NIST Proposes New Definition for Optical Watt

Researchers at the National Institute of Standards and Technology (NIST) seek to develop a more precise definition for the standard unit of light power, the optical watt. Instead of the current definition, which is based on comparisons to electrical heating, the NIST study suggests that the optical watt could be determined from light’s radiation force and speed. “We are pursuing research to enable the radiation pressure approach to make the most accurate measurement of laser power in the world,” physicist Paul Williams said.

To measure optical power the conventional way, scientists aim a laser at a coated detector, measure the detector’s temperature change, and then determine the electrical power needed to generate an equivalent amount of heat. NIST’s equipment for measuring the optical watt in this way is large and is not portable. With the new approach, laser power would be measured by comparing it to the force of gravity on a reference mass or an equivalent force.

NIST has developed a technique to measure a laser’s force, or the push exerted on a mirror by a stream of photons. The result, measured in either milligrams (mass) or micronewtons (force), is traceable to the International System of Metric Units (SI) base unit, the kilogram, and can be used to calculate optical power, the NIST team said. Calculations of mass require knowledge of the exact amount that gravity accelerates objects at a particular altitude and location. Calculations of force do not, making them simpler to use as a primary standard.

According to NIST scientists, the new definition for the optical watt could be:

One watt of optical power is that which, upon normal reflection from a perfect mirror, produces a force whose magnitude (in newtons) is equal to 2 divided by the speed of light; or

One newton of force is that which is produced when an optical power (in watts) of a magnitude equal to the speed of light divided by 2 reflects normally from a perfectly reflecting mirror.

NIST scientists established the validity of the mass/force approach by comparing results using this approach to those using the conventional method. The researchers believe that the new approach offers several advantages, specifically, a portable primary standard in the form of a reference mass and the potential for improved precision.

Optical power comparisons. Courtesy of N. Hanacek/NIST.

The mass/force approach could be especially helpful for high-power lasers used in manufacturing and in the military because it is simpler, faster, less costly, and more portable than conventional methods.

The study was done in the context of NIST’s laser power measurement and calibration services, where the range of light power that can be measured ranges from very low light intensity of a few photons per second to 100-kilowatt lasers emitting 100,000,000,000,000,000,000,000 photons per second. Primary standards that define measurement units based on fundamental constants, such as the new measurement standard proposed for the optical watt, are in sync with the recent redefinitions from SI, which offer more reliable measurements based on unchanging properties of nature.

The research was published in Metrologia (www.doi.org/10.1088/1681-7575/ab4641).