3. Making light work of light MeasureMent
About uDt instruments
hiStory The early begnnngs of UDT Instruments can be traced to 1967 when
a small group of nventors at Unted Detector Technology (UDT)
began manufacturng the frst commercally avalable transmpedance
amplfers for planar-dffused and Schottky barrer slcon photosen-
sors . Over the next several years, ths same group of people went on
to poneer leadng-edge technologcal nnovatons for photometers,
radometers, fber-optc power meters and optcal poston-sensng
nstruments . By the early 1980‘s, ths hghly sklled and successful
group grew nto an autonomous entty known as UDT Instruments .
Drawng on the momentum generated by UDT‘s precson photomet-
rc nstruments,the company developed an nventve handheld color-
meter for the growng televson and computer perpherals markets .
The development of UDT‘s SLS9400 colormeter promses to strength-
en our company‘s poston as a leader n precson electro-optcs
nstrumentaton, whle meetng the strngent demands of a multtude
of CRT calbraton requrements . UDT s posed and ready to excel to
greater technologcal excellence wth only one goal n mnd: to meet
and exceed the ever-changng needs of ts customers worldwde .
ServiCe We at UDT Instruments stand behnd our products and the companes
who use them . For ths reason, we contnue to servce those same
lght-measurng nstruments that we bult twenty years ago . By offer-
ng these servces to our customers, both new and establshed, we
stay nvolved wth our products and extend a personal touch to our
busness relatonshps . We know of no other company n our ndustry
that hres more qualfed sales engneers, people who really under-
stand lght measurement prncples and practces . By hrng such
knowledgeable engneers, we ensure you that you wll get the best
electro-optc nstruments to ft your applcaton and budget .
QuAlity The nstrument you receve s certan to be relable and accurate . We
mantan a Qualty program that affects every ndcator module, sen-
sor head, and optcal accessory we sell . And when t comes tme for
re-calbraton, upgrades, or repars, you’ll dscover that our servce and
metrology departments reflect ths same commtment to qualty and
personalzed servce .
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4. Making light work of light MeasureMent
About uDt instruments
teChnoloGy UDT Instruments has always been and contnues to be at the forefront
of lght measurement technology . We hold U .S . and worldwde pat-
ents on our QED products, whch are absolute radometrc reference
standards n the vsble and near IR spectrum . Our QED-200 product
won a prestgous IR-100 award as one of the 100 most sgnfcant
U .S . nventons n 1986 . These products were developed n conjunc-
ton wth the Natonal Insttute of Standards Technology (NIST) and
the Natonal Physcal Laboratory (NPL) . UDT Instruments contnues to
work wth the NIST under Cooperatve Research And Development
Agreements (CRADA) n order to develop even more state-of-the-art
products nto the 21st Century .
PubliCAtionS In addton to our comprehensve Gude To tutoral seres, UDT regu-
larly publshes artcles n trade journals and other scentfc lterature
whch we've made avalable as applcaton notes to explan subtle
detals and applcatons of our technology .
ProfeSSionAl UDT s commtted to supportng the ndustry through ts professonal
socety afflates . We are proud to be sustanng members of:
SoCietieS • Socety of Photo Optcal Instrumentaton Engneers (SPIE)
• Optcal Socety of Amerca (OSA)
• Natonal Assocaton of Broadcasters (NAB)
• Laser Insttute of Amerca (LIA)
• Illumnatng Engneerng Socety of Amerca (IES)
• Socety For Informaton Dsplay (SID)
UDT also actvely partcpates n the Councl for Optcal Radaton
Measurement (CORM) and the Commsson Internatonale l'Eclarage
(CIE) .
WArrAnty UDT Instruments warrants that ts products are free from defects
n materal and workmanshp under normal use and servce for a
perod of one year from the date of shpment from our factory . UDT
Instruments‘s oblgaton under ths warranty s lmted to the replace-
ment or repar of any product determned to be defectve durng the
warranty perod, provded the product s returned to the factory pre-
pad . Ths warranty does not apply to any equpment that has been
repared or altered, except by UDT Instruments, or whch has been
subject to msuse, neglgence, or accdents . It s expressly agreed
that ths warranty wll be n leu of all warranty of merchantablty . No
other warranty s expressed or mpled . UDT Instruments s not lable
for consequental damages .
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5. Making light work of light MeasureMent
Application information
introDuCtion
Photometry s the scence concerned wth
measurng human vsual response to lght .
Because the eye s a hghly complex
Green
organ, ths s by no means a smple task . It
nvolves the meetng of many dscplnes:
Relative Response
UV IR psychology, physology, and physcs
among them .
