Influence of the temperature dependent spectral power distribution of light-emitting Diodes on the illuminance responsivity of a photometer

• Junction temperature dependent photometer deviation of LEDs was estimated by c(St,Ss) factor.
• Junction temperature dependent c(St,Ss) variation of the commercial photometer was small.
• Junction temperature dependent c(St,Ss) variation of industrial photometer was more than 7%.
• Photometers with excellent spectral responsitivities must be used for accurate measurement.
• Careful calibration must be done when using a photometer with poor spectral responsitivities.

Accurate optical measurements of LEDs are crucial because of the increasing popularity of LEDs. However, a photometer with a V(λ) filter spectrum curve may yield large errors when it is used for photometric measurements of colored LEDs. The junction-dependent light output and spectral distribution of LEDs also introduce measurement errors of the measured photometric characteristics. For the accurate measurements of LEDs, the c(St,Ss) factors were used to estimate the possible deviation in the photometric measurement of colored LEDs with various junction temperatures using commercial and industrial grade photometer heads. The spectral measurements of LEDs with specified junction temperature were conducted using a miniature fiber-optic spectrometer, and the relative spectral power distributions of LEDs were used to calculate the spectral mismatch correction c(St,Ss) factors of the photometer heads. Therefore, the c(St,Ss) factors of colored LEDs were calculated according to the temperature dependent spectral power distributions with various junction temperatures, and these factors were used to estimate the possible deviation in the photometric measurement of colored LEDs. The estimation of the possible deviation in the photometric measurement shows that photometers with excellent relative spectral responsitivities must be used for accurate measurement; otherwise, careful calibration must be conducted when using a photometer with inferior relative spectral responsitivity of the photopic filter.