Uncertainty is a primary consideration when performing LED measurements, but where do you get started? Should you believe manufacturer’s performance claims?

Start with the lowest uncertainty standards that you can afford. Aim for better than +/- 0.5 nm Dominant Wavelength repeatability and better than 1 nm Dominant Wavelength uncertainty. The key parameters for array detector based standards are dynamic range (electrical and optical), thermal control and stability. LEDs have highly saturated narrow spectral emissions, where 14 to 16 bit or greater optical resolution is required for low uncertainty color determination.

Always verify performance, do not believe test equipment manufacturer’s claims. Proper use of a well characterized lab grade photometer can give < 3% uncertainty for luminous intensity measurements and cost about $6,000. However, an inadequate spectral system can cost up to $15,000 and provide highly inaccurate results. Excellent precision spectral systems are available and can provide extremely accurate results at a price of $20,000 – $30,000.

Performance Verification

The quality of spectral measurements of LEDs depends significantly on the quality of the array detector based spectroradiometer used. This NM-7DH LED 470nm plot shows the results of measurements taken with five different types of spectroradiometers.

Guidelines-for-LED-Measurements-with-Low-UncertaintyThe reference instrument is the dark blue curve which was accumulated with a scanning double grating system and four other array detector spectroradiometers. The data is plotted on a logarithmic scale on the vertical axis that gives a much more detailed view of the low level reading not discernable in the linear plot of the same data.

The key fact to take away is the difference caused by the out of band integrated scatted light in the optical system, usually referred to as stray light. This creates significantly different chromaticity coordinates for each of the data sets, which is not acceptable in the color accuracy and binning requirements required by today’s LED production facilities.

Gamma Scientific Verified Solutions

Gamma Scientific can supply the highest quality temperature stabilized array detector based spectroradiometer systems including integrated stray light characterization and correction. This translates into not only stable and repeatable, but precise measurements of color for unmatched quality in binning of LED production. The 450nm slc plot shows an example of the amount of correction that is routinely obtained.

Guidelines-for-LED-Measurements-with-Low-Uncertainty-Chart.pngGamma Scientific’s spectroradiometer systems for LED measurement provide high accuracy and near-perfect repeatability via RadOMA back-thinned CCD array technology. The results are highly repeatable, precise, millisecond measurements of LED spectra of all colors.

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