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Photoelectric spectrophotometry by the null method
NOTES
FROM
PHOTOELECTRIC
THE
U. S. BUREAU
OF STANDARDS
SPECTROPHOTOMETRY METHOD.’ By
BY
THE
XC NULL
K. S. Gibson. [ABSTRACT.]
IN connection with the color standardization work of the National Bureau of Standards 2 it is desired to have available a number of independent methods of making spectrophotometric determinations, especially in the visible part of the spectrum; for it is generally admitted that the fundamental basis of color To supplement the other specification is spectrophotometry. methods at present in use at the Bureau and especially to overcome the well-known uncertainty of measurements by these other methods in the blue and violet, the author in 1917 was given the problem of developing a method for accurate and convenient photoelectric spectrophotometry suitable for routine determinations. This was especially desirable for the measurement of spectral transmission,. The sensitive potassium-hydride photoelectric cells now on the market (made by Kunz) when used with an incandescent lamp and a glass dispersing prism, give a maximum response usually near 460 millimicrons, and are thus The making and assembling of very suitable for the purpose. apparatus was completed in April, 1918; and since that time it has been in continual use, Boeing very satisfactory as to speed of operation, ease of keeping in working condition, and accuracy of measurement. In the null method as used, two photoelectric, cells and the proper batteries are connected in a sort of Iliheatstone bridge with the electrometer as the indicator. Radiant arrangement,3 energy from a 6oo-watt Mazda C moving-picture lamp, after dis* Communicated by the Director. Paper No. 349. ‘I. G. Priest, “The Work of the National Bureau Establishment of Color Standards and Methods of Traits. I. E. s., xiii, p. 38, 1918. a F. I(. Richtmyer, Phys. Rrv., (2) vi, p. 66, 1915.
’Scientific
of Standards on the Color Specificatinns.”
547
548
U. S. BUREAU OF STANDARDS NOTES.
[J. 1;. I.
persion through a Hilger constant deviation spectrometer, is incident upon one of the photoelectric cells. Radiant energy from a Iq-watt Mazda C lamp is incident on the other photoelectric cell. The photcelectric currents generated in the two cells by the radiant energy from the two lamps tend to nullify each other so far as charging the electrometer is concerned; and by proper adjustments, the two currents may be made exactly equal, the zero motion of the spot of light from the electrometer indicating the balanced condition. Measurements of the spectral transmission of a specimen are made by observing at any desired wave length the distances of the’ 6oo-watt lamp from the spectrometer slit necessary to obtain a baltince of the electrometer with and without the specimen in position, all other factors such as slit widths, currents, etc., being kept constant. The ratio of the squares of these two distances respectively is the transmission. By this use of the null method all errors are eliminated, as well as the necessity of any kind of calibration, in connection with the relation between photoelectric current and incident radiant power, with the dark currents through the photoelectric cells, and with electrometer deflection methods. The accuracy has been tested in various ways, chief of which are the measurements of rotating sectors of known transmission and comparisons with values obtained by other methods. It is considered that the uncertainties of values obtained from 410 to 550 millimicrons, inclusive, are not greater than 0.01 for any values of transmission between 0.03 and I .oo and not greater Beyond than 0.003 for transmissions between 0.00 and 0.10. these wave lengths just given, as far as 390 and 603 millimicrons. inclusive, the uncertainties of measurement are not greatly increased, being at 390 and 600 not more than twice as great as Measurements can be made from throughout the better range. 380 to 650 millimicrons. The apparatus has also been used for the measurement of spectral diffuse reflection relative to that of a standard such as magnesium carbonate, and is adapted to the measurement of the relative distribution of radiant power of two sources, to the measurement of fluorescence, and to extension into the ultraviolet if quartz parts instead of glass were used.
FROM
PHOTOELECTRIC
THE
U. S. BUREAU
OF STANDARDS
SPECTROPHOTOMETRY METHOD.’ By
BY
THE
XC NULL
K. S. Gibson. [ABSTRACT.]
IN connection with the color standardization work of the National Bureau of Standards 2 it is desired to have available a number of independent methods of making spectrophotometric determinations, especially in the visible part of the spectrum; for it is generally admitted that the fundamental basis of color To supplement the other specification is spectrophotometry. methods at present in use at the Bureau and especially to overcome the well-known uncertainty of measurements by these other methods in the blue and violet, the author in 1917 was given the problem of developing a method for accurate and convenient photoelectric spectrophotometry suitable for routine determinations. This was especially desirable for the measurement of spectral transmission,. The sensitive potassium-hydride photoelectric cells now on the market (made by Kunz) when used with an incandescent lamp and a glass dispersing prism, give a maximum response usually near 460 millimicrons, and are thus The making and assembling of very suitable for the purpose. apparatus was completed in April, 1918; and since that time it has been in continual use, Boeing very satisfactory as to speed of operation, ease of keeping in working condition, and accuracy of measurement. In the null method as used, two photoelectric, cells and the proper batteries are connected in a sort of Iliheatstone bridge with the electrometer as the indicator. Radiant arrangement,3 energy from a 6oo-watt Mazda C moving-picture lamp, after dis* Communicated by the Director. Paper No. 349. ‘I. G. Priest, “The Work of the National Bureau Establishment of Color Standards and Methods of Traits. I. E. s., xiii, p. 38, 1918. a F. I(. Richtmyer, Phys. Rrv., (2) vi, p. 66, 1915.
’Scientific
of Standards on the Color Specificatinns.”
547
548
U. S. BUREAU OF STANDARDS NOTES.
[J. 1;. I.
persion through a Hilger constant deviation spectrometer, is incident upon one of the photoelectric cells. Radiant energy from a Iq-watt Mazda C lamp is incident on the other photoelectric cell. The photcelectric currents generated in the two cells by the radiant energy from the two lamps tend to nullify each other so far as charging the electrometer is concerned; and by proper adjustments, the two currents may be made exactly equal, the zero motion of the spot of light from the electrometer indicating the balanced condition. Measurements of the spectral transmission of a specimen are made by observing at any desired wave length the distances of the’ 6oo-watt lamp from the spectrometer slit necessary to obtain a baltince of the electrometer with and without the specimen in position, all other factors such as slit widths, currents, etc., being kept constant. The ratio of the squares of these two distances respectively is the transmission. By this use of the null method all errors are eliminated, as well as the necessity of any kind of calibration, in connection with the relation between photoelectric current and incident radiant power, with the dark currents through the photoelectric cells, and with electrometer deflection methods. The accuracy has been tested in various ways, chief of which are the measurements of rotating sectors of known transmission and comparisons with values obtained by other methods. It is considered that the uncertainties of values obtained from 410 to 550 millimicrons, inclusive, are not greater than 0.01 for any values of transmission between 0.03 and I .oo and not greater Beyond than 0.003 for transmissions between 0.00 and 0.10. these wave lengths just given, as far as 390 and 603 millimicrons. inclusive, the uncertainties of measurement are not greatly increased, being at 390 and 600 not more than twice as great as Measurements can be made from throughout the better range. 380 to 650 millimicrons. The apparatus has also been used for the measurement of spectral diffuse reflection relative to that of a standard such as magnesium carbonate, and is adapted to the measurement of the relative distribution of radiant power of two sources, to the measurement of fluorescence, and to extension into the ultraviolet if quartz parts instead of glass were used.