understand units of radiometric measurement convert between different radiometric units

8/14/2019 Understand Units of Radiometric Measurement Convert Between Different Radiometric Units

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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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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



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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


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



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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



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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



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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


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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Spatial gradients

Spectral gradients

Relatively weak


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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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Spatial gradients

Spectral gradients

Relatively weak


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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



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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.



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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.



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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



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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