A Photoelectric Photometer for Rapid Grading of Naval Stores Products ROBERT H. OSBORN, Hercules Experiment Station, Hercules Powder Company, Wilmington, Del.
A photoelectric color grader, especially suited to naval stores plant laboratories, is described. Its design is such that a single meter reading indicates the ratio of the transmissions of a sample for light beams of two colors. By proper choice of variables, any class in a wide range of transparent materials may be graded rapidly and without the necessity of avoiding dirt, haze, or surface imperfections in the samples.
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differences for rosin colors are well represented by differences in z trichromatic coefficients. Accordingly, the official U. S. rosin standards have been spaced systematically on a scale of 2 trichromatic coefficients.
Theory of Instrument Figure 1 is a schematic diagram showing the essential parts of a typical instrument designed for measuring the color of naval stores products. If a sample be placed in the space between the filter holder and the barrier layer photocell, using a filter having characteristics such that the response of the photocell is proportional to the sum of the X , Y , and 2 tristimulus values of the sample, the indicating instrument (galvanometer or microammeter) may be set at some predetermned value by adjusting the iris diaphragm. Then, without changing this adjustment, a filter having characteristics such that the response of the photocell is proportional to the X tristimulus value alone is inserted in place of the first filter. The indicating instrument will then give a reading which will be proportional to the ratio of the transmission of the sample for the X beam to its transmission for the X Y 2 beam. This ratio, and hence the latter reading, will be proportional to the z trichromatic coefficient of the sample, as may be shown by the following analysis: Let il = photocell current output with the sample and the X Y + 2 filter in osition ti = photocell output with t i e sample and the X filter in position E = relative spectral energy of the lamp s = relative spectral response of the photocell to a source having an e ual energy spectrum ~ X + y + z= transmission of + Y + z filter Tx = transmission of X filter T = transmission of sample X = wavelength k = a constant
LARGE number of photoelectric instruments have been described in the literature in which transmission or reflection factors for light of two or more colors are measured (1, 9, 10, 11). For certain types of colorimetric work, such as color grading, quantities involving ratios of such factors are required. These ratios must be calculated from the data furnished by instruments of the foregoing type. I n general, these instruments will also measure quantities proportional to the ratios of transmission or reflection factors of two different materials for light of the same color. Another instrument (28) measures quantities proportional to the differences between the transmission or reflection factors of materials for light of two different colors. None of the photoelectric colorimeters investigated by the author has been suitable for the rapid routine color grading which is done in naval stores plants. Heretofore, the method of visual comparison with artificial standards has been used. Recognizing the limitations and inaccuracies inherent in such a procedure, means were sought whereby quantities proportional to the ratios of transmission factors of materials for light of two different colors could be obtained by means of single meter or dial readings from an instrumint of the photoelectric type. FurtherLAMP more, the optical, mechanical, and electrical characteristics of the design which was finally chosen were so arranged that the readings were proportional to either the 2, y, or z trichromatic coefficients (6) of the samples being graded, depending on the combination of VOLTM€T€.e characteristics chosen for that particular series of measurements. The justification for this procedure is based on the observation made in this laboratory that the grades of samples having more or less uniform clarity and color characteristics can be based on one of their trichromatic coefficients. The choice of a coefficient for a particular type of sample depends on the position on the I. C. I. color mixture diagram (6) of the locus of points representing the complete range of colors of samples of that type. For example, FIGURE 1. SCHEMATIC DIAGRAM OF PHOTOELECTRIC GRADERFOR TRANSPARENT Brice (8) has shown that the chromaticity AND TRANSLUCENT SAMPLES 572
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Hor low light intensities and low external resistance in tne photocell circuit, the photocell current is known to be practically a linear function of illumination @)-that is,
The integral in Equation 2 is equal to a constant times the
X tristimulus value of the sample (6),while the integral of Equation 1 is equal to another constant times the sum of the X , Y , and Z tristimulus values of the sample. Dividing Equation 2 by Equation 1:
" , U , U 8 ~ " . Y ~ " r u l n ~ . . , o " r u l r r " ~ v r u r v ""*"
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erly placed in the drawer and pushed into the light beam,, the compartment is tightly closed. A circular filter holder provided with a handle permits rapid changes from ,one filter to an. other. Space for eight filters has been allowed in anticipation 01 possible future requirements. The photocell current is indicatec on a Rawson Type 507C microammeter, graduated with 10( divisions, and giving full-scale deflection st 30 mioroamperes, For the comfort of the operator an adjustable mirror is provider for reading the meter while the operator is in B sitting position A threeposition selector switch permits the operator (1) t< make an initial adjustment of the bucking voltage; (2) to maki color measurements using the bucking voltage; and (3) to make measurementswithout the bucking voltage, or with the photocel connected directly to the meter. Once a day, !he operator turns the selector,switch toposi$o? 1. ,He then adjusts the rheostitl on t scal colc is t urned to' pdsition 3 for the meashement of dark-colored Sam~ples. With the seleotor switch in this position, the color scale is a compressed version of the scale obtained with the selector swiitch in position 2. ~~~~~I~
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In order to avoid the effect of variahles such as a ~ n ofa the , on the filters, possible small changes in the spectral lam~ pdust characteristic of the uhotocell. etc., calibrations are ureuared Nhere
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