m SPECIAL Yadav Dr. Bharat Kumar National Physical Laboratory, Council of Scientific & Industrial Research, New Delhi, India Illuminance Meter and their suitability Recently I participated in “The Third Country Training Programme on Standard photometers are used to Strengthening of Measurement Standards calibrate a variety of luxmeters that are sent Institutes of Asia Pacific Countries”, Jan 15 for calibration. In recent years it has been – Feb 25, 2012 at NIMT, Thailand. In this found that a large number of luxmeter of8 training Photometry Standard was the one varying shapes and sizes are being received of the five parameters. In this short note I in NMIs. It is also becoming increasingly going to share my knowledge and experience more frequent that signal devices have light in the field of Illuminance measurement and emitting diodes (LEDs) as source and these discuss a technique to determine the quality devices are coming up in a big way. To May-June 2012/Vol.14, No.67 of Illuminance meter (or lux meter). measure the Illuminance produced by such a source and also the other photometric It has been found that most of the properties of the source, it is necessary that commercial Illuminance meters that are used the Illuminance meter must have a well for measuring the Illuminance at various corrected V(λ) characteristics. Where V(λ) places are, in fact, no Illuminance meters. is the spectral luminous efficiency function The result of using such devices could be for photopic vision. We carried out an unwarranted if such meters are used to experimental study at NPLI where we measure the Illuminance being produced by introduced a filter system in the optical path signaling devices specially made of LED and in between the source (illuminant-A) and the also to measure low level Illuminance. In this detector (standard detector with the best short note I am going to discuss how we CIE response function). We repeated this could experimentally arrive in NPLI at the experiment with different type of standard fact that a device which is termed as detectors. One of the filters is a blue filter Illuminance meter is, in scientific terms, no which transmits a spectral profile peaking Illuminance meter. 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.
Detector outputs of the standardphotometers were measured with the bluefilter in the optical path and then without anyfilter and then the red filter introduced in theoptical path. The ratios of the response ofthe detectors with red filter to the blue filterswere calculated and the ratios for the abovedetectors were found close to each otherand were in accordance with their matchwith the CIE response function. When theother detectors (attached in ordinarycommercial luxmeter) were put under thesame test, not only were the responses ofthe detectors with red filter to blue filtersdiffering drastically but also the Illuminancevalue. This means that none of the detectorswas corrected for V(λ) function. Observations: The ratio of Fig. 1. Transmittance profile of blue filterthe responses with red filter and blue filter 9termed as R/B ratio was taken as the May-June 2012/Vol.14, No.67theoretically estimated value. This valuecame out to be 0.30. Now a few detectorshaving V(λ) response match (know as f1number) were taken. The f1 number wasvarying from 1.5% to 2.5% for the threestandard detectors taken for the study. TheR/B ratio were again measured for thesedetectors and it was found in between 0.25to 0.35 which is in accordance with themismatch of V(λ) response with that of theCIE value. When the R/B ratio was taken forsome commercial luxmeters, the valueswere found very much scattered. It is recommended that all theluxmeter must have the spectral responsei.e., V(λ) response which should match theCIE value as close as possible. Best casewould be if the R/B of the test detector fallsin between 0.25 to 0.35. Reference: B.K. Yadav, D.P. Fig. 2. Transmittance profile of red filterBahuguna, Jai Bhagwan and H.C. Kandpal,“Illuminance meter – An important devicefor photometry,” Mapan – Journal ofMetrology Society of India, Vol. 19, Nos. 1-2,2004; pp. 103-108.