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  1. H. E. Ives, A Proposed Standard Method of Colorimetry, J.O.S.A. & R.S.I.,  5, p. 469; 1921.
    [Crossref]
  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 colorimetric 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. Lloyd A. Jones, An instrument (Densitometer) for the Measurement of High Photographic Densities. J.O.S.A. & R.S.I.,  7, p. 231; 1923.
    [Crossref]
  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. T. W. Case, “Thalofide Cell—A New Photoelectric Substance,” Phys. Rev.,  15, p. 289; 1920.
    [Crossref]
  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. 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. J. B. Johnson, The Schottky-Effect in Low Frequency Circuits, Phys. Rev.,  26, p. 71; 1925.
    [Crossref]
  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. K. S. Gibson, Photoelectric Spectrophotometry by the Null Method, ; 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; April1910. 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.
    [Crossref]

1927 (1)

V. Bush and H. L. Hazen, “Integraph Solutions of Differential Equations,” Frank. Inst. J.,  204, p. 575; 1927.
[Crossref]

1925 (3)

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.

Ferrie, Quelques applications scientifique des lampes a 3 et 4 electrodes associees a des cellules photoelectriques. L’onde Electrique,  4, p. 97; 1925.

J. B. Johnson, The Schottky-Effect in Low Frequency Circuits, Phys. Rev.,  26, p. 71; 1925.
[Crossref]

1923 (1)

Lloyd A. Jones, An instrument (Densitometer) for the Measurement of High Photographic Densities. J.O.S.A. & R.S.I.,  7, p. 231; 1923.
[Crossref]

1921 (1)

H. E. Ives, A Proposed Standard Method of Colorimetry, J.O.S.A. & R.S.I.,  5, p. 469; 1921.
[Crossref]

1920 (1)

T. W. Case, “Thalofide Cell—A New Photoelectric Substance,” Phys. Rev.,  15, p. 289; 1920.
[Crossref]

1910 (1)

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; April1910. 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.

Bush, V.

V. Bush and H. L. Hazen, “Integraph Solutions of Differential Equations,” Frank. Inst. J.,  204, p. 575; 1927.
[Crossref]

Case, T. W.

T. W. Case, “Thalofide Cell—A New Photoelectric Substance,” Phys. Rev.,  15, p. 289; 1920.
[Crossref]

Ferrie,

Ferrie, Quelques applications scientifique des lampes a 3 et 4 electrodes associees a des cellules photoelectriques. L’onde Electrique,  4, p. 97; 1925.

Gibson, K. S.

K. S. Gibson, Photoelectric Spectrophotometry by the Null Method, ; 1919.

Hazen, H. L.

V. Bush and H. L. Hazen, “Integraph Solutions of Differential Equations,” Frank. Inst. J.,  204, p. 575; 1927.
[Crossref]

Ives, H. E.

H. E. Ives, A Proposed Standard Method of Colorimetry, J.O.S.A. & R.S.I.,  5, p. 469; 1921.
[Crossref]

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; April1910. 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.

Johnson, J. B.

J. B. Johnson, The Schottky-Effect in Low Frequency Circuits, Phys. Rev.,  26, p. 71; 1925.
[Crossref]

Jones, Lloyd A.

Lloyd A. Jones, An instrument (Densitometer) for the Measurement of High Photographic Densities. J.O.S.A. & R.S.I.,  7, p. 231; 1923.
[Crossref]

Frank. Inst. J. (1)

V. Bush and H. L. Hazen, “Integraph Solutions of Differential Equations,” Frank. Inst. J.,  204, p. 575; 1927.
[Crossref]

J.O.S.A. & R.P.I. (1)

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.

J.O.S.A. & R.S.I. (2)

Lloyd A. Jones, An instrument (Densitometer) for the Measurement of High Photographic Densities. J.O.S.A. & R.S.I.,  7, p. 231; 1923.
[Crossref]

H. E. Ives, A Proposed Standard Method of Colorimetry, J.O.S.A. & R.S.I.,  5, p. 469; 1921.
[Crossref]

L’onde Electrique (1)

Ferrie, Quelques applications scientifique des lampes a 3 et 4 electrodes associees a des cellules photoelectriques. L’onde Electrique,  4, p. 97; 1925.

