Abstract

A new type of measuring and computing engine has been constructed which determines wave-lengths and wave numbers directly from spectrograms made by crossing the dispersion of a Fabry-Perot interferometer with that of a concave diffraction grating. The wave-length of a spectrum line is first determined to six figures from the grating dispersion by the method used in the author’s automatic comparator, and this wave-length is then converted to wave number by a mechanical continuous-function computing arrangement. The thickness of the etalon separator used having been pre-set into the machine, all wave-lengths are automatically divided into twice this thickness to give on dials the order of interference corresponding to the instantaneous wave-length setting of the comparator. The interference pattern of each spectrum line is projected on a screen in front of the operator with a magnification proportional to the square root of the order of interference, and falls on a set of parabolic fiducial lines drawn to fit the optical system used to project the interference fringes on the slit of the grating spectrograph. The operator moves the screen carrying these parabolic lines up or down until they coincide with the density maxima of the interference pattern; the partial order of interference in the center of the pattern can then be read to three figures from a dial. By turning the plate control handle of the machine slightly the operator then makes the reading of the machine coincide with that of this dial; cams having introduced corrections for the dispersion of air and the variation of phase change with wave-length, the dials then give the wave-length and wave number of the line correctly to seven or eight figures, depending on its sharpness. The speed of reduction of Fabry-Perot patterns is by this means increased at least 20-fold over that of previous methods, and the precision of setting appears to be considerably improved also. Repeat readings on sharp lines are found consistent to within a few ten-thousandths of an angstrom. The precision of the wave-length values obtained is limited principally by the diffuseness of the interference pattern arising from the breadth of a line.

Improvements now under development include automatic photoelectric scanning of interferometer patterns, and the direct recording by an electric typewriter of eight-figure wave-length and wave-number values for each line. The new device is being used to determine standard wave-lengths needed for the more effective utilization of the original automatic comparator.

© 1946 Optical Society of America

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References

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  1. G. R. Harrison, J. Opt. Soc. Am. 25, 6 (1935); Rev. Sci. Inst. 9, 15 (1938); G. R. Harrison and J. P. Molnar, J. Opt. Soc. Am. 30, 343 (1940).
    [CrossRef]
  2. M.I.T. Wavelength Tables edited by G. R. Harrison (John Wiley and Sons, Inc., New York, 1939).
  3. K. W. Meissner, J. Opt. Soc. Am. 31, 405 (1941).
    [CrossRef]
  4. For a special comparator which reads directly the squares of fringe diameters, see K. Burns, J. Opt. Soc. Am. and R.S.I.,  19, 250 (1929).
    [CrossRef]
  5. H. Kayser, Tabelle der Schwingunszahlen (S. Hirzel, Leipzig, 1925; revised edition J. W. Edwards, Ann Arbor, 1944).

1941 (1)

1935 (1)

G. R. Harrison, J. Opt. Soc. Am. 25, 6 (1935); Rev. Sci. Inst. 9, 15 (1938); G. R. Harrison and J. P. Molnar, J. Opt. Soc. Am. 30, 343 (1940).
[CrossRef]

1929 (1)

For a special comparator which reads directly the squares of fringe diameters, see K. Burns, J. Opt. Soc. Am. and R.S.I.,  19, 250 (1929).
[CrossRef]

Burns, K.

For a special comparator which reads directly the squares of fringe diameters, see K. Burns, J. Opt. Soc. Am. and R.S.I.,  19, 250 (1929).
[CrossRef]

Harrison, G. R.

G. R. Harrison, J. Opt. Soc. Am. 25, 6 (1935); Rev. Sci. Inst. 9, 15 (1938); G. R. Harrison and J. P. Molnar, J. Opt. Soc. Am. 30, 343 (1940).
[CrossRef]

Kayser, H.

H. Kayser, Tabelle der Schwingunszahlen (S. Hirzel, Leipzig, 1925; revised edition J. W. Edwards, Ann Arbor, 1944).

Meissner, K. W.

J. Opt. Soc. Am. (2)

G. R. Harrison, J. Opt. Soc. Am. 25, 6 (1935); Rev. Sci. Inst. 9, 15 (1938); G. R. Harrison and J. P. Molnar, J. Opt. Soc. Am. 30, 343 (1940).
[CrossRef]

K. W. Meissner, J. Opt. Soc. Am. 31, 405 (1941).
[CrossRef]

J. Opt. Soc. Am. and R.S.I. (1)

For a special comparator which reads directly the squares of fringe diameters, see K. Burns, J. Opt. Soc. Am. and R.S.I.,  19, 250 (1929).
[CrossRef]

Other (2)

H. Kayser, Tabelle der Schwingunszahlen (S. Hirzel, Leipzig, 1925; revised edition J. W. Edwards, Ann Arbor, 1944).

M.I.T. Wavelength Tables edited by G. R. Harrison (John Wiley and Sons, Inc., New York, 1939).

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Figures (8)

Fig. 1
Fig. 1

Optical system used for projecting Fabry-Perot etalon fringes on the slit of a stigmatic grating spectrograph. Quartz-fluorite achromat Q focuses on the slit the fringes formed at infinity, and at the same time focuses the interferometer aperture on the collimator mirror of the spectrograph.

Fig. 2
Fig. 2

Portion of a typical interference spectrogram of iron and neon.

Fig. 3
Fig. 3

Oblique photograph of the machine, known as WINMAC (wave-length interferometric measurement and computation), with which wave-lengths and wave numbers can be determined directly from Fabry-Perot spectrograms.

Fig. 4
Fig. 4

Appearance of a set of interference fringes (Ne λ6217) on the screen in front of the operator, after he has made a setting of the parabolic fiducial lines on fringes a, b, c, and d. Owing to distortion of the image produced by an unsatisfactory projecting lens the other fringes are improperly located. A lens was later procured and tested which showed less distortion than 0.1 percent.

Fig. 5
Fig. 5

Variable-magnification projection system used to form on the screen an enlarged image of the interference patterns so that they are always on the same scale of vs. d, no matter by what wave-length produced.

Fig. 6
Fig. 6

Block diagram of the mechanical parts of WINMAC. The various rectangles marked with conversions indicate units of which a simplified version is shown in Fig. 7.

Fig. 7
Fig. 7

Simplified sketch of non-linear conversion unit (in this case from wave-length λ to wave number σ). A linear portion of the λ − σ relation goes directly through gears; the non-linear remainder is controlled by a cam which provides the proper instantaneous speed ratio from an integrator unit through a differential.

Fig. 8
Fig. 8

Curve A + B gives the relation between wave-length λ and wave number σ in the range 6500–5000A. Curve A shows a linear approximation to this, obtained through a gear train and chosen so as to leave a minimum non-linear residue involving only one change in sign. Curve B shows the remainder, fed in through a unit of the type shown in Fig. 7.

Tables (1)

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Table I Plate 43.2t = 2.00851596 ± .00000005 cm.

Equations (2)

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P = P 0 + = 2 t λ + 2 t · d 2 λ · 8 f 2 ,
/ D 2 = P 0 / 8 f 2 m 2 .