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SI Length and Traceability


We maintain and develop methods to provide traceability to the meter via laser frequency/wavelength. The most significant source of uncertainty in most very high accuracy length measurements is the value of the index of refraction of air. The development of accurate refractometers will increase the accuracy of nearly all laser-based length measurements and is crucial to meet increasing demands from industry for high accuracy traceability.

Technical Goals:

  • To realize the meter in air to less than 1 part in 108
  • To install a less than 1 part in 108 air-wavelength reference in NIST M48 room
  • To explore meter-based realizations of the pascal and kelvin


Laser interferometry, which measures distances in terms of light wavelength, provides the backbone of top-level length metrology in industry and science. This project develops techniques to facilitate the tie between interferometer-based length measurements and the SI definition of length (in terms of the second). Integral to providing this link are:

  1. Optical frequency combs, which can use GPS timing signals to provide optical frequency/vacuum wavelength standards at almost any desired wavelength and anywhere in the world. These wavelengths are directly traceable to the definition of the meter and can be of arbitrarily high accuracy. The broad range of wavelengths available from a comb allows calibration of new sources at useful wavelengths for dimensional metrology, thus enabling innovation.
  2. Refractometers, linking vacuum wavelength to air wavelength. Determination of vacuum wavelength is straightforward via the comb, but practical interferometry must be done in air, and the air refractive index is often a limiting factor for ultra-high accuracy measurements. To overcome this problem, we are developing refractometers to measure the refractive index of air with a target uncertainty below 1 part in 108.


  • Our customers demand ever-lower calibration uncertainties, which for longer lengths are often limited by uncertainty in air refractive index. The semiconductor industry envisages a need for fractional accuracies of dimensional measurement in the 10-8 regime (ITRS roadmap).
  • Currently, this low level of uncertainty can only be attained working in vacuum, but this option is not attractive (low throughput, inconvenience, and expense) whether in a production environment (such as a stepper) or in the laboratory (such as the next-generation NIST Linescale).
  • If we are to maintain leadership and provide tomorrow's tools for length metrology at the highest levels of accuracy, we must solve the problem of air refractive index measurement, reducing uncertainties well below 1 part in 108.
  • This project has made important steps toward reaching that goal. Excellent results have been achieved for dry gases (absolute refractometry to ±3 parts in 109), and current efforts focus on overcoming problems associated with air humidity.

Major Accomplishments:




  • Published a "weak value thermostat" journal paper that used simple optics in a weak measurement configuration and achieved 0.2 mK sensitivity
  • Awarded IMS funding for "Realizing pressure, length, and temperature"

Lead Organizational Unit:



NIST Sensor Science Division


  • Primary cal labs
  • Manufacturers of ultraprecise translation stages
  • Manufacturers of pressure transducers
  • Semiconductor positioning and process control
  • Commercial and military aviation
  • Mercury reduction

Facilities/Tools Used:

  • Class 1000 cleanroom (silicate bonding)
  • Nanofab (grating mirror)
  • Optical frequency comb, referenced to GPS disciplined-oscillator
  • High-accuracy weather station (thermometer, barometer, hygrometer)

Figure 1. FLOC (fixed-length optical cavity) is a dual Fabry-Perot refractometer. It works by measuring the difference in wavelength between the bottom cavity (pumped to high vacuum) and the top cavity (which is filled with gas). If the refractivity and temperature of the gas are known, the FLOC can function as a highly precise, multidecade pressure sensor.


Figure 2. MIRE (monolithic interferometer for refractometry) features a gas cell integrated with a fiber-fed heterodyne interferometer. All parts of the interferometer are silicate-bonded to a baseplate and form a quasi-monolithic structure, which delivers the highest stability. MIRE can track the wavelength of air by comparing the pathlength of a laser beam passing through the cell interior (at vacuum) to the pathlength of a laser beam passing through the cell exterior (in lab air).

 PLT logo

Measuring Pressure with Light



Physical Measurement Laboratory (PML)
Engineering Physics Division (683)

General Information:
301-975-3468 Telephone
301-869-0822 Facsimile

100 Bureau Drive, M/S 8211
Gaithersburg, Maryland 20899-8211