Phys. Rev. (3)

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; April1910. 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.

J. B. Johnson, The Schottky-Effect in Low Frequency Circuits, Phys. Rev.,  26, p. 71; 1925.
[Crossref]

T. W. Case, “Thalofide Cell—A New Photoelectric Substance,” Phys. Rev.,  15, p. 289; 1920.
[Crossref]

Other (14)

This is true also of the improved cell which was later developed.

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.

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 colorimetric 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.

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.

This result could have been accomplished equally well with a constant deviation type of monochromator system.

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.

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.

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.

K. S. Gibson, Photoelectric Spectrophotometry by the Null Method, ; 1919.

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.

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.

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.

See Report of Committee on Colorimetry for 1920–21, J.O.S.A. & R.S.I., 6, p. 548; 1922.

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.

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Figures (12)

Fig. 1
Fig. 1

The optical system of the color analyser with a schematic representation of the recording mechanism.

Fig. 2
Fig. 2

The flicker disk shown on the left consists of alternate silvered and clear sectors. The rotation of this disk causes the light reaching the photoelectric cell to come alternately from the standard and the sample. Changes in the sensitivity of the cell are of no consequence with this flicker method of comparison. The shutter shown on the right controls the illumination of the standard.

Fig. 3
Fig. 3

This curve shows the photoelectric current obtained at each wave-length with the present optical system from the magnesium carbonate standard with the shutter wide open.

Fig. 4
Fig. 4

Diagram of the conventional method of connecting the photoelectric cell to the first stage. The equivalent circuit is shown on the right.

Fig. 5
Fig. 5

Diagram of a method of connecting the photoelectric cell to the first stage which allows a dark sample to be measured with the same accuracy as a light one. This method incidentally simplifies the design of the amplifier.

Fig. 6
Fig. 6

Curves showing the variation of plate and grid currents in the UX-222 tube used under the conditions shown in Fig. 5.

Fig. 7
Fig. 7

Interstage netword of resistance-coupled amplifier used between the first stage and the power stage. By the choice of the proper values for C2 and C3, the amplifier is “tuned” to the frequency of the signal.

Fig. 8
Fig. 8

Curves showing computed performance of the amplifier shown in Fig. 7.

Fig. 9
Fig. 9

Photograph of first model of color analyser.

Fig. 10
Fig. 10

A typical curve representing in this case the analysis of the color of a piece of green silk. This specimen was measured twice by the instrument ad the differences between the two curves are less than the width of the pen line.

Fig. 11
Fig. 11

Results of a test of accuracy using a piece of amber glass as a specimen. The lower curve was determined by the present instrument while the plotted points represent data furnished by the Bureau of Standards. The cause of the error at 410 millimicrons has been discovered and eliminated.

Fig. 12
Fig. 12

An integrating attachment for the color analyser which mechanically determines the three primary sensation values.

Equations (12)

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I c = 2 π · Δ I c ( sin ω t + 1 3 sin 3 ω t + 1 5 sin 5 ω t + )
e = 2 · Δ I c π ( 2 ) 1 / 2 · Z = 0.45 · Δ I c · Z
Z = 1 ( 1 R + 1 r + 1 R g ) 2 + ω 2 C g 2 .
e = 0.45 Δ I c ω C g .
e = 0.45 6 × 10 - 13 ω C g = 2.7 × 10 - 13 × 1 ω C g .
log 10 I g = 4.0 E g - 6.2
d E g = 0.25 d I g I g .
Q = Δ I g · t = C g · E g
V . A . = μ ( a 2 + b 2 ) - 1 / 2
a = 1 + R p R 1 + R p R 3 + R p C 2 R 3 C 3
b = ω R p C 2 - R 1 + R p ω R 1 R 3 C 3 .
ω 2 = R 1 + R p R 1 R 3 R p C 2 C 3 .