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RadiometryRadiometry
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GOALS
understand units of radiometric measurement
convert between different radiometric units
choose appropriate light measurement equipment
walk safely through the minefield of LED descriptions
select the appropriate light source for a particular purpose
specify light in conventional, understandable terms
understand the basics of the radiometry of materials
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Light is an electromagnetic wave just like:
What is light?
Ultraviolet (~100 to ~350 nm)Visible (~350 to ~830 nm)
Infrared (~830 nm to 20+ microns)
X-rays (very short wavelength)
Microwaves (cm wavelength)Radio waves (meters to kilometers)
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Radiometry
Ultraviolet (~100 to ~350 nm)Visible (~350 to ~ 830 nm)
Infrared (~830 nm to 20+ microns)
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Baseball quiz
a. The manager leaves a pitcher in the game long after hehas become too tired. The pitcher allows 8 runs in asingle inning.
b. On a ground ball, the shortstop throws to first base in
time to get the batter, but the first baseman drops the ball.The batter is safe.
c. With bases loaded and no outs, the batter hits an easyground ball to the pitcher. Instead of throwing home andgetting the lead runner, the pitcher throws to first base.The batter is out, but the runner from third scores.
d. All of the above.
In baseball, which of the following is correctly called anerror?
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Baseball quiz
a. The manager leaves a pitcher in the game long after hehas become too tired. The pitcher allows 8 runs in asingle inning.
b. On a ground ball, the shortstop throws to first base in
time to get the batter, but the first baseman drops the ball.The batter is safe.
c. With bases loaded and no outs, the batter hits an easyground ball to the pitcher. Instead of throwing home andgetting the lead runner, the pitcher throws to first base.The batter is out, but the runner from third scores.
d. All of the above.
In baseball, which of the following is correctly called anerror?
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GEOMETRY
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FLUX (Watts)
Flux is the total power emitted in all directions.
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Irradiance (watts/m2) E
Irradiance is the flux per unit area striking a surface.
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Irradiance (watts/m2) E
Irradiance says nothing about the angle of incidence.
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Plane angle
radians
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Solid angle
steradians
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Solid angle
Ice cream cone 0.1sr
Egyptian pyramid 1sr
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Intensity (watts/sr) I
Intensity is the flux per unit solid angle from a source.
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Radiance (watts/m2sr) L
Radiance is the flux per unit solid angle, per unit
projected area from an extended source.
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SPECTRUM
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SPECTRORADIOMETRIC FLUX(Watts/nm)
(also E, I, L)
Power (or irradiance, intensity, or radiance) per
wavelength interval.
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250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000
0
0.02
0.04
0.06
0.08
Wavelength (nm)
Spectralflux(W/nm)
100 W Blackbody3000K temperature
Heated source Black body
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There are16.3 watts inthe band
from 750nmto 1000nm.
d=
2
1Total flux (watts)
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For sources that are limited to a
specific small wavelength band, thetotal power in the source is generally
the appropriate quantity for total flux.
d=
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Sometimes, for very spectrally narrowsources such as lasers and other line
sources, it is common to speak of the power
at a particular wavelength.
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PHOTOMETRYHuman visual response
Includes:
Luminous flux (illuminance, intensity, luminance)
Color
Color temperature
Color rendering
Luminous efficacy
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Luminous Flux (lumens) is an engineering
representation of the visual response to opticalpower.
350 400 450 500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
1.2
wavelength (nm)
Luminousefficiency
Luminous efficiency, V
dVLumens = )()(683
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Conversion Factors
1 footcandle (lumen / ft2)
=10.764 lux (lumen / m2)
1 footlambert (candela / ft2)
=
3.426 nit (candela / m2)
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COLOR
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Chromaticity is an engineering representation of the humanperception of color.
350 400 450 500 550 600 650 700 750
0
0.5
1
1.5
2
y-bar
x-bar
x-bar
z-bar
w avelength (nm)
Standard colorimetric observer functions
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dxX = )()(
dyY = )()(
dzZ = )()(
ZYXXx++
=
ZYXYy++
=
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BLUE
WHITE
GREEN
YELLOW
ORANGE
RED
PURPLE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x,y chromaticity diagram
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x,y chromaticity diagram
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BLUE
WHITE
GREEN
YELLOW
ORANGE
RED
PURPLE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x,y chromaticity diagram
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BLUE
GREEN
YELLOW
ORANGE
RED
PURPLE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x,y chromaticity diagram
Mixing light of any two points on this diagram produces light that
lies along the straight line between them.
