. Visual photometry uses
the eye as a comparison detector, while physical photometry
uses either optical radiation detectors constructed to mimic
the spectral response of the eye, or spectroradiometry coupled
with appropriate calculations to do the eye response weighting.
Typical photometric units include lumens, lux, and candelas.
Plane light source
Being watched from a short distance, any light source is
a plane light source. To achieve reliable results, light
sources used to be measured from a distance where they can
be regarded as point light sources.
PWM – Pulse width modulation
Pulse width modulation is often used for LED applications
related to vehicle lights. The PWM serves to create a different
level of intensity to realize different functions of a signal
lamp (rear lamp, brake lamp, rear fog lamp) with one light
source and to reduce thermal emission.
R top
radiant energy
A form of energy consisting of the electromagnetic spectrum,
which travels at 299,792 kilometers/second (186,206 miles/second)
through a vacuum, and more slowly in denser media (air,
water, glass, etc.). The nature of radiant energy is described
by its wavelength or frequency, although it also behaves
as distinct quanta ("corpuscular theory"). The
various types of energy may be transformed into other forms
of energy (electrical, chemical, mechanical, atomic,
thermal, radiant), but the energy itself cannot be destroyed.
reflectance
The ratio of the intensity of reflected radiant flux to
that of incident flux. In popular usage, it is considered
the ratio of the intensity of reflected radiant energy to
that reflected from a defined reference standard.
reflectance, specular
See specular reflectance.
reflectance, total
See total reflectance.
Retroreflection
Retroreflection is used on road surfaces, road signs, vehicles
and clothing (large parts of the surface of special safety
clothing, less on regular coats). When the headlights of
a car illuminate a retroflective surface, the reflected
light is directed towards the car and its driver, and not
wasted by going in all directions as with diffuse reflection.
However, a pedestrian can see a retroreflective surface
in the dark only if there is a light source directly between
them and the reflector, e.g. a torch they carry, or directly
behind them, e.g. a car approaching from behind. The formula:
R = I/E. Retroreflection is measured in mcd/lx. Specific
applications require measuring the intensity reflected by
a certain surface with dimensions known.
Retroreflectometer
A photometer equipped with a light source regulated to a
certain illuminant (usually A or D65 or C) that measures
the coefficient of retroreflection in mcd/lx or in cd/lx/m²
of retroreflectors. See retroreflector.
Retroreflector
A retroreflector is a device that sends light or other radiation
back where it came from regardless of the angle of incidence,
unlike a mirror, which does that only if the mirror is exactly
perpendicular to the light beam. A retroreflector may consist
of many very small versions of these structures incorporated
in a thin sheet or in paint. In the case of paint containing
glass beads, the paint glues the beads to the surface where
retroreflection is required, and the beads protrude, their
diameter being about twice the thickness of the paint. Retroreflection
is used on road surfaces, road signs, vehicles and clothing
(large parts of the surface of special safety clothing,
less on regular coats). When the headlights of a car illuminate
a retroflective surface, the reflected light is directed
towards the car and its driver, and not wasted by going
in all directions as with diffuse reflection. However, a
pedestrian can see a retroreflective surface in the dark
only if there is a light source directly between them and
the reflector, e.g. a torch they carry, or directly behind
them, e.g. a car approaching from behind.
S top
SAE - The Society of Automotive Engineers
The SAE has more than 90,000 members - engineers, business
executives, educators, and students from more than 97 countries
- who share information and exchange ideas for advancing
the engineering of mobility systems. The SAE also works
on regulations for automotive lamps (Ground Vehicle Lighting
Standards Manual) and issued the photometry accuracy guidelines:
SAE-J 1330 MAR2000.
saturation
The attribute of color perception that expresses the amount
of departure from a gray of the same lightness. All grays
have zero saturation (ASTM). See chroma/chromaticity.
scattering
Diffusion or redirection of radiant energy encountering
particles of different refractive index. Scattering occurs
at any such interface, at the surface, or inside a medium
containing particles.
spectral power distribution curve
Intensity of radiant energy as a function of wavelength,
generally given in relative power terms.
spectrophotometer
Photometric device that measures spectral transmittance,
spectral reflectance or relative spectral emittance.
spectrophotometric curve
A curve measured on a spectrophotometer; a graph with relative
reflectance or transmittance (or absorption) as the ordinate,
plotted with wavelength or frequency as the abscissa.
spectrum
Spatial arrangement of components of radiant energy in order
of their wavelengths, wave
number or frequency.
specular gloss
Relative luminous fractional reflectance from a surface
in the mirror or specular direction. It is sometimes measured
at 60° relative to a perfect mirror.
specular reflectance
Reflectance of a beam of radiant energy at an angle equal
but opposite to the incident angle; the mirror-like reflectance.The
magnitude of the specular reflectance on glossy materials
depends on the angle and the difference in refractive indices
between two media at a surface. The magnitude may be calculated
from Fresnel's Law.
specular reflectance excluded (SCE)
Measurement of reflectance made in such a way that the specular
reflectance is excluded from the measurement; diffuse reflectance.
