Abstract: In a method wherein sets of measured values of the intensity of electromagnetic radiation, which has been subject to optical interference, absorption or scatter by a sample, are correlated with different sets of known values derived from either a model of the optical properties of the sample, or from an analogue technique, correlation is by means of either a zero dependent correlation function, or a normalized residual function. Both functions are unaffected by gain factors, thereby avoiding any need to determine and to maintain absolute sensitivities of optical detectors; provide greater variation of correlation than with techniques employing a conventional correlation coefficient, thereby increasing the precision with which the optimum correlation can be determined, especially when either the measured value, or known values are subject to error; enable the method to be carried out with only two radiation components; and can enable a reduction in computing time.

Abstract: In the method and apparatus, optimum correlation between measured and known values of transmittance or reflectance determines the property or identity of a sample. Three or more discrete radiation components can be selected which are transmitted or reflected by relatively different amounts due to interference, absorption or scatter, thereby enabling fewer radiation components to be used to provide an accurate result. The known values can be derived by analogue or digital techniques, the digital technique employing a model or models taking account of absorption, interference and/or scatter. Techniques are disclosed for reducing computing time by range ajustment, identification of a zero correlation value in a differential function of the model or models and computation of correlation coefficients for two or more groups of radiation components.

Abstract: The apparatus includes means for directing a plurality of beams of radiation of different spectral compositions to a sample such as a film or coating. A part of the beams is transmitted through the sample and received by a radiation detector. Another part of the beams reflected by the sample is in turn reflected, preferably by a concave mirror, back through the sample onto another radiation detector. The signals `a` and `b` from the detectors are combined in accordance with the relationship a/(1-b/a) to suppress optical interference effects.

Abstract: The calibration standard is attached to a sensing head of an infrared absorption gauge for the purpose of calibrating the gauge. The calibration standard includes spectrally selective absorption means having a spectrum closely resembling that of a substance to be measured, such as water sorbed onto a paper sample. The calibration standard also includes optical means for returning infrared radiation, which has passed through the absorption means, to the absorption gauge substantially along the same path as would normally be followed by the radiation reflected from the sample in the absence of the standard. The spectrally selective absorption means is preferably made of anhydrous material, such as glass containing a rare earth substance, or polypropylene, which is stable. The optical means may include diffusing means and a reflective plane.

Abstract: The apparatus employs two or more beams of radiation of different spectral composition which are transmitted through a sample and received by a detector which provides signals for computing the required parameter by ratio measurements. The beams are directed to a sample zone where a part is transmitted through, and a part is reflected by a sample. The invention provides diffusing means to receive the transmitted and reflected parts and pass the same onto a radiation responsive detector. The diffusing means may be a ground glass plate or a member made of fused alumina. A concave mirror may be used to reflect the reflected part back through the sample zone which is close to or at the center of curvature of the mirror. A beamsplitter may also be used so that the beams are directed substantially normal to a plane in the sample zone.