Detector/filter provides higher accuracy alternative to spectral LEDs

Measuring the average intensity of the visible radiation emitted from an LED is a critical issue, not just for developers of electro-optical instrumentation and photonics measuring devices, but also federal and international regulatory agencies.

There are numerous technical complications in measuring the average luminosity of a color LED. These include detection geometry, minimization of stray light, error corrections, and application specific requirements, such as determining cockpit lighting compatibility with night vision goggles (NVG) used in military aircraft. This has resulted in the need for international standards for the measurement and reporting of the visible output of color LED's.

Historically, this meant expensive equipment and challenging spectroradiometric measurements, but new technical advances in the form of detector/filters for photometric measurements have yielded extraordinary results at a fraction of the cost. Such advances are questioning the spectroradiometry will remain the true industry standard for LED measurements.

The visual brightness of a light source depends not only on the amount of electromagnetic radiation emitted, but also on the spectral composition of the LED and the visual response of the human eye, which is not constant throughout the visible spectrum. For LED applications, one isn't just trying to measure the photons being emitted by the LED, but how a human eye would "see" these photons. The eye response peaks at 555 nanometers, falling off sharply in the blue, ending at 360 nm, and in the red at 830 nm.

This, along with the complicated nature of luminous measurements, has lead the International Commission on Illumination, Commission Internationale de l'Eclairage (CIE), to set international standards for the practice of measuring visible light. The CIE has established the standard curve for the photopic response of the normal human eye. This curve has been adopted as the International Standards Organization (ISO) standard for the human eye response. In addition, the CIE has developed the standard method for measuring the average luminous intensity, or flux (lumen) per unit solid angle (steradian) directed at the observer. The standard unit for measuring light intensity is the candela (lumen/steradian).

The two standard techniques for high precision measurements of LED average luminous intensity are spectroradiometry and photometry. Photometry relates to measurements of visible radiation as the normal human eye responds to it. In contrast, radiometry is not limited to the eye response, and may be used for measurements in the ultraviolet and in the infrared as well as the visible portion of the spectrum.

Historically, the industry standard for light measurement was a spectroradiometer, calibrated by light sources with known spectral power distribution provided by the National Institute for Standards and Technology (NIST) or an equivalent. Radiometric measurements are usually wavelength specific; this means the systems must be calibrated, with a specific calibration factor applied for each individual wavelength. In addition, spectral purity or rejection of out-of-band energy in the spectroradiometer is critical for obtaining useful radiometric information at each wavelength. Quality spectroradiometric measurements, therefore, require expensive equipment, complicated measurements and precise wavelength calibration, but this technique may be replaced by something simpler and much less expensive.

Engineers at Gamma Scientific have developed new detector/filter combinations for photometry and have done a head-to-head testing of their new photometer against spectroradiometers. The key technical advancement that makes photometry less expensive and highly accurate is the newly designed detector filters. The recently developed detector/filter combinations for photometry match the CIE photopic response curve very closely. The determination of the closeness of match is based on measurements taken at NIST, which measure the sensitivity of the detector/filter combination at each wavelength in five nanometer bands throughout the visible spectral range. This detector/filter combination with additional calibration from the photometry laboratory at NIST is the primary standard with the lowest uncertainty (highest accuracy) for measuring photometric quantities at Gamma Scientific. The detector-based candela standard for photometry is the international primary standard for measuring photometric quantities with the lowest uncertainty.

A head-to-head comparison of photometric versus spectroradiometric light measurements shows very close agreement to the best of the spectroradiometric results. Of significant interest is the blue region of the spectrum, where accurate light measurements are the most challenging for photometric instruments. At 470 nm, the photometric measurement yielded 1.3% difference compared with the spectroradiometer, without compensating for the residual difference between the photometric standard curve and the detector/filter fit. By using the relative spectral response curve of the LED and applying a correction factor for the small residual spectral mismatch of the detector/filter, the overall measurement uncertainty can be less than 1%.

By developing a new detector/filter combination that very closely matches the CIE human eye response function, a new option for measuring average luminous intensity of LEDs is available. These new photometers provide lower uncertainty than current spectroradiometer technology, measuring average LED luminous intensity to less than 1% uncertainty at a cost of 15-35% of current standard spectroradiometric measurement systems, making this technique simpler and more cost effective.