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1800K3000K6500K
500 1000 1500 2000 2500
Wavelength (nm)
Black Body Spectra
BLUE
GREEN
YELLOW
ORANGE
RED
PURPLE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x,y chromaticity diagram
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0 0.2 0.4 0.6 0.8
0
0.2
0.4
0.6
0.8
1
BLUE
WHITE
GREEN
YELLOW
ORANGE
RED
PURPLE
BLACK BOIES
x
y
x,y chromaticity diagram
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COLOR TEMPERATURE
Any light source that has the same chromaticity
coordinates as a black body can be described ashaving the color temperature of that black body.
The terms color temperature and black body
temperature are not synonymous. Color temperature
is derived from colorimetric calculations. There are
limitless different spectra that possess a particular
color temperature and have little or no resemblance tothe blackbody curve for that temperature.
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CORRELATED COLOR TEMPERATURE
The correlated color temperature of a light source is
the color temperature of the point on the black body
locus that is closest to the chromaticity coordinatesof the light source.
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0 0.2 0.4 0.6 0.8
0
0.2
0.4
0.6
0.8
1
BLUE
GREEN
YELLOW
ORANGE
RED
PURPLE
x
y
x,y chromaticity diagram
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COLOR
Rendering
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Color rendering describes how well aparticular light source displays
surface colors.
Color rendering index is an numerical
representation of color rendering.
Color rendering & color rendering index
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SOURCE, ()
REFLECTANCE
FACTOR, R()
STIMULUS, ()R()
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SOURCE,
1()
REFLECTANCE
FACTOR, R()
STIMULUS,
1()R()
REFLECTANCE
FACTOR, R()
SOURCE,
2()
STIMULUS,
2()R()
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COLOR RENDERING INDEX
Compares the light source with a reference
illuminant, which is a blackbody or sunlightsource that has the same correlated color
temperature as the source.
Computes the average chromaticity differenceon 8 reference surfaces between the actual
source and the reference illuminant.
Produces a number that is difficult to interpret.
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LUMINOUS
EFFICACY
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Measures the effectiveness of a lightsource in producing luminous flux
from physical power.
Usually, the physical power that we
care about is the power from the
wall plug.
LUMINOUS EFFICACY
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MAKINGRADIOMETRIC
MEASUREMENTS
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GEOMETRY
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DETECTOR
INTEGRATING
SPHERE
FLUX
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Calibrated
baredetector
Calibrated
detector with
diffuser
Irradiance
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Irradiance detector
d
Intensity
I=E x d2
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Spot meter
Radiance
Aperture defines
solid angle
Detector
definesprojected
area
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SPECTRUM
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The spectral character of a source is measured
with a device called a spectroradiometer, which
can be outfitted with input optics to measure
spectral flux, spectral irradiance, or spectralradiance.
SPECTRORADIOMETRY
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1. For narrowband sources, the responsivity of the detector is nearly
constant over the band of the source, so a radiometer can be calibrated to
read watts directly.
2. Also, some detectors, especially thermal detectors, have a responsivity that
doesnt vary with wavelength. These detectors read directly in watts, even for
broadband sources.
Total Flux
3. Its possible to design a filter that, in combination with a detector, provides
a flat response over a specified spectral range. Such a filter-detector
combination, can be calibrated to read directly in watts for radiation in that
band.
4. Alternately, the total flux can be calculated numerically from
spectroradiometric measurements.
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Luminous Flux
1. Radiometers that measure luminous flux (called photometers) areusually equipped with a glass or plastic filter in front of the detector. The
combination of the filter and the detector provides a spectral response that
approximates the shape of the luminous efficiency curve.
2. Alternately, the luminous flux can be calculated numerically from
spectroradiometric measurements.
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1. The color of a light source can be measured with a colorimeter,
which consists of 3 (or 4) detectors that are filtered that approximatethe spectral response of the standard colorimetric observer
functions.
