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Adam Roberts

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Proceedings Article | 12 September 2008 Paper

Radiometry of flashing LED sources

Don Gregory, Stephanie Medley, Adam Roberts

Proceedings Volume 7062, 70621B (2008) https://doi.org/10.1117/12.795534

KEYWORDS: Light emitting diodes, Eye, Sensors, Luminous efficiency, Power meters, Convolution, Radiometry, Xenon, Lamps, Silicon

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A laboratory based technique has been devised for measuring the illumination characteristics of flashing light emitting diode (LED) sources. The difference between the photopic measurement of a continuous source and a flashing source is that some analytic method must be incorporated into the measurement to account for the response of the eye. Ohno et al have devised an analytic expression for the impulse response of the eye, which closely matches existing forms used for finding effective intensity¹. These other forms are the Blondel-Rey equation, the Form Factor method, and the Allard method.^4,5,6 Ohno's research suggests a modified Allard method, but offers no procedure for actually making the measurement. In this research, the modified Allard¹ method approach has been updated using standard laboratory equipment such as a silicon detector in conjunction with a digital multi-meter and Labview® software to make this measurement. Labview® allows exact computation of the modified Allard method. However, an approximation scheme for the conversion from radiometric units to photopic units must be adopted. The LED spectral form is approximately a Gaussian line shape with full width at half maximum of about 15 to 30nm. The Gaussian curve makes converting from radiometric to photopic units difficult. To simplify, the technique presented here estimates the spectral form of the LEDs to be a Dirac delta function situated at the peak wavelength. This allows the conversion from watts to lumens to be a simple application of the luminous efficiency curve.² For LEDs with a full width half maximum of 20nm, this scheme is found to be accurate to ± 5%.

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