Photometry can be sad to have become
Blue Red a modern scence n 1924, when the
Commsson Internatonale de l‘Eclarage
(CIE) met to defne the response of the
average human eye . The Commsson
measured the lght-adapted eyes of a sz-
Wavelength in Nanometers able sample group, and compled the data
Cie phototopic response curve. nto the photopc curve . Smply stated,
the curve reveals that people respond
strongest to the color green, and are less
senstve to the spectral extremes, red and
volet .
Scotopic
Vision The eye has an altogether dfferent
Relative Response
response n the dark-adapted state, where-
n t also has dffculty determnng color .
Ths gave rse to a second set of measure-
ments, and the scotopc curve .
Yellow
Havng defned the eye‘s spectral response,
Orange
CIE sought a standard lght source to serve
Green
as a yardstck for lumnous ntensty . The
Violet
Red
Blue
frst source was a specfc type of candle,
gvng rse to the terms footcandle and
Wavelength in Nanometers candlepower . In an effort to mprove
repeatablty, the standard was redefned
Cie scotopic response curve.
n 1948 as the amount of lght emtted
from a gven quantty of meltng platnum .
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6. Making light work of light MeasureMent
Application information
bASiC ConCePtS
The basc unt of photometry s the lumen, whch Scannng can be accomplshed wth dscrete-
s related to ts radometrc analog, the Watt, by: wavelength, scannng monochromators, or
lm = 683 x W x Vλ mult-channel detectors . In ether case, the
ntensty of a lght source s measured wave-
Where Vλ s the relatve lumnosty, a coeffcent
length-by-wavelength, and then the results are
scaled to vsual response . Unty occurs at the
mathematcally ftted to the photopc curve .
eye‘s peak response wavelength, 555 nanome-
For ths reason, such technques do not occur n
ters .
real tme, and requre mcroprocessor control .
Two useful laws n photometry recur: the nverse Scannng approaches offer hgh accuracy, but
square law and the cosne law . The frst defnes tend to be costly, and complex to operate .
the relatonshp between llumnaton from a
Optcal flterng offers a smple and cost-effec-
constant-ntensty lght source and ts dstance
tve soluton . Wth only one photo-current sgnal
from a surface . It states that the ntensty per
to process, sngle-channel electroncs can be
unt-area on the surface, vares n nverse propor-
used . Also, recent advances n flter desgn, and
ton to the square of the dstance between the
mprovements n sold-state detectors, allow ths
source and surface, or:
method to rval scannng systems for photomet-
∆lm/M2 α 1/∆d2 rc accuracy .
Accordngly, successve llumnance measure-
ments are only as accurate as the control of
source to surface dstance . Further, f llumnance
s known at one dstance, t can, barrng nterfer-
ence, be calculated for any dstance .
The cosne law ndcates the ntensty of lght on
a surface of fxed area, vares wth ncdent angle .
In fact, the ntensty falls off as the cosne of the
angle . Ths results because the projected surface
area, n the plane perpendcular to ncdence, s
proportonally reduced .
Thus n measurements of envronmental lghtng,
sensors requre cosne correcton to account for
off-axs lght . Wthout t, consderable errors wll
occur, especally wth brght sources at low nc-
dent angles (e .g ., wndows) . Ths often accounts
for the dfference n readngs between two pho-
tometers .
The cardnal challenge n photometry s to recre-
ate the spectral response of the human eye . But
electronc sensors have dstnct response char-
acterstcs whch bear no resemblance to the CIE
standard observer . Therefore, these sensors must
be spectrally corrected . Two technques are con-
ventonally used to accomplsh ths: wavelength
scannng, and detector/flter matchng .
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8. Making light work of light MeasureMent
Application information
bASiC ConCePtS
Ths flter-matchng technque nvolves the
To Surface layerng of colored-glass flters over an optcal
detector . Each element functons to attenu-
θ ate selectve wavelengths untl the detector‘s
Effective Area presented response smulates the CIE curve . Planar df-
to Incident Flux is fused slcon photododes offer the best pho-
Measurement Area x Cos φ
tosensor characterstcs, snce they afford hgh
senstvty and lnearty throughout the vsble
spectrum . Usng slcon photodetectors, and
Irradiation = Area x Cos φ
x Incident Flux advanced flter desgns, UDT Instruments
matches the CIE human eye response curve
wthn 1% total area error . Ths s the best
the intensity of off-axis light decreases
match achevable, accordng to CIE .
relative to the cosine of incident light.