2. Alternately, the chromaticity can be calculated numericallyfrom spectroradiometric measurements.
COLOR
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COLOR TEMPERATURE
The color temperature of a light source can be
calculated numerically from the chromaticity.
For sources that have the spectral shape of a blackbody (such as tungsten lamps), the color
temperature can be measured with a calibrated pair
of detectors (red/blue).
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Color Rendering Index
Color rendering index is calculated from
the spectral power distribution of a
source.
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LEDsLEDs
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Single numberdescriptors for
LEDs
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The angular distance between 50%
points on an intensity plot
Viewing angle
GEOMETRY
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Viewing angle
Typical LED spectra
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300 350 400 450 500 550 600 650 700 750 800
BLUE GREEN RED
w avelength (nm)
spectrum(1/nm)
LEDs
Typical LED spectra
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The wavelength of the maximum
spectral density
Peak wavelength
SPECTRA
The mean wavelength
Centroid wavelength
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SPECTRA
The wavelength halfway between the
half peak points
Center wavelength
All derived from a wavelength plot
Full width, half max (FWHM)
The distance between the half peak
points
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
y
x,y chromaticity diagram
LED
white point
dominant
wavelength
Purity is the
relative distanceof the LED from
the white point to
the spectrum
locus.
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BLUE LED Spectrum with Characteristic Wavelengths
0.0
0.5
1.0
430 450 470 490 510 530
wavelength - nm
dominantcentroidpeak
RED LED Spectrum with Characteristic Wavelengths
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RED LED Spectrum with Characteristic Wavelengths
0.0
0.5
1.0
600 620 640 660 680 700
wavelength - nm
dominant centroidpeak
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Radiometry of LEDsRadiometry of LEDs
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An LED is just a light source
BUT
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Why are LEDs different?Sharp gradients
Spatial gradients
Spectral gradients
Relatively weak
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LED Measurements
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Why are LEDs different?Sharp gradients
Spatial gradients
Spectral gradients
Relatively weak
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Why are LEDs different?Sharp gradients
Spatial gradients
Spectral gradients
Relatively weak
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GEOMETRY
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IntensitySignal LEDs viewing the LED directly
Direct illumination flashlight
Control IREDs TV remote control
Traffic signalsTail lights
Lighted signs
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Irradiance detector
d
Intensity
I=E x d2
source location
is knowndetector is in1/r2 region
irradiance is
uniform
across face
of detector
detector is in the
correct direction
from the source
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AXES
optical
peak
mechanical
Problems
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It is difficult to measure true intensity.
It is difficult for two labs to get the same
measurement on the same LED.
Solution
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CIE publication 127 (1997) defines a
new term, Averaged LED Intensity
CIE 127 completely defines twomeasurement geometries, Condition A
and Condition B
Averaged LED intensity according
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to CIE conditions A or B
mechanical axis
10 cm Condition B (0.01 sr)
31.6 cm Condition A (0.001 sr)
LED detector
Circular aperture of
area, A=100 mm2
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FLUX
Backlighting
Room illumination
LED development
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Correct Desired
Most LEDs have significant back flux.
CIE and flux
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CIE has not yet published a
recommendation for flux measurementthat addresses these concerns.
A recommendation is probably one to
two years away.
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Why are LEDs different?Sharp gradients
Spatial gradients
Spectral gradients
Relatively weak
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Why are LEDs different?Sharp gradients
Spatial gradients
Spectral gradients
Relatively weakRelatively weak
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SPECTRUM
Typical LED spectra
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300 350 400 450 500 550 600 650 700 750 800
BLUE GREEN RED
w avelength (nm)
spectrum
(1/nm)
LEDs
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Conventional sources
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SUNLIGHT
TUNGSTE
N
LAMP
300 350 400 450 500 550 600 650 700 750 800
w avelength (nm)
spectrum
(1/nm)
Sunlight and tungsten lampCIE illuminants D65 and A
Conventional sources
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300 350 400 450 500 550 600 650 700 750 800
w avelemgth (nm)
spectrum
(1/nm)
Fluorescent lamp38WT8/750
Data courtesy of Osram Sylvania , Inc.
Conventional sources
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350 400 450 500 550 600 650 700 750 800
w avelength (nm)
spectrum(1/nm)
High pressure sodium lamp400 Watt
Data courtesy of Osram Sylvan ia, Inc.
