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Smart Mirror

Patent Number: 10,234,309


Radiation pressure is a force that results when light reflects from a mirror. By measuring the force on a mirror, the power in a laser beam can be assessed. We have invented a miniaturized mirrored force sensor that combines several key elements to make a sensor that is smaller, faster, more sensitive to force and less sensitive to noise, gravity, and thermal effects.

New Enabling Technology

A disk-shaped device is smaller than the half-dollar coin underneath it.
A smart mirror prototype placed on a half-dollar coin. The mirrored center is suspended from Archimedean spiral springs. Different amounts of optical power displace the mirror and spring ensemble by different amounts.
Credit: NIST

This invention measures the power of a laser beam by detecting the displacement caused by photon pressure on a mirrored surface. Accurate measurement and control of laser power measurement is essential to manufacturing. However, to date there has been no way to precisely measure laser power during a manufacturing process. Without this information, some manufacturers may have to spend more time and money assessing whether their parts meet manufacturing specifications after production.

NIST’s photon momentum sensor, or smart mirror, by contrast, is designed to create a compact laser power sensor that could be included in the laser head and potentially be used in fast inline process monitoring. Mounted beneath the top reflecting plate is a second plate. The two plates form a capacitor, the capacitance of which depends on how much the top place is displaced by photon pressure. 

Smaller, faster, more sensitive to force and less sensitive to noise, gravity and thermal effects

It is a radiation-pressure power meter (RPPM) that employs a dual spring detector concept. Two identical springs are used in a tandem configuration that mitigates environmental vibration signals, as well as errors due to changes in the sensor tilt. This method improves both sensitivity and speed (250 times faster than RPPM) and makes it a strong candidate for use in applications requiring a small compact sensor.

Compact size and consistent performance.


  • Nondestructively measures photonic power from 2.5 W to (eventually) 2 kW
  • Chip-scale
  • Measurements traceable to the SI volt
  • Compensates for distortion in its components due to thermal effects


  • Medical and dental technology
  • Aerospace applications
  • Additive manufacturing
  • Welding


The smart mirror provides new opportunities for manufacturers of lasers used in additive manufacturing (interchangeably known as 3D printing) and welding, which require low-beam intensity in the midrange. This approach allows the sensor to be embedded at the end of a robotic arm or in additive manufacturing and laser welding systems where the laser head will move and rotate. The size and reliability of this device enables it to be incorporated within the laser itself and measure performance while the laser is being used. This will help accelerate the parts qualification process. 


A demonstration of the basic working principles of the smart mirror, a new chip-sized device designed to measure the power of a laser in real time. Smart mirrors could one day be integrated into the manufacturing processes that create airplanes, cars, cellphones and more. Researchers say the device could help manufacturers speed up the process of part qualification, which ensures that their goods meet engineering specifications. Credit: Jennifer Lauren Lee/NIST
Created March 23, 2020, Updated February 11, 2021