The exclusion may be accomplished by using 0° (perpendicular)
incidence on the samples. This then reflects
the specular component of the reflectance back into the
instrument by use of black absorbers or light traps at the
specular angle when the incident angle is not perpendicular,
or in directional measurements by measuring at an angle
different from the specular angle.
specular reflectance included (SCI)
Measurement of the total reflectance from a surface, including
the diffuse and specular reflectances.
standard
A reference against which instrumental measurements are
made.
standard illuminants (CIE)
Known spectral data established by the CIE for four different
types of
light sources. When using tristimulus data to describe a
color, the illuminant must also be defined. These standard
illuminants are used in place of actual measurements of
the light source.
standard observer (CIE)
1) A hypothetical observer having the tristimulus color-mixture
data recommended in 1931 by the CIE for a 2° viewing
angle. A supplementary observer for a larger angle of 10°
was adopted in 1964.
2) The spectral response characteristics of the average
observer defined by the CIE. Two such sets of data are defined,
the 1931 data for the 2° visual field (distance viewing)
and the 1964 data for the annular 10° visual field (approximately
arm's length viewing). By custom, the assumption is made
that if the observer is not specified, the tristimulus data
has been calculated for the 1931, or 2° field observer.
The use of the 1964 data should be specified.
subtractive primaries
Cyan, magenta and yellow. Theoretically, when all three
subtractive primaries are combined at 100% on white paper,
black is produced. When these are combined at varying intensities,
a gamut of different colors is produced. Combining two primaries
at 100% produces an additive primary, either red, green
or blue:
100% cyan + 100% magenta = blue
100% cyan + 100% yellow = green
100% magenta + 100% yellow = red
T, U top
tint
1) verb:To mix white pigment with absorbing (generally chromatic)
colorants.
2) noun:The color produced by mixing white pigment with
absorbing (generally chromatic) colorants. The resulting
mixture is lighter and less saturated than the color without
the white added.
total reflectance
Reflectance of radiant flux reflected at all angles from
the surface, thus including both diffuse and specular reflectances.
transparent
Describes a material that transmits light without diffusion
or scattering.
tristimulus
Of, or consisting of, three stimuli; generally used to describe
components of additive
mixture required to evoke a particular color sensation.
tristimulus colorimeter
An instrument that measures tristimulus values and converts
them to chromaticity components of color.
tristimulus values (CIE)
Percentages of the components in a three-color additive
mixture necessary to match a color; in the CIE system, they
are designated as X, Y and Z. The illuminant and standard
observer color-matching functions used must be designated;
if they are not, the assumption is made that the values
are for the 1931 observer (2° field) and illuminant
C. The values obtained depend on the method of integration
used, the relationship of the nature of the sample and the
instrument design used to measure the reflectance or transmittance.
Tristimulus values are not, therefore, absolute values characteristic
of a sample, but relative values dependent on the method
used to obtain them. Approximations of CIE tristimulus values
may be obtained from measurements made on a tristimulus
colorimeter that gives measurements generally normalized
to 100. These must then be normalized to equivalent CIE
values. The filter measurements should be properly designated
as R, G and B instead of X, Y and Z.
V, W top
V(λ) Function
In order to have also a well defined photometer, an "artificial
eye" has been constructed to simulate the light sensitivity
of the human eye. The relative response of the normal human
eye to monochromatic light at the different spectral frequencies
was determined experimentally by the CIE and standardized
in 1924. This is known as the photopic luminous efficiency
function. The symbol of this function is V(λ) and it is
usually expressed as a function of the wavelength of light
(in air).
value
Indicates the degree of lightness or darkness of a color
in relation to a neutral gray scale. The scale of value
(or V, in the Munsell system of color notation) ranges from
0 for pure black to 10 for pure white. The value scale is
neutral or without hue.
Visible light
is only a small section of electromagnetic radiation which
produces a sensation of brightness and color in the human
eye. Electromagnetic radiation is a form of energy. The
spectrum of such radiation provides information on its energy
composition. The entire spectrum of electromagnetic radiation
ranges from X-ray radiation at the high-energy, short-wave
end to radio waves at the low-energy, long-wave end. Radiometry
is the measurement of optical radiation, which is electromagnetic
radiation within the frequency range between 3×1011
and 3×1016 Hz. This range corresponds to
wavelengths between 0.01 and 1000 micrometers (mm), and
includes the regions commonly called the ultraviolet (UV),
the visible (VIS), and the infrared (IR). Two of the many
typically encountered units are watts/m² and photons/sec-steradian.
X top
X
1) One of the three CIE tristimulus values; the red primary.
2) Spectral color-matching functions of the CIE standard
observer used for calculating the X tristimulus value.
3) One of the CIE chromaticity coordinates calculated as
the fraction of the sum of the three tristimulus values
attributable to the X value.
Y top
Y
1) One of the three CIE tristimulus values, equal to the
luminous reflectance or transmittance; the green primary.
2) Spectral colormatching function of the CIE standard observer
used for calculating Y tristimulus value.
3) One of the CIE chromaticity coordinates calculated as
the fraction of the sum of the three tristimulus values,
attributable to the Y value.
Z top
Z
1) One of the three CIE tristimulus values; the blue primary.
2) Spectral color-matching function of the CIE standard
observer used for calculating the Z tristimulus value.
3) One of the CIE chromaticity coordinates calculated as
the fraction of the sum of the three tristimulus values
attributable to the Z primary.
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