Device for calibrating the photometric linearity of optical instruments

Abstract

A device and method are disclosed for checking photometric linearity of intensity measuring optical instruments which operate by passing a beam of light through samples undergoing analysis. Variable and step attenuators in optical series are introduced into the path of the light beam used by such instruments. The variable attenuator completely obstructs a segment of the light beam and the area of this segment is manually adjustable. The step attenuator, of known accuracy, homogeneously attenuates the full area of the remaining beam. To use the device, the instrument is first adjusted to respond correctly at zero and full scale. The variable attenuator is then inserted into the beam to its selected location and the output response of the instrument is recorded. The output response is again recorded after the step attenuator is inserted into the remaining or unobstructed segment of the light beam. Linearity is checked by comparing output response at diverse settings of the variable attenuator with and without the presence of the step attenuator. The ratios of output response with and without the step attenuator should remain constant irrespective of the variable attenuator setting and will correspond in value to the accurately known effect of the step attenuator in a linear instrument. If sought, an accurate input-output response can be reconstructed by an iterative sequence which begins at the full scale point. From that point the input magnitudes are repetitively reduced by the known effects of the step attenuator, while the output magnitudes are repetitively reduced by smoothed values of the apparent steps obtained during the linearity analysis.

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