There s another more mportant specfcaton
of the qualty of a photometrc detector and
that s the f₁, value . Ths s defned by the CIE
and s a numercal value assgned to the aver-
age devaton of the photometrc detector‘s
response from the CIE curve . An f₁, 1 .5% s
the best possble laboratory grade detector
whle an f₁, 3% s consdered sutable for
most applcatons .
However, the relatonshp between a gven
detector and flter s delcate . Once the two
have been matched, they should not be
nterchanged wth other photometrc detec-
tor/flter pars . Each detector exhbts unque
response characterstcs that requre a specfc
combnaton of flter layers and thcknesses .
the typical spectral response of silicon
photodetectors.
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9. Making light work of light MeasureMent
Application information
bASiC ConCePtS
PHOTOMETRIC FILTER RESPONSE Once the detector‘s response s fxed, t s
calbrated usng the transfer of standards
CIE Response curve technque . Ths requres a detector of known
response, whch can be obtaned from the
Relative Response
Sensor Design Shape
Natonal Insttute of Scence and Technology
(NIST) . A detector/flter par s postoned
before an optcal source wth constant wave-
length and ntensty characterstcs (usually a
tungsten halogen lamp) . The electrcal output
of the detector under test s then compared to
the standard detector‘s output .
Once the sensor‘s lumnous response s deter-
Wavelength in Nanometers mned, t can be matched to a precson gan-
uDt instruments photometric filters match controlled electronc amplfer and readout
the Cie curve to within 1% total area error. system .
Calibration by transfer of Standards
Rt = Responsvty of the test detector (A/lm)
Rr = Responsvty of the reference detector (A/lm)
It = Measurement of the test detector (A)
Ir = Measurement of the reference detector (A)
( )
A A
Rt lm = Rr lm ( )( ) lt ( A)
lr ( A)
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10. Making light work of light MeasureMent
Application information
imPortAnt termS
luminous flux
Lumnous flux s expressed n lumens, the fun-
Detector
damental unt of photometry . It s a measure
of the total optcal output of a vsble lght
source .
The measurement requres all of a source‘s
power to be concentrated on a detector . Ths
Detector can be a problem wth dvergent sources lke
LEDs, and lamps . In these cases, ntegratng
spheres are often used .
illuminance
Illumnance s a measure of the amount of vs-
in illuminance measurements, area ble lght ncdent upon a prescrbed surface
area . In Englsh unts, one lumen of flux fallng
is determined by the detector unless
on one square foot s termed a footcandle .
there is an external aperture. The metrc equvalent, one lumen per square
meter, s called a lux (10 .76 lux = 1 footcan-
dle) .
Of course, detectors don‘t have such large areas . So the area of the detector s multpled proporton-
ally . Specal attenton s due when the detector s under-flled or used behnd correctve optcs, snce
the sensor‘s area no longer defnes the surface beng llumnated .
For example, llumnance measurements are partcularly susceptble to errors ntroduced by off-axs
lght . So cosne-correctng dffusers are used wth the detector head . Snce the cosne dffuser s
essentally maged onto the sensor, the dffuser‘s area, not the sensor‘s, represents the measurement
surface .
Photometric Quantities and units
Quantity Symbol Units Abbreviations
Luminous energy Q lumen•second…talbot lm•s…talbot
Luminous Density U lumen•second/m3 lm•s/m3
Luminous Flux F lumen lm
Illuminance E lumen/m2…lux lm/m2…lx
lumen/cm2…phot lm/cm2…ph
lumen/ft2…footcandle lm/ft2…fc
Luminous Exitance M same units as illuminance
Luminance (brightness) L candela/m2…nit cd/m2…nt
candela/cm2…stilb cd/cm2…sb
candela/π ft2…footlambert cd/π ft2…fl
candela/π m2…apostilb cd/π m2…asb
candela/π cm2…lambert cd/π cm2…L
Luminance intensity Iu lumen/steradian…candela lm/st…cd
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11. Making light work of light MeasureMent
Application information
imPortAnt termS
luminous exitance
Lumnous extance s an ntrnsc property of
a lght source . It s calculated by measurng
lumnous flux (lumens), and dvdng by the
surface area of the source . Ths measurement
s also expressed n lumens per square meter,
but s not to be confused wth llumnance
measurements or lux . The area referred to n
lumnous extance s that of the lght source,
not the llumnated surface . Ths measure-
ment s most applcable to emtters wth flat
surfaces .
luminous exitance is calculated by
measuring luminous flux and divid-
ing by the source‘s area.
Detector
luminous intensity
Lumnous ntensty s also a source
property, but one where the source‘s
LED drecton and dvergence come nto
Ω
play . Defned as the quantty of lum-
nous flux emtted unformly nto a
sold angle, the basc unt of lumnous
ntensty s the candela, equal to one
lumen per steradan .
