chapter 01 - introduction optical measurement
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Chapter One Introduction
Contents
1. Measurement of Optical Fiber and Optical Components
2. Radiometry and Photometry
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Optical Measurements
Introduction
Early fiber optic systems need only modest test.Now the industryis eolin!" thus optical fibre systems and measurement technolo!y need
to be improed.
Narrow wavelength spacing#
$%M systems with 1&& '()
E.!. power" si!nal*to*noise ratio" waelen!th
(i!h data rates#
+ 1& 'b,s re-uires compatible components characteristic
E.!. spectrum width" dispersion" bandwidth response
Optical amplifier#
Enablin! $%M systems
E.!. !ain" noise fi!ure
uestion
$hy need accurate and reliable optical test / measurement techni-ues0
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Optical Measurements
Introduction
Expansionof optical communication systemsReplacin! copper cables eerywhere" towards access area
Complexfibre optic systems
ll optical networks3 passie and actie
Self-reviewof the basic features of a fiber*optic communication lin4 are necessary.
Fibre optic lin4 measurements determine if the system meets its end desi!n goals.
ll of the components contained within the lin4 must be characteried and specified to
!uarantee system performance.
uestion
$hat are the thin!s to 4now before proceedin! with fiber optic test / measurement0
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Optical Measurements
Introduction
Optical fibres#Singlemode fibres 3 6tandard fibre" %ispersion*shifted fibre" Non*)ero %ispersion*
shifted fibre" Polari)ation Maintainin! fibre" Erbium*doped fibre
Multimodefibres 3 6tep inde7" 'raded*8nde7
Optical components#
!wo-port optical components# hae optical input and optical output" E.!. $%Mcoupler" 9andpass filter" 8solator
Single-portcomponents. E.!. :ransmitter" Receier
:his chapter will briefly introducethe types of measurements that can be made to the fibre
optic and optical components.
:he detailsof each measurement will be discussed in the dedicated chapters.
uestion
$hat are the parameters to measure0
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Measurement of Optical #i$re and !wo-port Components
Insertion %oss
9oth a sourceand receiverare necessary6ource 3 a waelen!th tuna$le laseror a $road$and source
Receier 3 an optical power meter
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Measurement of Optical #i$re and !wo-port Components
Insertion %oss
Optical power meterCali$ratedoptical to electrical conerter
&owaelen!th information
Optical spectrum analy)er
:unable $andpass filter? power meter
uestions
%oes an optical spectrum analy)er proide waelen!th information and why0
(ow to use an OPM but still !ettin! the waelen!th information0
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Measurement of Optical #i$re and !wo-port Components
Insertion %oss
:A6 ? OPMAar!e measurement ran!e" but B 2&&nm
#inewaelen!th resolution
Maor limitation 3 broadband noisefrom :A6
uestions
$hat is the noise referrin! to0
(ow to improe the measurement usin! the :A60
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Measurement of Optical #i$re and !wo-port Components
Insertion %oss
:A6 ? O6'ighestperformance solution
:A6 proides narrowspectral width
O6 proides additional filteringof the broadband noise emission
uestions$hat is the direct effect on the measured spectrum by usin! the aboe confi!uration0
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Measurement of Optical #i$re and !wo-port Components
Insertion %oss
9roadband emission source ? O6(idewaelen!th ran!e coera!e
Moderatemeasurement ran!e
#astmeasurement speed
:un!sten lamp emitters 3 entirefibre*optic communication waelen!th ran!e
Optical amplifiers 3 narrower waelen!th ran!es" but with much higherpower
uestion
$hat is the disadanta!e of a tun!sten lamp source0
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Measurement of Optical #i$re and !wo-port Components
)mplifier *ain and &oise #igure
'ain measurementsOften done in large signalconditions 3 !ain saturation
Re-uires a high-powere7citation source
Characteri)ation of noise
Optical domain3 measure the leel of 6E comin! from the amplifier
Electrical domain 3 use a photodetector and an electrical spectrum analyser tocharacteri)e the total amount of detected noise produced by the system
uestion
$hat is the potential error in the measurement of the amplifier noise0
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Measurement of Optical #i$re and !wo-port Components
)mplifier *ain and &oise #igure
:he fi!ure below shows a test configuration used to measure !ain and noise fi!ure ofoptical amplifier
For $%M systems 3 characteri)ation needs the same si!nal*loadin! conditions as in the
actualapplication
uestion
$hy is there a difference in the optical amplifier characteri)ation between sin!le* and
multi*channel systems0
