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Electrical-Substitution Radiometer

Patent Number: 9,291,499


A radiometer includes a substrate; a radiation absorber disposed on the substrate to absorb radiation; a thermal member disposed on the substrate to change electrical resistance in response to a change in temperature of the radiometer; and a thermal link to connect the radiometer to a thermal reference, wherein the radiation absorber, the thermal member, or a combination comprising at least one of the foregoing includes a plurality of carbon nanotubes, the carbon nanotubes being mutually aligned with respect to the substrate, and the radiometer being configured to detect optical power.


A flat pink device with a round black patch lies over a quarter.
A black circular patch of vertically aligned carbon nanotubes used in initial versions of NIST’s electrical substitution radiometer is shown with a quarter coin for scale.
Credit: NIST

This invention is an electrical-substitution radiometer (ESR) — a thermal detector for optical and infrared radiation — based on a novel configuration of arrays of vertically aligned carbon nanotubes. An ESR is a primary reference standard used worldwide to measure optical power. It typically measures modulated optical power incident on a conical radiation-absorbing surface by sensing changes in electrical resistance caused by temperature change. The temperature change may be produced by either by optical (and infrared) absorption and heating or electrical heating. An increase in the applied heater power when the incident light is blocked is a measure of the optical power absorbed by the radiometer.  

However, many ESRs can be bulky and are hand-assembled from individual components.  Further, the time response of many ESRs is not optimal for detecting transient optical signatures. Moreover, some conical surfaces are not sensitive to low optical power or certain wavelengths. 

High-speed efficiency and accuracy in a chip-scale package

The NIST ESR eliminates these limitations. It employs a novel absorber in the form of a mat containing a dense array of vertically aligned carbon nanotubes shown to absorb 99.9% of incident optical power. The nanotubes can vary in size but are often about 10 nm thick and 150 µm long. The mats are grown on a micro-machined silicon chip, an instrument design that is easy to modify and duplicate. 

World’s most accurate and robust absolute radiometer.


  • Operates over a wide range of wavelengths (200 nm to 500 µm)
  • Makes measurements orders of magnitude faster than conventional designs
  • Fabricated using familiar semiconducting manufacturing processes on silicon wafers
  • Many variations on the basic design can be produced 
  • Low cost and simple to duplicate
  • Can be electrically or optically connected to different kinds of devices
  • Can be configured to work at cryogenic or room temperature
  • Size can be readily scaled to other detector technology such as imaging arrays and multi-element sensors.
  • Uniform spectral responsivity


  • Fiber-optic telecommunications calibrations
  • Photonics industry applications
  • Detector systems for optical radiation/power meters
  • Hyperspectral imaging for medical uses
  • Thermal imaging systems for civilian and defense uses, long-range surveillance
  • Measurement of power in wavelengths from the UV to THz
  • CubeSat radiometer applications for solar irradiance and Earth radiance


Scores of industries rely on various forms of radiation power measurement, and new applications arise continuously in telecommunications, laser-power measurement and many more areas. NIST’s invention solves the problem of disseminating high-accuracy and infrared measurement capability, which is beneficial for industrial, scientific and defense applications, sensing visible and infrared radiation, thermal imaging, and other sensing.  


A NIST electrical substitution radiometer prototype. Carbon nanotubes oriented perpendicular to the penny coin make up the black disk. The heater runs around the circumference of the disc, and the thermistor is at the bottom of the disk, just above the green and gray thermal link to the base.
Created March 23, 2020, Updated February 11, 2021