Several thngs are suggested by ths
defnton . One, ths measurement
luminous intensity is a measure of the flux s not applcable to collmated lght
emitted into a solid angle. sources . Two, t s naccurate for non-
unform emtters .
To calculate lumnous ntensty, the
detector‘s area (or the area prescrbed by the
aperture n front of t), and ts dstance from the
lght source must be known . From these, the
sold angle can be calculated, and then dvded
nto the flux readng .
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12. Making light work of light MeasureMent
Application information
imPortAnt termS
Measurement Detector
Plane
one luminous measurement technique involves
fixing the detector‘s field-of-view through the use
of a lens.
luminance luminous energy
Also known as photometrc brghtness, lum- Lumnous energy s a measure of the rate of flow
nance s a measure of the flux reflected by, or of flux, and so s expressed n lumen-seconds .
emtted from, a relatvely flat and unform sur- Generally, t s appled to flashed or pulsed sourc-
face . The technque takes nto account the area of es .
the surface measured, and the angle subtended It s also possble to measure any photometrc
by an observer lookng at t . quantty on a tme-dependent bass . For nstance,
Lumnance may be thought of as lumnous the llumnance of a rotatng beacon n one drec-
ntensty per unt area, and so n metrc terms s ton could be ntegrated over tme to yeld foot-
expressed as candle-seconds .
candelas per square meter . But a host of other
terms are used for ths measurement, some to
descrbe a
crcular measurement area rather than a square
one (see Photometrc Quanttes and Unts chart) .
To measure lumnance, the detector feld-of-
vew must be restrcted, and ts angle calculated .
Usually, a lens or baffle s used to acheve ths . In
fact, the human eye, wth ts lens and aperture,
functons as a lumnance meter .
Note that so long as the detector‘s feld-of-vew
s flled, ths measurement s ndependent of the
dstance between the detector and measurement
planes . That‘s because feld sze and source nten-
sty vary n drect proporton to one another as a
functon of dstance .
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13. Making light work of light MeasureMent
Application information
hoW to SPeCify A Photometer SyStem
Specfyng a photometer system s best
Photopic Filter approached n three steps . Frst, evaluate the
Luminance Lens
Detector source to determne whch measurement
technque best apples . Then, select a detec-
tor and optcal system (detector head) that
sut the measurement . And fnally, match the
detector head to the partcular electroncs
whch provde the most effectve user nter-
Photometer
face for the applcaton .
Crts and other displays are typically Consider the Source
measured in terms of luminance. Common sense goes a long way n determn-
ng the rght measurement for an applcaton .
After all, photometry s concerned wth the
relaton of lght to the human eye . So, the frst
queston s: how wll people be affected by the
source to be measured?
For nstance, measurements of ambent or
envronmental lghtng are concerned wth
Integrating Sphere people‘s ablty to read prnt or safely see
objects n an area . It s not the power of a
partcular source that s of concern, but rather
how well the source lghts the area of nter-
est . For ths reason, lghtng for the outdoors,
offces, factores, and photography are mea-
LED sured n terms of llumnance .
Filter
However, f n the same room or space one
wshed to determne the brghtness of walls,
Detector fabrc, or panted surfaces, the measurement
changes altogether . Because now the amount
Photometer of reflected lght receved by the eye s of con-
cern . Snce all of these surfaces are dffuse and
relatvely unform, a lumnance measurement
integrating spheres are the most accu- would best apply .
rate means of measuring small, divergent
sources like leDs.
SEE ALSO: The Guide to Photometer
Radiometer System Configuration,
available as a free PDF download at
www.udtinstruments.com
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14. Making light work of light MeasureMent
Application information
hoW to SPeCify A Photometer SyStem
entfc applcatons . But when ther potental
damage to the eye s of concern, they would
probably be measured for lumnous flux . A
lensed LED, however, s a dvergent, though
drectonal, source . Lumnous ntensty would
best characterze t . But wth surface or edge
emttng LEDs, emsson as a functon of sur-
face area s sgnfcant . Ths descrbes a lum-
nous extance measurement .
Lumnous energy measurements apply to any
perodc source . Pulsed LEDs, photographc
flash unts, strobe lghts, arc lamp systems,
for luminance measurements requiring and rotatng or scannng lghts are sev-
eral examples of sources whose flux s tme
small fields-of-view, a lens system with
dependent .
view-through optics is essential.
