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Recently I participated in “The
Third Country Training Programme on
Strengthening of Measurement Standards
Institutes of Asia Pacific Countries”, Jan 15
– Feb 25, 2012 at NIMT, Thailand. In this
training Photometry Standard was the one
of the five parameters. In this short note I
goingtosharemyknowledgeandexperience
inthefieldofIlluminancemeasurementand
discussatechniquetodeterminethequality
of Illuminance meter (or lux meter).
It has been found that most of the
commercialIlluminancemetersthatareused
for measuring the Illuminance at various
places are, in fact, no Illuminance meters.
The result of using such devices could be
unwarranted if such meters are used to
measuretheIlluminancebeingproducedby
signalingdevicesspeciallymadeofLEDand
alsotomeasurelowlevelIlluminance.Inthis
short note I am going to discuss how we
could experimentally arrive in NPLI at the
fact that a device which is termed as
Illuminance meter is, in scientific terms, no
Illuminance meter.
Illuminance Meter
and their suitability
SPECIAL
Dr. Bharat Kumar Yadav
National Physical Laboratory, Council of Scientific & Industrial Research, New Delhi, India
Standardphotometersareusedto
calibrateavarietyofluxmetersthataresent
for calibration. In recent years it has been
found that a large number of luxmeter of
varyingshapesandsizesarebeingreceived
in NMIs. It is also becoming increasingly
morefrequentthatsignaldeviceshavelight
emittingdiodes(LEDs)assourceandthese
devices are coming up in a big way. To
measuretheIlluminanceproducedbysuch
a source and also the other photometric
propertiesofthesource,itisnecessarythat
the Illuminance meter must have a well
corrected V(λ) characteristics. Where V(λ)
is the spectral luminous efficiency function
for photopic vision. We carried out an
experimental study at NPLI where we
introducedafiltersystemintheopticalpath
inbetweenthesource(illuminant-A)andthe
detector (standard detector with the best
CIE response function). We repeated this
experiment with different type of standard
detectors. One of the filters is a blue filter
which transmits a spectral profile peaking
at 480nm and the other is a read filter.
Response of these filters alongwith a V(λ)
filter are shown in Fig. 1 and Fig. 2.
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Detector outputs of the standard
photometers were measured with the blue
filterintheopticalpathandthenwithoutany
filterandthentheredfilterintroducedinthe
optical path. The ratios of the response of
thedetectorswithredfiltertothebluefilters
werecalculatedandtheratiosfortheabove
detectors were found close to each other
and were in accordance with their match
with the CIE response function. When the
other detectors (attached in ordinary
commercial luxmeter) were put under the
same test, not only were the responses of
the detectors with red filter to blue filters
differingdrasticallybutalsotheIlluminance
value.Thismeansthatnoneofthedetectors
was corrected for V(λ) function.
Observations: The ratio of
the responses with red filter and blue filter
termed as R/B ratio was taken as the
theoretically estimated value. This value
came out to be 0.30. Now a few detectors
having V(λ) response match (know as f1
number) were taken. The f1 number was
varying from 1.5% to 2.5% for the three
standard detectors taken for the study. The
R/B ratio were again measured for these
detectors and it was found in between 0.25
to 0.35 which is in accordance with the
mismatch of V(λ) response with that of the
CIE value. When the R/B ratio was taken for
some commercial luxmeters, the values
were found very much scattered.
It is recommended that all the
luxmeter must have the spectral response
i.e., V(λ) response which should match the
CIE value as close as possible. Best case
would be if the R/B of the test detector falls
in between 0.25 to 0.35.
Reference: B.K. Yadav, D.P.
Bahuguna,JaiBhagwanandH.C.Kandpal,
“Illuminance meter – An important device
for photometry,” Mapan – Journal of
MetrologySocietyofIndia,Vol.19,Nos.1-2,
2004; pp. 103-108.
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Fig. 1. Transmittance profile of blue filter
Fig. 2. Transmittance profile of red filter