# Practical realization of the watt from Planck’s constant using radiation pressure

## TL;DR

Scientists built a device to measure laser power using radiation pressure and Planck’s constant, achieving high accuracy for watt measurements.

## Contribution

A primary force standard using radiation pressure enables the lowest uncertainty multi-kW watt measurement based on Planck’s constant.

## Key findings

- The HALO device measured laser power from 100 W to 5000 W with 0.12% uncertainty at 5 kW.
- The HALO results matched a thermal standard within 0.78% ± 1.12% at 2 kW.
- This is the first radiation pressure-based watt measurement at the kilowatt level.

## Abstract

A primary force standard is implemented to realize the watt through Planck’s constant by means of radiation pressure at the kilowatt level. The high amplification laser-pressure optic, or HALO, is a multiple reflection radiation pressure apparatus used for absolute radiometry of high-power lasers. In this work, a primary standard electrostatic force balance is used to measure the reflection-enhanced optical forces. With this configuration, the HALO is used to measure laser powers in the range of 100 W–5000 W from a 1070 nm fiber laser. The expanded uncertainty of the 5 kW measurement is 0.12%, which is both the lowest uncertainty multi-kW measurement and radiation pressure-based measurement to-date. The HALO result was validated against a thermal primary standard using a calibrated transfer standard at 2 kW. The degree of equivalence was 0.78% ± 1.12%, which demonstrates agreement within the uncertainties of these two primary standards.