AS NZS 1301.436:2014 Methods of test for pulp and paper
In the case of filter colorimeters, the spectral characteristics are determined by the filters inserted into the light beams in combination with the characteristics of the receptor, the sphere lining, the lamps and other optical parts of the instrument, or by a set of individual optical filters with different specific wavelengths. The filters shall be chosen so that the overall characteristics of the instrument agree with the spectral functions specified in the test methods relating to the determination of specific optical properties.
In the case of abridged spectrophotometers, the spectral characteristics are determined by the accuracy to which the individual receptors represent the nominal wavelengths assigned to them, the band-width associated with each receptor, and the values given to the mathematical functions used in the subsequent calculations. For colorimetry, the instrument shall incorporate not less than 16 receptors uniformly spaced over at least the range from 400 to 700 nm.
In instruments providing spectral data, the manufacturer shall indicate the optical bandpass of the instrument. Colorimetric data shall be computed only from spectral data presented at wavelength intervals equal to the instrument’s optical bandpass-width. The centroid wavelength of each band shall not differ from its nominal wavelength by more than ± 0,5 nm.
No procedure for calibration to eliminate errors in spectral characteristics is currently available. The spectral characteristics can be checked using suitable coloured reference standards. For specific purposes where close inter-instrument agreement is required, product-specific IR3 reference standards may be used to calibrate the instrument in relation to a reference instrument.
To calculate tristimulus values as specified by the CIE illuminant and standard observer functions (1931 or 1964), the appropriate tables of weighting factors presented in ASTM E308-06 1 ) for measurement at e.g. 10 nm or 20 nm intervals shall be used. The tristimulus values shall be calculated by direct summation using these tabulated values with no attempt at interpolation using e.g. a cubic spline function. The actual tables of values to be used are given in the relevant test methods for determining specific optical properties.
In ASTM E308-06 the tables to be used are those which assume that the spectral bandpass of the instrument used to obtain the data is equal to the measurement interval and is triangular in shape. These tables are to be used together with the data for which the manufacturer has identified the instrument bandpass as previously mentioned.