Abstract

The multipurpose reflectometer was developed primarily to measure apparent reflectance, specular gloss and trichromatic coefficients. These measurements are useful in the ceramic, paint, textile, paper and chemical industries to indicate lightness, gloss and color of finished articles. In the reflectometer, two light beams from a single source are directed along separate paths to two barrier-layer photo-cells. Various types of these photo-cells were studied to find which could be used most advantageously. The reflectometer employs a substitution null method and requires a galvanometer to indicate equality of the currents generated by the two photo-cells. For each sample tested, there is a photometric adjustment to restore equality of the currents. The amounts of photometric adjustment are measured on the direct-reading scales; one of which is used for apparent reflectance and the other for specular gloss. Because of its high precision, the instrument is well suited for measuring small differences in apparent reflectance, gloss or color of nearly identical samples. However, for greatest accuracy, it is necessary to correct the scale readings by calibration.

© 1940 Optical Society of America

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Figures (13)

Fig. 1
Fig. 1

Spectral response curves reported by manufacturers for three barrier-layer photo-cells together with I.C.I. luminosity (visibility) function. The “Photronic” curve is for Photronic type 1 cells. References to data on cells are given in Table I.

Fig. 2
Fig. 2

I.C.I. luminosity function times I.C.I. illuminant C compared with (1) spectral response of G.E. cell times spectral transmission of green filter times spectral energy of 3100°K; and (2) spectral response of Photox cell times spectral energy of 2840°K. The green filter is Corning 330 Signal Yellow, 2.7 mm of melt of 11–9–31 plus Corning 428 Light Shade Blue Green, 3.5 mm of melt of 11–2–37, reference 7. (The areas under each of the three curves are equal.)

Fig. 3
Fig. 3

Currents of different barrier-layer photo-cells as a function of (1) duration of exposure and (2) temperature. Cell terminals at zero potential, incandescent source with no filters, cell illumination of two footcandles. Numbers identify curves of individual cells.

Fig. 4
Fig. 4

Currents of one of each of four types of barrier-layer photo-cells as a function of (1) duration of exposure and (2) temperature. Cell terminals at zero potential, incandescent source with no filters, cell illuminations of 0.5, 2.5 and 11 footcandles as indicated on curves.

Fig. 5
Fig. 5

Null-method visual reflectometer (reproduced from circular describing instrument, reference 25).

Fig. 6
Fig. 6

Diagram of Photox 0°,45° reflectometer. A—Test Photox cell, B—shutter, C—comparison Photox cell, D—lamp and scale attached, E—knob to move lamp and scale, and F—index.

Fig. 7
Fig. 7

Diagram of multipurpose reflectometer.

Fig. 8
Fig. 8

Three reflectometer attachments for special functions. A. For approximate values of A45°, 0° of nonflat surfaces and surfaces of small area; B. for 60°, −60° specular gloss; C. for transmission.

Fig. 9
Fig. 9

Small-scale copy of the reflection- and gloss-scale calibration curves of one multipurpose reflectometer. The circles are relative values of F1/F2 calculated from Walsh’s equation with r=2.5 cm.

Fig. 10
Fig. 10

Corrections applied to the reflection-scale settings of one instrument when the values of A45°, 0° of the samples measured are roughly 1.0, 0.5, 0.1 or 0.01 times settings.

Fig. 11
Fig. 11

Corrections applied to the gloss-scale settings of one instrument when the A45°, 0° of the comparison surface is 0.135, 0.011, or 0.004.

Fig. 12
Fig. 12

Different operations of the multipurpose reflectometer showing functions of both the test and comparison specimens for each operation. (The green filter, required with the G.E. cell for all measurements according to the luminosity function, is not shown.)

Fig. 13
Fig. 13

Form used at the National Bureau of Standards for recording and reducing multipurpose reflectometer data. The figures entered illustrate the determination of A45°, 0° of four porcelain-enamel plaques.

Tables (4)

Tables Icon

Table I Currents generated by barrier-layer photo-cells in current-balancing circuits (potential across cell terminals at zero).

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Table II Errors in multipurpose reflectometer measurements, their causes and control.

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Table III Luminous apparent reflectance (A45°, 0°) for ten porcelain-enamel plaques chosen as master standards for colors of kitchen and bathroom accessories, references 46 and 47, measured with (1) a multipurpose reflectometer using G.E. cell and green (luminosity) filter and (2) a spectrophotometer, reference 23, by computation.

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Table IV The measuring operations of the multipurpose reflectometer, some applications of each, arrangements for operation, and the standards used.

Equations (3)

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F 2 F 1 = 1 2 r 2 [ 2 r 2 + d 2 - d ( 4 r 2 + d 2 ) 1 2 ] ,
G c ( type 1 ) A 45 ° , - 45 ° A 45 ° , 0 ° .
G c ( type 2 ) A 45 ° , - 45 ° - A 45 ° , 0 ° A 45 ° , - 45 ° ,