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Measurement of Optical #i$re and !wo-port Components
Chromatic +ispersion
Measurement is accomplished by analy)in! the group delaythrou!h the fiber,componentsas function of wavelength
Procedure
waelen!th tunable optical source is intensity modulated
:he phase of the detected modulation si!nal is compared to that of the transmitted
modulation
:he waelen!th of the tunable source is then incrementedand the phase comparison
is made a!ain
:he phase delay is conerted into the !roup delay
uestion
$hat is the waeform shape of the modulation si!nal0
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Measurement of Optical #i$re and !wo-port Components
Chromatic +ispersion
:he fi!ure shows the resultfor the measurement of the !roup delay with waelen!th
uestion
(ow can the !roup delay be calculated from the phase delay0
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Measurement of Optical #i$re and !wo-port Components
Chromatic +ispersion
:he fi!ure shows the chromatic dispersion measurement set-upfor two*port optical deices
ccurate characteri)ation of the minimum fibre dispersion waelen!th is important in the
desi!n of high-speed:%M and $%M communication systems
%ispersion compensationcomponents also re-uire accurate measurement of dispersion
uestion
$hy is it important to characteri)e chromatic dispersion of fibre0
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Measurement of Optical #i$re and !wo-port Components
,olariation
Polari)ation of the li!htwae si!nal refers to the orientation of the electric fieldin spaceE.!. insertion loss and !roup delay of a two*port optical component varyas a function of the
input polari)ation
Polari)ation transfer function characteri)ation
,olariation analy)er measures the polariation state
:he polari)ation state is represented by a ones polariation-state vector
ones state ector contains two comple7 numbers that -uantify the amplitude and
phaseof the ertical and hori)ontal components of the optical field
uestion
(ow does the polari)ation state of a linearly polari)ed li!ht eole in a fibre0
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Measurement of Optical #i$re and !wo-port Components
,olariation
:he ones matri7 measurementpply threewell*4nown polari)ation states at the input
Characteri)e the resultin! output polari)ation state in the polariation analyer
:he ones matri7 of the polari)ation transfer function will predict the output polari)ation state
for anyinput polari)ation state
:he fi!ure below illustrates a measurement techni-ue to characteri)e the polari)ationtransfer function of optical fibre and components.
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Measurement of Optical #i$re and !wo-port Components
.eflection
Optical time-domain reflectometry
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Measurement of Optical #i$re and !wo-port Components
.eflection
:he fi!ure shows an e7ample O:%R display
:he locations and ma!nitudes of faults
%etermined by measurin! the arrival timeof the returnin! li!ht
Reduction in .aleigh scatteringand occurrence of #resnelreflection
uestion
(ow to determine the locations and ma!nitudes of faults0
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Measurement of !ransmitter and .eceiver
,ower
:he fi!ure illustrates a basic power*meter instrument dia!ram
Process
6ource 3 optical fibre 3 photodetector 3 electrical current
.esponsivity
:he conversion efficiencybetween the input power and the output current
Gnits of mps,$att
function of wavelengthfor all photodetectors
Must be cali$ratedin order to ma4e optical power measurements
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Measurement of !ransmitter and .eceiver
,ower
:hermal*detector headsMeasure the temperaturerise caused by optical si!nal absorption
Hery accurate and are wavelength-independent
6uffer from poor sensitivity
:hermal detectors are used to cali$ratephotodetectors
Gpper power limit
%etermined by saturationeffects
Responsiity decreasesbeyond this point
Aower power limit
Aimited by the averagingtime of the measurement and the dark current
%esi!n considerations
Power meters hae to be independent of the input polariation
:he reflectivityof the optical head has to be eliminated
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Measurement of !ransmitter and .eceiver
,olariation
Ai!ht sourcesAaser sources are predominantly linear polariedsources
AE%s hae no preferred direction of polari)ation and are predominantly unpolaried
Polari)ation effects
Polari)ation*dependent loss" !ain" or elocity
:hese are influenced by the am$ient conditions" e.!. stress" temperature
:hus" a polari)ed input will perform unpredicta$ly
Polari)ation measurement
:o determine the fraction of the total li!ht power that is polaried
:o determine the orientationof the polari)ed component
uestion
'ies the names for the polari)ation effects0
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Measurement of !ransmitter and .eceiver
,olariation
