In colonial times, lamp makers relied on whale oil-based candles as a standard measure of light. Today, a new "electronic eye" developed at the National Institute of Standards and Technology is giving the lighting industry its most accurate measure of brightness ever.
The new electronic eye is twice as accurate as the lighting standard it replaces, explains NIST physicist Chris Cromer. Scientists now are using the electronic eye to maintain the candela, the international base unit for measuring light. The electronic eye, a photometer with a green filter, silicon photodiode and an electronic circuit for signal processing, has an aperture that works much the same way as the human iris.
Technology has come a long way since the time of the whale oil candle standard, and so has the demand for precise and accurate lighting. For example, lighting in cockpits and dashboards must be at specific brightness levels for safety reasons. Proper lighting is also important for accurate color perception. Even federal grain inspection programs carry specific lighting requirements.
This new candela standard will help the lighting industry meet new light bulb labeling requirements for brightness, energy efficiency and color rendering. It also will help ensure proper illumination for vehicle control displays.
"NIST's electronic eye is a detector which mimics the human eye," Cromer says. "It's more sensitive in the green region of visible light, just as the eye is."
Using a detector as the candela standard has advantages over previous methods, which were based on a light source, Cromer explains. The electronic eye replaces a multistep method based on radiation from a black surface at the melting point of gold.
The previous system used standard candela lamps as primary reference standards. Manufacturers could purchase these lamps from NIST for calibrating their own products. However, all the intermediate steps in developing the standard lamps increased the uncertainty of their calibration. The brightness of the standard lamps would drift slightly over the life of the lamp, Cromer explains.
With the electronic eye, NIST has shifted from a light source to a light detector as a primary standard. Although manufacturers can still purchase standard bulbs from NIST, they can now send their own detectors to NIST for on-site calibration. NIST has eight standard detectors that are used to calibrate detectors for the lighting industry.
The new method of calibrating detectors offers lighting manufacturers a more direct comparison to NIST's primary lighting standard. The NIST device's stability fluctuates by no more than 0.1 percent per year, Cromer says.
The candela is one of the international base units of measure maintained in NIST's Physics Laboratory. The laboratory also maintain standards for the second and the kelvin.
As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.