Conventional sources
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350 400 450 500 550 600 650 700 750 800
w avelength (nm)
spectrum
(1/nm)
Metal halide lamp100 Watt
Data courtesy of Osram Sylvania, Inc.
Typical LED spectra
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300 350 400 450 500 550 600 650 700 750 800
BLUE GREEN RED
w avelength (nm)
spectrum
(1/nm)
LEDs
LED and photopic filter
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0
0.2
0.4
0.6
0.8
1
1.2
300 400 500 600 700 800
Photopic efficiency Achieved photopic filter Blue LED
Spectrograph bandpass model
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0
1
-20 -10 0 10 20wavelength - nm
bandpass
in FWHM
BP
center
Conventional sources
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300 350 400 450 500 550 600 650 700 750 800
w avelemgth (nm)
spectrum
(1/nm)
Fluorescent lamp38WT8/750
Data courtesy of Osram Sylvania , Inc.
Effect of Spectrometer Bandpass on
measured LED Dominant Wavelength
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-2
-1
0
1
2
400 500 600 700
wavelength - nm
LED FWHM = 20 nm
step = 1nm, 2nm, 5nm
step = 10nm
bandpass
20nm
10nm
5nm
1nm
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RADIOMETRIC
PROPERTIESOF
MATERIALS
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AB
C
Transmittance = B/A
Reflectance = C/A
Absorptance = (A-B-C)/A
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Bidirectional reflection
distribution function
BRDF = L/E
Lambertian
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cosI
Reflectance Factor
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PRD
A A
B C
100% reflectance
Lambertian
R=B/C R=BRDF x
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COLOR OF
MATERIALS
SOURCE, ()
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( )
REFLECTANCE
FACTOR, R()
STIMULUS, ()R()
Standard colorimetric observer functions
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350 400 450 500 550 600 650 700 750
0
0.5
1
1.5
2
y-bar
x-bar
x-bar
z-bar
w avelength (nm)
dxRKX = )()()( Y
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dyRKY = )()()(
dzRKZ = )()()(
dyK = )()(
100ZYX
Xx
++=
ZYX
Yy
++=
dX )()(X
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dxX = )()(
dyY = )()(
dzZ = )()(
ZYXx
++=
ZYX
Yy
++=
dxRKX = )()()( Y
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dyRKY = )()()(
dzRKZ = )()()(
dyK = )()(
100ZYX
Xx
++=
ZYXy
++=
x andy contain the chromaticity information.
Ycontains the brightness information.
A particular material will have different
chromaticity, depending on the spectrum of
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y, p g p
the source.
It is possible for two different materials to have
the same chromaticity and the same perceived
color under a particular source spectrum. Such
pairs of materials are called metamers. They
will generally have different chromaticity and
perceived colors under another source
spectrum.
Other coordinate systems
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u, v, Y- approximately uniform two dimensional
color space
L*, u*,v* andL*, a*, b* - approximately uniformthree dimensional color spaces.
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TYPICAL VALUES
Irradiance and Illuminance
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Direct sunlight 1,000 w/m2
Direct sunlight 100,000 lux (lumen/m2
)Shade 10,000 lux
Overcast day 1,000 luxOffice space 300 - 600 lux
Full moon 0.2 lux
LUMINOUS INTENSITY
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Automobile headlight 5000 10,000 cd
Household flashlight 100 1,000 cd
LED 1 mcd 5 cd
RADIANCE and LUMINANCE
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Sun 2 x 107 w/(m2 sr)
Sun 2 x 109 nit (cd/m2)
Frosted light bulb 100,000 nit
Fluorescent lamp 5,000 nit
Computer screen 100 nit
Ensure that your radiometer actually measures
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Ensure that your radiometer actually measures
the quantity that you intend to measure.
Ensure that the quantity that you intend to
measure is the quantity that you reallyneed to measure.
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DOCUMENTS
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CIE Publication 13.3 (1995) Method of Measuring
and Specifying Color Rendering of Sources
CIE Publication 15.2 (1986) Colorimetry, 2nd Edition
www.cie.co.at/cie/index.html
CORM Directory of Reference Documents on
Photometry 2001 www.CORM.org