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Monday, February 15, 2016

Making Optical Instruments Agree

If two optical instruments are calibrated with the same standards they must agree, right? Unfortunately, the answer is "no".  

In order for two optical instruments to agree, regardless of the material being measured, they must be identical in geometry, photometry, and spectral response.

Instrument geometry includes things such as size and shape of viewed sample area, size and shape of sample illuminated area, cone angles, distances between elements, lenses, filters, stops, etc. In the Paper Industry these items are typically defined in the appropriate ISO, TAPPI or other standard.  However, there are instances where some elements of geometry are not well defined and one manufacturer may treat things different than another.  Also, diffuse geometry can mean different things. As an example, one instrument may have diffuse illumination and 0° viewing and another may have diffuse illumination and 8° viewing.

Instrument photometry can best be described as the linearity of a measuring instrument's response to light.  Fig. 1, below, shows that an instrument with perfect photometric linearity will double its reading when the amount of light striking the photoreceptor is doubled.  Many optical instruments only offer calibration standards for a one-point calibration. However, it is good to have at least a two-point calibration (zero and some other point in the measurement range) or a calibration and then verification standards that can be used to confirm linearity over the measurement range of the instrument.  Generally speaking, it is the responsibility of the instrument manufacturer to insure photometric linearity, but there should at least be ways to check this.

Fig. 1 - Double the reading when amount of light striking photoreceptor is doubled.
Spectral Response
Instrument spectral response is the ability to light as a function of the wavelength of the signal. In basic terms, does the instrument accurately measure with respect to wavelength e.g. 50% at 500 nm should be the same on all instruments. Fig. 2, below, shows some basic optical elements that can affect spectral response. Spectral response is normally set by the manufacturer. However, it can be affected if any of the elements in Fig. 2 are changed. Therefore, extreme caution should be taken when changing any of these elements and the instruction manual or manufacturer should be referenced.
Fig. 2 - Matching the wavelength response of instruments can involve many elements
If data is being compared to other labs, facilities or over time, it is important to understand if the same make and/or model of instrument is being used to ensure geometric compatibility. Likewise, instruments even of the same make and model will not agree if the photometry and spectral response are not correct. There are some calibration materials that can be used to check these parameters, but often the manufacturer may have to be involved if it is suspected that geometry, photometry or spectral response is incorrect.

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