About a year ago, engineering consultant Mike Wood was trying to measure the output of a tri-chromatic LED luminaire. Its pure blue output looked bright to his eyes, but his top-of-the-line photometer showed very little output. To make the matter more confusing, his least expensive light meter gave a reading that was closer to what he was seeing. What was going on?

The problem is that the standard CIE photopic luminous efficiency function, originally established in 1924 and slightly revised in 1951, doesn’t match the response of the human eye well at the very short wavelengths. This efficiency function is a correction factor for converting electromagnetic power, which is what a light meter reads, to brightness, which is what people see. Our eyes are very sensitive to green, but not so sensitive to light at the red and violet ends of the spectrum. The luminous efficiency function, built into the response of a good lux or footcandle meter, makes the correction from light power to brightness. An expensive meter follows the CIE function closely, a less expensive meter, not so closely, but the function is not accurate for short wavelengths. The error in the function makes little difference between meter reading and brightness sensation with continuous or near-continuous spectrum sources, such as incandescent or metal halide lamps, but the difference can be significant with narrow-band sources, such as red-green-blue LED luminaires. This makes measuring LED luminaire output and comparing it to the output of more continuous spectrum luminaires difficult. What to do?

The ESTA Technical Track session, “Photometric Errors with LEDs, or ‘Caught by the Short Waves,’” scheduled for Saturday, October 25 at LDI, from 4:00 to 5:30pm will explore the errors at the short end of the spectrum and a possible (although probably not practical) solution. Mike Wood of Mike Wood Consulting will work with modern state-of-the-art luminaires and photometers, while Karl G. Ruling of ESTA will demonstrate a photometer originally developed for measuring the output of gas lamps.

For additional information on ESTA sessions at LDI, visit the ESTA technical track.