:he fi!ure illustrates a polariation analyerinstrument
Polari)ation analy)er
Four power meterswith polariation characteriingoptical components
8t measures the Stokes parameters# S&" S1" S2" S
S&3 total powerof the si!nal
S13 power difference between verticaland horiontalpolari)ation components
S23 power difference between /01and -01de!rees linear polari)ation
S3 power difference between right-handand left-handcircular polari)ation
S1and S2are measured with polariersin front of detectors
Sis measured with a waveplatein front of a detector
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Measurement of !ransmitter and .eceiver
,olariation
:he polari)ation state of a source is coneniently isuali)ed usin! a ,oincar2 spherePoincarI sphere
:he a7es are the 6to4es parameters normaliedto S&3 alues are between & and 1
Polari)ation state is represented by the three-dimensional coordinates
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Measurement of !ransmitter and .eceiver
,olariation
:he de!ree of polari)ation
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Measurement of !ransmitter and .eceiver
Optical Spectrum )nalysis
n optical spectrum analy)er
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Measurement of !ransmitter and .eceiver
Optical Spectrum )nalysis
O6Consists of a tuna$le $andpass filterand an optical power meter
:he li!ht from the input fibre is collimatedand applied to the diffraction !ratin!
:he diffraction !ratin! separates the input li!ht into different angles dependin! on
waelen!th
:he li!ht from the !ratin! is then focused onto an output slit:he !ratin! is rotatedto select the waelen!th that reaches the optical detector
uestion
$hat are the components in the O6 that constitute to the tunable bandpass filter0
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Measurement of !ransmitter and .eceiver
Optical Spectrum )nalysis
:he filter bandwidth is determined bythe diameterof the optical beam that is incident on the diffraction !ratin!
the aperturesi)e at the input and output of the optical system
Fabry*Perot
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Measurement of !ransmitter and .eceiver
Optical Spectrum )nalysis
:he fi!ure below shows a spectral plot for a +#3 laserthat is modulated with 2.; 'b,s di!italdata
ccurate spectral measurement
:he O6 must hae a ery narrowpassband and steeps4irts
filter stopband should be K 14 d3down to measure the smaller sidelobes.
O6s do not hae sufficient resolution to loo4 at the detailed structure of a laser
lon!itudinal mode
uestion
$hat determines the alue of the stopband0
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Measurement of !ransmitter and .eceiver
)ccurate (avelength Measurement
:he fi!ure below illustrates a method by which ery accuratewaelen!th measurementscan be made
Michelson interferometer confi!uration
:he li!ht from the un4nown source is splitinto two paths
9oth are then recom$inedat a photodetector
One of the path len!ths is varia$leand the other is fi7ed in len!th
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Measurement of !ransmitter and .eceiver
)ccurate (avelength Measurement
s the ariable arm is moed" the photodetector current aries
:o accurately measure the waelen!th of the un4nown si!nal" a reference laser with a
4nown waelen!th is introduced into the interferometer
uestion
$hy does the photodetector current ary0
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Measurement of !ransmitter and .eceiver
)ccurate (avelength Measurement
:he waelen!th meter comparesthe interference pattern from both lasers to determine thewaelen!th
:his procedure ma4es the measurement method less sensitiveto enironmental chan!es
Reference lasers
(elium*neon
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Measurement of !ransmitter and .eceiver
%inewidth and Chirp Measurement
'eterodyneand homodyneanalysis tools are used to e7amine the fine structure of optical
si!nals
:hese analysis methods allow the measurement of modulatedand unmodulatedspectral
shapes of the lon!itudinal modes in laser transmitter
'eterodyne
:he fi!ure illustrates a heterodyne measurement setup
:he un4nown si!nal is combined with a stable" narrow*linewidth local oscillator
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Measurement of !ransmitter and .eceiver
%inewidth and Chirp Measurement
'eterodyne
:he AO must hae the same polariationfor best conersion efficiency
:he two si!nals mi7 in the photodetector to produce a difference fre-uency
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Measurement of !ransmitter and .eceiver
%inewidth and Chirp Measurement
'omodyne
%imitedinformation on the optical spectrum
Much easierto perform
AO is a time-delayed ersion of itself
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Measurement of !ransmitter and .eceiver
%inewidth and Chirp Measurement
:he fi!ure shows a homodyne measurement of an unmodulated %F9 laser
uestion
$hat is the measured linewidth of the %F9 laser0
