LEDs and Measurements – How Much Light Source Influence the Measurement Results

When measuring the lighting conditions in an indoor or outdoor environment (e.g. illuminance or luminance) one should always consider the conditions at which the photometer was calibrated and the used light source. In spite of ban of incandescent lamps years ago, photometers are still calibrated with the CIE standard illuminants A, which is realized by incandescent lamp at color temperature of 2865 K. But the measured light sources are usually not incandescent or tungsten halogen lamp. In most cases indoor lighting uses fluorescent lamps and outdoor lighting HID lamps. Beside that more and more LED lamps are used both indoor and outdoor. For cases where photometer is calibrated with different light source as the measured one the ISO/CIE 19476:2014 standard “Characterization of the Performance of Illuminance Meters and Luminance Meters” [1] recommends use of spectral mismatch correction factors for correction of results. Although useful, spectral mismatch correction factors are difficult to calculate and apply. Especially if one does not know the spectrum of measured light source and the spectral responsivity of the used instrument. Measurements without application of spectral mismatch correction factors may, on the other hand, result in larger measurement error. The paper deals so with a question: how large the measurement error, when measuring the illuminance under different light sources, could be with a lux meter of class C and without the mismatch correction factor correction.