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Correction of spectral mismatch

The consideration of the spectral correction factor is an important component of photometric measurements. With the exception of spectroradiometers all photometric detectors (photodiodes, luminance measuring cameras) have to be adapted to the spectral sensitivity of human visual perception of brightness V(lambda) function by means of full-glass or partial filters. The integral parameter f1' describes the quality of the spectral matching and has nothing to do with measurement errors. However, under certain circumstances (refer to [KB2013]) it is possible to estimate the measuring uncertainty and the basis of the f1' value.

spectral distribution of CIE Illuminant A
relative spectral power distribution of the detector

The impact of local variations of the spectral matching for narrowband or unfavorably spectral distributed light sources cannot be reflected by the f1 'parameter. Here it is required to calculate the spectral mismatch correction factor (SMCF) on the basis of the relative spectral detector sensitivity and the relative spectral power distribution of the DUT.

relative spectral power distribution of the DUT

The current standards for the measurement of SSL products (CIE S 025, EN13032-4, LM79) clearly refer to the need to take account of SMCF. White phosphor converted LEDs usually have a significant proportion of blue where the V(lambda) function is running out. In this range the local deviations of the spectral matching are often slightly higher which leads to an increased SMCF. Nevertheless, the level of the SMCF for white phosphor converted LEDs usualy lies in the range of the f1' value which is expounded in [KB2013] as well as in EN13032-4 (C.3.6). Here, the correction factors for white RGB LEDs are shown as well, which are expected to be slightly larger.

The luminous flux measurement of all RiGO801 goniophotometers is based on the integration of illuminances (CIE84:1989), which are measured with a photometer. The photometer heads have a very good spectral maching with an f1' < 1.5%. The SMCF of all common white light sources is here in a negligible range.

For calculating the SMCF an Excel file is available, where only the SPD of the DUT must be set. The SPD can also be measured by using RiGO801 Goniophotometer with included spectroradiometer option (gonio-spectroradiometer).

The following figure shows the spectrum of a white phosphor converted LED, the V (lambda) function and the spectral matching a RiGO801 photometer head.

For the following spectra the SMCF were calculated exemplary. Except to the factors of the blue LED (3.4%) the factors are negligible.

 

LED1 white

LED2 white

LED3 white

LED blue

OLED

SMCF

1.0009

1.0017

1.0018

0.9657

1.0025

Conclusion: When measuring white light sources with RiGO 801 goniophotometer the SMCF are negligibly small. Only when measuring monochromatic LED in the edge region of the V (lambda) function (blue / red) the measured luminous flux should be corrected by the SMCF.

[KB2013] KRÜGER, U., BLATTNER, P. (2013). Spectral mismatch correction factor estimation for white LED spectra based on the photometer's f1' value. CIE x038:2013 Proceedings of the CIE Centenary Conference “Towards a New Century of Light”, Paris, 2013.

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