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: #re;uency +omain
:his characteri)ation methods display information as a function of the modulation
fre;uency
:he fi!ure shows a dia!ram of a lightwave signal analyer
8t consists of a photodetector followed by a preamplifier and an electrical spectrum
analyer
:he modulation fre-uency response of these components must be accurately cali$rated as
a unit
:his modulation domain si!nal analy)er measures the followin! modulationcharacteristics#
%epth of optical modulation
8ntensity noise
%istortion
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: #re;uency +omain
:he fi!ure shows the power of the modulation si!nal as a function of the modulation
fre;uency3 a %F9 laser modulated at > '()
:he relative intensity noise
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: Stimulus-.esponse Measurement
:he fi!ure shows the instrument for measurin! the modulation response of optical
receiers" transmitters and optical lin4s
Electrical ector analy)er
8ts electrical sourceis connected to the optical transmitter
n optical receier is connected to the input
Compares both the ma!nitude and phase of the electrical si!nals enteringand leavingthe analy)er
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: Stimulus-.esponse Measurement
:he fi!ure shows measurements of a %F9 laser transmitter and an optical receier
Maor challen!es 3 cali$rationof the O,E and E,O conerters in both ma!nitude and phase
response
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: !ime +omain
:he shape of the modulation waveformas it pro!ress throu!h a lin4 is of !reat interest
n oscilloscopedisplays the optical power ersus time" as shown in the fi!ure below
'igh speed sampling oscilloscope
Often used in both telecommunication and data communication systems
%ue to the giga$itper second data rates inoled
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: !ime +omain
:he fi!ures below illustrate eye diagrammeasurement
Eye dia!ram
:he cloc4 waeform is applied to the triggerof the oscilloscope
:he laser output is applied to the inputof the oscilloscope throu!h a calibrated optical
receier
:he display shows all of the di!ital transitions overlaidin time
8t can be used to troubleshoot lin4s that hae poor bit*error ratio performance
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Measurement of !ransmitter and .eceiver
Modulation )nalysis: !ime +omain
8nternational standards such as 6ONE:
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Measurement of !ransmitter and .eceiver
Optical .eflection Measurements
:he fi!ure shows the apparatusto measure the total optical return*loss
Optical return*loss measurement
n optical source is applied to a deice under test throu!h a directional coupler
:he reflected si!nal is separatedfrom the incident si!nal in the directional coupler
9y comparing the forward and reerse si!nal leels" the total optical return*loss is
measured
uestion
$here are the possible reflections0
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Measurement of !ransmitter and .eceiver
Optical .eflection Measurements
:he fi!ure shows the return*loss ersus waelen!th for a pac4a!ed laser usin! a tunable
laser source for e7citation
Aar!e total return*loss
:he locationsof the reflectin! surfaces become important
Re-uires optical time*domain reflectometry
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Measurement of !ransmitter and .eceiver
Optical .eflection Measurements
Optical component characteri)ation re-uires very finedistance resolution in the milimeter to
micron ran!e
:he fi!ure illustrates a hi!h resolution O!+. measurement based on broadband source
interferometry
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Measurement of !ransmitter and .eceiver
Optical .eflection Measurements
(i!h resolution O:%R
Gses a Michelson interferometerand a $road$and light sourceto locate reflections
with 2&Lm accuracy
Constructie interference occurs only when the moable mirror to the directional
coupler distance e;uals the distance from the deice under test reflection to the
directional coupler
:he resolution of the measurement is determined by the spectral width of thebroadband li!ht source
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.adiometry and ,hotometry
Radiometry
:he science of measuringli!ht in any portion of the electroma!netic spectrum" in terms
of a$solutepower
8n practice" the term is usually limited to the measurement of infrared" isible" andultraiolet li!ht usin! optical instruments
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.adiometry and ,hotometry
Photometry
:he science of measurin! isible li!ht in units that are wei!hted accordin! to the
sensitiity of the human eye
8t is a -uantitatie science based on a statistical model of the human visual responseto li!ht * that is" our perception of li!ht * under carefully controlled conditions.
:he standardi)ed model of the eyes response to li!ht as a function of wavelength is!ien by the luminosityfunction.
