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Journal of the Optical Society of America

Journal of the Optical Society of America

  • Vol. 18, Iss. 2 — Mar. 1, 1929
  • pp: 96–116



JOSA, Vol. 18, Issue 2, pp. 96-116 (1929)

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  1. Ives, H. E., A Proposed Standard Method of Colorimetry, J.O.S.A. & R.S.I., 5, p. 469; 1921.
  2. No attempt has been made to include a bibliography of the subject in the present paper. This has been rendered unnecessary by the excellent reports of the Progress Committee on Spectrophotometry published in this Journal. See J.O.S.A. & R.P.I., 10, p. 169; 1925, and J.O.S.A. & R.P.I., 11, p. 357; 1925.
  3. The relative visibility of blue light as given by Gibson and Tyndall is only 0.043 at 440 millimicrons. Because of the difficulty of making accurate photometric settings when the visibility is low, it is quite common in the practice of visual spectrophotometry to ignore the violet region beyond 430 or 440 millimicrons. An attempt is sometimes made to justify this procedure on the grounds that light of low visibility is unimportant for calorimetric purposes. It should be remembered, however, that the color sensation evoked by a given specimen depends on the extent to which the three elementary color sensations are stimulated. The maximum of the violet sensation occurs at 440 millimicrons, and spectrophotometric data for colorimetric purposes should be determined to 400 millimicrons at least, and possibly beyond.
  4. Jones, Lloyd A., An instrument (Densitometer) for the Measurement of High Photographic Densities. J.O.S.A. & R.S.I., 7, p. 231; 1923
  5. For certain purposes it is desirable to do this anyway to average the color of a complicated pattern which is intended to be viewed at a great distance. For example, certain types of roofing material consist of a mixture of crushed green slate and red brick dust pressed into an asphalt-coated felt base. On close inspection, both the red and green particles are visible. When the material is in place on a roof, however, these particles cannot be resolved and the surface appears homogeneous. Consequently, rotating the specimen gives the average color that is desired.
  6. This result could have been accomplished equally well with a constant deviation type of monochromator system.
  7. As a practical matter, an automatic adjustment which keeps the photoelectric cell current constant is not far from the best compromise between purity and sensitivity when a caesium cell is used. This has certain advantages from the standpoint of the design of the amplifier.
  8. It is obviously impossible to adhere strictly to the conditions of normal illumination and 45° viewing, since this would require an illuminating and viewing system of zero numerical aperture. Consequently, it is more accurate to say that the chief ray of the pencils illuminating the center of the sample and the magnesium carbonate strike the surfaces normally, Similarly, the chief rays of the reflected pencils leave the surfaces at 45°. To specify the conditions completely, it is necessary to state also the numerical apertures of the incident and reflected pencils.
  9. Case, T. W., "Thalofide Cell—A New Photoelectric Substance," Phys. Rev., 15, p. 289; 1920.
  10. This is true also of the improved cell which was later developed.
  11. It should be noted that a change of 0.1% in a white specimen produces the same change in cell current as a 1% change in a sample reflecting only 10%. However, by the present method of coupling the cell to the amplifier, the output depends on the fractional change rather than the absolute value. It is thus possible to measure all specimens to the same percentage accuracy.
  12. General Ferrie, Quelques applications scientifique des lampes a 3 et 4 electrodes associees a des cellules photoelectriques. L'onde Electrique, 4, p. 97; 1925.
  13. The grid-filament capacity of the UX-222 tube alone is about 6×10-12 farads. The capacity of the entire circuit including the photoelectric cell is approximately 2×10-11 farads. With a screen-grid tube, the effective capacity is not increased by voltage amplification.
  14. Johnson, J. B., The Schottky-Effect in Low Frequency Circuits, Phys. Rev., 26, p. 71; 1925.
  15. The same result can be accomplished at the sacrifice of light by placing the shutter in the beam illuminating the sample rather than the standard.
  16. Gibson, K. S., Photoelectric Spectrophotometry by the Null Method, B. of S. Sci., Paper No. 349; 1919.
  17. Blocking of the amplifier is always the result of extraneous disturbances, such as a mechanical shock to the first tube, rather than too much signal.
  18. A sheet of this size affords ample precision for reading the record except for specimens of very low reflecting power. In the latter case, it is often convenient to place a circular stop in the beam of light illuminating the standard, thereby expanding the scale of reflecting power values.
  19. There is theoretically no lower limit to the precision of the flicker method. The practical limit depends on the ratio of the signal to the strays.
  20. This system conforms closely to the principles enumerated by H. E. Ives in his paper on "Scattered Light in Spectrophotometry and a New Form of Spectrophotometer," Phys. Rev., 30, p. 446; April 1910. When in proper adjustment, the error due to stray light is everywhere less than 0.1% as evidenced by the failure of filters inserted just before the cell to cause any perceptible change in the balance point.
  21. See Report of Committee on Colorimetry for 1920–21, J.O.S.A. & R.S.I., 6, P. 548; 1922.
  22. V. Bush and H. L. Hazen, "Integraph Solutions of Differential Equations," Frank. Inst. J., 204, p. 575; 1927.

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