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National Standard of Length

Name of Standard: National Standard of Length

Code designation: ECM 110-1/08-036

Year of publication: 2008, updated 2016

Department: 8014 CMI LPM Praha 5

Guarantor: RNDr. Petr Balling, Ph.D.

Number of  CMCs: 24

National Standard of Length consists of several primary wavelength standards and equipment enabling the realization of the SI definition of the meter: The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs


Measurand:                                         Range:                         Expanded Uncertainty (k = 2):

Primary wavelength standards (MeP method c):

vacuum wavelength                              ≈ 532,245 nm            0,000 000 002 0 nm

vacuum wavelength                              ≈ 543,516 nm            0,000 000 005 4 nm

vacuum wavelength                              ≈ 632,991 nm            0,000 000 006 7 nm

vacuum wavelength                            ≈ 1 542,384 nm           0,000 000 080 nm

fs comb (MeP mehtod b):

frequency (optical)                              (281 – 576) THz          2,0∙10-13∙f

vacuum wavelength                            (520 – 1064) nm          2,0∙10-13∙λ


displacement L (comparator IK-1)      (0 – 1,8) m                   Q[1 nm, 1,2∙10-7L]

length L (gauge blocks)                       (0,1 – 1) m                   Q[70 nm, 8,5∙10-8L]

thermal expansivity α of long gauge block

-1∙10-5/K – 3∙10-5/K     Q[3∙10-9/L, 2∙10-8, 8∙10-4∙α]/K, α in 1/K

line scales, line spacing L                 10µm – 200 mm             Q[28 nm, 3,77∙10-7L]

line scales, line spacing L                 (200 – 500) mm              Q[68 nm, 2,13∙10-7L]


Primary standards of length - helium-neon lasers 633 nm (red) stabilized to hyperfine  spectral components of iodine molecule – were developed in ČSMÚ and later in ČMI by group lead by Ing. Jan Blabla, CSc. since 1978. These standards were first internationally compared in BIPM in 1981 with very good agreement with reference value which was later (in 1983) used for adopting the current SI definition of the metre. The group of primary standards of wavelengths 633nm served as National wavelength standard of Czechoslovakia since 1980s.

The other primary wavelength standards were developed in following years: iodine stabilized He-Ne lasers 612 nm (orange, 1994) and 543,5 nm (green, 1996), iodine stabilized Nd:YAG laser 1064 nm/532 nm (infrared/green, 2001) and acetylene-stabilized DFB laser 1542 nm / 771 nm (infrared, 2004).

The combined expanded relative uncertainty of these primary standards is in the order of 10‑11 and stability in the order 10‑11 to 10‑14. These standards have participated in more than 10 international comparisons, always successfully.

The properties of all primary wavelength standards were studied carefully and some new methods enabling the increase of precision were developed. ČMI has contributed several times to the improvement of uncertainty of the MeP by participation of newly developed standards in absolute frequency measurement.  Wavelength standards 543 nm, 532 nm, 633 nm and 1542 nm developed in CMI were absolutely measured by fs combs in BIPM, BEV and MPQ (2001-2004). In 2005 CMI acquired femtosecond frequency comb generator MenloSystems FC 8004 and supporting instruments and started absolute frequency measurements of optical frequency standards. The CMI comb was also internationally compared several times and since 2009 its CMC is recognized and published in KCDB with relative expanded uncertainty 2∙10‑13.

The radiation of above mentioned wavelength standards is used as reference for dimensional measurements using interferometers. Two such interferometers – IK-1 and IDKM, both developed in CMI are also part of National Standard of Length.  The vacuum wavelength has to be corrected for refractive index of air according to actual temperature, pressure, humidity and CO2 content in air which limits the relative uncertainty of dimensional measurements to few parts in 108 in best case. Interferometric comparator IK-1 is mainly used for calibration of industrial interferometers but can be also used for precision calibrations of length transducers, actuators and linescales. The long gauge block interferometer IDKM was developed as complementary to short gauge block interferometer NPL-TESA which is operated in ČMI Liberec. IDKM measures up to four gauges up to 1m long; both the central length and the thermal expansivity are measured. IK-1 and IDKM took part in international comparison EURAMET 610, EUROMET.L-K2, EURAMET.L-K1.2011, EURAMET.L-K1.2019  and APMP.L-S1.

The National Standard of Length is described in more detail in references below.