:he eye has different responses as a function of waelen!th when it is adapted to li!htconditions
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.adiometry and ,hotometry
%ifference
Radiometry includes the entireoptical radiation spectrum" while photometry is limited to
the visi$lespectrum as defined by the response of the eye.
uantities
:here are two parallel systems of -uantities 4nown as photometric and radiometric-uantities.
Eery -uantity in one system has an analogous-uantity in the other system.
:his table !ies the radiometric and photometric -uantities" their usual sym$olsand theirmetric unitdefinitions.
oule" $ watt" lm lumen" m meter" s second" sr steradian
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.adiometry and ,hotometry
Proected area is defined as the rectilinear proection of a surface of any shape onto a planenormalto the unit ector
where is the an!le between the local surface normaland the line of sight
:he radian is the plane anglebetween two radii of a circle that cuts off on the circumferencean arc e-ual in len!th to the radius
uestion
%erie the proected area for the shapes of flat rectan!ular" circular disc and sphere0
. di t d ,h t t
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.adiometry and ,hotometry
One steradian
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.adiometry and ,hotometry
>uantities and ?nits ?sed in .adiometry
Radiometric units can be diided into twoconceptual areas#
:hose hain! to do with poweror energy" and
:hose that are geometricin nature.
Ener!y
8t is an 8nternational 6ystem of Gnits
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.adiometry and ,hotometry
Now" incorporatin! power with the !eometric -uantities areaand solid angle.
8rradiance
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.adiometry and ,hotometry
Radiant intensity
8t is another 68 deried unit and is measured in (@sr.
8ntensity is power per unit solid an!le" d/d. :he symbol is I.
Radiance
8t is the last 68 deried unit we need and is measured in (@m7sr.
8t is power per unit proected area per unit solid an!le" d/ddAcos
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.adiometry and ,hotometry
>uantities and ?nits ?sed in ,hotometry
:hey are basically the same as the radiometric units e7cept that they are wei!hted for the
spectral responseof the human eye
:he symbols used are identical to those radiometric units" e7cept that a subscript isadded to denote visual.
Candela
8t is the luminous intensity" in a !ien direction" of a source that emits monochromaticradiation of fre-uency 104AB4B7hertand that has a radiant intensity in that direction ofB@56 watt per steradian.
:he candela is abbreiated as cd and its symbol is Iv.
. di t d ,h t t
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.adiometry and ,hotometry
Aumen
:he lumen is an 68 deried unit for luminous flux. :he abbreiation is lm and the
symbol is v.
:he lumen is deried from the candela and is the luminous flu7 emitted into unit solidan!le
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.adiometry and ,hotometry
8lluminance
8t is another 68 deried unit which denotes luminous flux density.
:he unit has a special name" the lux" which is lumens per s-uare metre" or lm,m2.
:he symbol is Ev
Auminance
8t is notincluded on the official list of deried 68 units.
8t is analo!ous to radiance" differentiatin! the lumen with respect to both area and
direction.
:his unit also has a special name" the nit" which is cd,m2or lm,m2sr if you prefer.
:he symbol is Lv.
8t is most often used to characteri)e the $rightness of flat emittin! or reflectin!surfaces.
.adiometry and ,hotometry
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.adiometry and ,hotometry
Properties Of :he Eye
:he eye has two !eneral classes of photosensors" conesand rods.
Cones
:he cones are responsible for light-adapted vision they respond to colorand haehigh resolutionin the central fovealre!ion
:he li!ht*adapted relatie spectral response of the eye is called the spectral luminousefficiency functionfor photopic ision" V
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.adiometry and ,hotometry
Rods
:he rods are responsible for dark-adapted vision" with no color information and
poor resolutionwhen compared to the foeal cones.:he dar4*adapted relatie spectral response of the eye is called the spectral luminousefficiency functionfor scotopic ision" V
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.adiometry and ,hotometry
Photopic
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.adiometry and ,hotometry
Conersion 9etween Radiometric and Photometric Gnits
$e 4now from the definition of the candela that there are >D lumens per watt at a fre-uency
of ;5&:()" which is ;;; nm
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.adiometry and ,hotometry
8n order to conert a source with non-monochromatic spectral distribution to a luminous-uantity" the spectral natureof the source is re-uired.
:he e-uation used is in a form of#
where Xv is a luminous term" Xis the correspondin! spectral radiant term" and V
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