LANDMEETKUNDE
K. D. FROOME, Ph. D„
The Use of Light for Measuring Distance
I would like to say how grateful I am to the N.L.F. for this
opportunity to speak to you and to see something of your Labora
tory and field work.
Light, or strictly speaking electromagnetic waves, offers us two
possible length standards. First, we may select a standard in the
form of the wavelength of a pure spectral line emitted by some
special source, and then by the techniques of interferometry use
it to measure gauges or other forms of working standards. Secondly,
we may derive a standard from the velocity of light, if we know
the time light takes to traverse the distance which we want to
measure, and if the velocity itself is known to the required accuracy.
The wavelength standard is by far the most widely applied of
the two, and is especially useful for the highly accurate measurement
of distances less than about 50 cm, although as will be shown later
it is quite possible to measure much greater distances by means of a
special technique using white light for the purpose of multiplying
a smaller length established by interference of monochromatic light.
Light sources for interferometry
The use of a lightwave as a standard of length was first suggested
by J. Babinet in 1829 but it was not until 1893 after A. A. Michel-
son and J. R. Benoit made the first direct determination of the
metre in wavelengths of the red line of the spectrum of natural
cadmium, that the suggestion was shown to be realizable in practice.
Up to 1935 some nine determinations (1) were made of the metre in
terms of this cadmium line and although the mean of the results
was accepted internationally as a secondary length standard it was
not until 1948 that the General Conference of Weights and Measures
(the supreme authority in the international metric organization)
adopted a resolution recognising the possibility of re-defining the
metre in terms of a wavelength standard.
In September 1957, after several years of work by many labora
tories, the International Consultative Committee for the Definition
of the Metre decided to recommend the adoption of the wavelength
of the orange-red line of the krypton-86 isotope spectrum and that
there should be 1 650 763.73 waves per metre, giving the value of
its wavelength in vacuo to be 0.605 78° 21 am. If the Committee's
suggestion is adopted by the General Conference in i960, then
Babinet's suggestion will have come to fruition at last.
Physicist of the National Physical Laboratory, Teddington, Middx.
Voordracht gehouden op de geodetische studiedag van de Nederlandse
Landmeetkundige Federatie te Wageningen op 23 mei 1958.