Selecting the right detector head
Electronc dsplays such as CRTs, avoncs, and The measurement type dctates your choce
automotve panels are ncdent drectly upon the of detector head assembles .
eye too . But alpha-numerc characters and lne
UDT Instruments offers a modular photometrc
detal are generally small . So the measurement
sensor-head desgn approach . In all cases, a sl-
system‘s feld-of-vew must be lmted or focused
con photodetector, detector housng, and photo-
n order to measure only the lghted portons
metrc flter assembly are provded . And for those
of the dsplay . Ths s, by defnton, a lumnance
lumnous flux measurements where all ncdent
measurement . So dsplay brghtness s usually
lght s collmated or focused onto the detector,
specfed n footlamberts .
ths smple head wll suffce .
Lamps are used n so many applcatons that t
However, f flux levels exceed 70 lumens per
s mpossble to defne just one way to measure
square centmeter, the detector may become
them . As prevously mentoned, lamps and lamp
saturated, and ts output nonlnear . In such
systems for area lghtng (rooms, streets, stad-
nstances, attenuaton s recommended . Neutral-
ums) call for llumnance measurements . But n
densty flters, apertures, or ntegratng spheres
automotve exteror lghtng, headlghts are usu-
acheve the desred effect . The correct selec-
ally measured for llumnance, tallghts for lum-
ton depends upon the amount of attenuaton
nance . There are a number of mnature, lensed
desred: t should be enough to avod detector
lamps on the market, and snce ther dvergence
saturaton, but not so much as to lose senstvty
s of concern, they would be measured for lum-
and dynamc range .
nous ntensty . Incandescent and fluorescent
lamp manufacturers specfy products n terms
of lumnous flux (or the radometrc equvalent,
watts) snce these wll be placed n fxtures
SEE ALSO: The Guide to Photometer
meant to dffuse and measure ther total output . Radiometer System Configuration,
available as a free PDF download at
Lasers and LEDs also requre a careful approach .
They are measured n radometrc terms for sc- www.udtinstruments.com
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15. Making light work of light MeasureMent
Application information
hoW to SPeCify A Photometer SyStem
The smple detector/flter arrangement s also mcroscopc or telescopc .
effectve for ambent measurements f all lght s UDT Instruments offers a wde range of optcal
at normal ncdence . But when off-axs lght, such accessores for out-of-the-ordnary measure-
as from wndows and perpheral sources, contrb- ments . These nclude: fber optc probes, for
utes to the total flux, a cosne dffuser s needed . convenence n measurng sources hdden n
In addton to beng wdely appled by lamp hard-to-reach places; LED measurement systems
manufacturers, ntegratng spheres are useful for specfc to ether segmented or dscrete LEDs;
measurements of small dvergent sources lke low-profle sensors for
lensed LEDs or mnature lamps . These can be slppng nto tght spaces, such as n photolthog-
nserted rght nto the sphere‘s entrance port to raphy exposure systems; and a varety of sensor
ensure that all lght s collected . heads customzed for Display lumnance measure-
Lumnance measurements requre a prescrbed ments .
sensor-head feld-of-vew . The sze of the source
n the measurement-feld plane, and the sensor- SEE ALSO: The Guide to Photometer
to-subject dstance determne the angle . Wth
Radiometer System Configuration,
large, but close felds, a smple baffle (steradan
shade or aperture) wll do . But small mages, such available as a free PDF download at
are those on CRTs or avoncs, call for a lens sys- www.udtinstruments.com
tem, as do measurements at a dstance . A varety
of lens assembles and optcal accessores are
avalable from UDT Instruments, to accommo-
date most any lumnance measurement, whether
uDt model 1120 telephotometer
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16. Making light work of light MeasureMent
Application information
hoW to SPeCify A Photometer SyStem
Choosing electronics matched to the Your choce of electroncs depends upon the
answers to a few basc questons:
application
1 . Is feld portablty needed?
The lght sensor n each UDT Instruments photo-
metrc head s a slcon photodode . Though sen- 2 . Wll the nstrument be nterfaced wth a
sor sze may vary, the output wll n all cases be a computer?
low ampltude current sgnal . Ths sgnal wll be 3 . Is a vsual dsplay desred, or wll an analog
converted nto a output suffce?
voltage by a transmpedance amplfer crcut, 4 . Wll more than one measurement be con-
and then used accordng to the requrements of ducted concurrently?
the partcular applcaton .
UDT Instruments offers photometer controllers
SEE ALSO: The Guide to Photometer and electronc amplfers that satsfy any comb-
Radiometer System Configuration, naton of answers to these questons . The nstru-
available as a free PDF download at ments range from smple analog amplfers and
www.udtinstruments.com hand held photometers, to multchannel com-
puter-controllable laboratory nstru-
ments . Versons are avalable whch
sut most any budget .
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