  1. CONSULTATIVE COMMITTEE FOR LENGTH. Mise en pratique for the definition of the metre in the SI [online]. B.m.: BIPM. 20. květen 2019 [vid. 2019-09-20]. Dostupné z:
  2. BIPM, “Recommended values of standard frequencies” (2018).
  3. BALLING, P.:„Primární etalonáž délky“, Metrologie, Tematická příloha č. 4/2009, p. 3-10
  4. BLABLA J., PICARD-FREDIN S., RAZET A.: "On the Fifth-Derivative Spectrum of the Hyperfine Structure of 127I2 at the 633nm Wavelength of the Helium-Neon Laser", Journal of Molecular Spectroscopy, 159 (1993) p. 282-285  (reference)
  5. BALLING P., BLABLA J., CHARTIER A., CHARTIER J.-M., ZIEGLER M.: "International Comparison of 127I2 - Stabilized He-Ne Lasers at l»633nm Using the Third and the Fifth Harmonic Locking Technique", IEEE Transactions on Instrumentation and Measurement, , Vol. 44 No.2 (1995),  p.173-176
  6. BALLING P.:, “Simple Measurement of Frequency Modulation of Laser with 0.1% Precision”, Metrologia 2001, 38(4), pp. 297-299
  7. MATUS M., BALLING P., ŠMÍD M., WALCZUK J., BÁNRÉTI E., TOMANYICZKA K., POPESCU G.H., CHARTIER A. and CHARTIER J.M.: International comparisons of He-Ne lasers stabilized with 127I2 at l » 633 nm (September 1999), Metrologia, , 39 n°1 (2002), 83-89
  8. PICARD S., ROBERTSSON L., MA L.-S., NYHOLM K., MERIMAA M., AHOLA T. E., BALLING P., KŘEN P., WALLERAND J.-P.: A comparison of 127I2-stabilized frequency-doubled Nd:YAG lasers at the BIPM, comparison in May 2001, Applied Optic, 42, No.6 (2003)
  9. MA L. S., PICARD S., ZUCCO M., CHARTIER J.-M. and ROBERTSSON L., BALLING P. and KREN P., QIAN J., LIN ZHONG Y., SHI CH., ALONSO M. V., XU G. and TAN S. L., NYHOLM K., HENNINGSEN J. and HALD J., WINDELER R.: Absolute Frequency Measurement of the R(12) 26-0 and R(106) 28-0 Transitions in 127I2 at l=543 nm, IEEE Transactions on Instrumentation and Measurement,  Vol. 55 No.3 (2006).
  10. BALLING P.,  FISCHER M.,  KUBINA P., AND HOLZWARTH R.: "Absolute frequency measurement of wavelength standard at 1542nm: acetylene stabilized DFB laser," Opt. Express, 13, 9196-9201 (2005),
  11. BALLING P., KREN P.: Absolute frequency measurements of wavelength standards 532 nm, 543 nm, 633 nm and 1540 nm, Eur. Phys. J. D, 48 (2008).
  12. KŘEN P., BALLING P.: Common path two-wavelength homodyne counting interferometer development, Meas. Sci. Technol., 20 084009 (2009) (4pp)
  13. M. Matus et al., „The CCL-K11 ongoing key comparison: final report for the year 2010", Metrologia, roč. 48, č. 1A, s. 04001–04001, led. 2011.
  14. P. Balling, P. Mašika, P. Křen, a M. Doležal, „Length and refractive index measurement by Fourier transform interferometry and frequency comb spectroscopy", Measurement Science and Technology, roč. 23, č. 9, s. 094001, zář. 2012.
  15. P. Balling, „Laser frequency stabilization and measurement of optical frequencies". Lambert Academic Publishing, 03-led-2012.
  16. M. Pisani et al., „Comparison of the performance of the next generation of optical interferometers", Metrologia, roč. 49, č. 4, s. 455–467, srp. 2012.
  17. M. Zeitouny et al., „Multi-correlation Fourier transform spectroscopy with the resolved modes of a frequency comb laser: Multi-correlation Fourier transform spectroscopy with the resolved modes of a frequency comb laser", Annalen der Physik, roč. 525, č. 6, s. 437–442, čer. 2013.
  18. M. Wisniewski et al., „Final report on supplementary comparison EURAMET.L-S20: Comparison of laser distance measuring instruments", Metrologia, roč. 51, č. 1A, s. 04002–04002, led. 2014.
  19. M. Doležal et al., „Analysis of thermal radiation in ion traps for optical frequency standards", Metrologia, roč. 52, č. 6, s. 842–856, pro. 2015.
  20. H. Wu et al., „Absolute Distance Measurement Using Frequency Comb and a Single-Frequency Laser", IEEE Photonics Technology Letters, roč. 27, č. 24, s. 2587–2590, pro. 2015.
  21. M. Matus et al., „Measurement of gauge blocks by interferometry", Metrologia, roč. 53, č. 1A, s. 04003–04003, led. 2016.
  22. P. B. R. Nisbet-Jones et al., „A single-ion trap with minimized ion–environment interactions", Applied Physics B, roč. 122, č. 3, bře. 2016.
  23. H. Wu, F. Zhang, T. Liu, P. Balling, J. Li, a X. Qu, „Long distance measurement using optical sampling by cavity tuning", Optics Letters, roč. 41, č. 10, s. 2366, kvě. 2016.
  24. H. Wu, F. Zhang, T. Liu, P. Balling, a X. Qu, „Absolute distance measurement by multi-heterodyne interferometry using a frequency comb and a cavity-stabilized tunable laser", Applied Optics, roč. 55, č. 15, s. 4210, kvě. 2016.
  25. S. Quabis et al., „Intercomparison of flatness measurements of an optical flat at apertures of up to 150 mm in diameter", Metrologia, roč. 54, č. 1, s. 85, led. 2017.
  26. P. Balling, Z. Ramotowski, R. Szumski, A. Lassila, P. Křen, and P. Mašika, ‘Linking the optical and the mechanical measurements of dimension by a Newton’s rings method’, Metrologia, vol. 56, no. 2, p. 025008, Apr. 2019, doi: 10.1088/1681-7575/ab00ae.
  27. T. Nordmann, A. Didier, M. Doležal, P. Balling, T. Burgermeister, and T. E. Mehlstäubler, ‘Sub-kelvin temperature management in ion traps for optical clocks’, Review of Scientific Instruments, vol. 91, no. 11, p. 111301, Nov. 2020, doi: 10.1063/5.0024693.
  28. M. Matus et al., ‘The CCL-K11 ongoing key comparison. Final report for 2021’, Metrologia, vol. 59, no. 1A, p. 04004, Jun. 2022, doi: 10.1088/0026-1394/59/1a/04004.
  29. E. Prieto et al., ‘Measurement of short gauge blocks by interferometry’, Metrologia, vol. 59, no. 1A, p. 04002, Jan. 2022, doi: 10.1088/0026-1394/59/1a/04002.


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