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When to use the Polarization filter

Different materials reflect light in distinct ways, that can be characterized with a reflectance function. The reflectance function of a glossy material has a strong specular peak (see Fig. 1), which indicates that most of the light reflected by the surface has with the surface the same angle as the incident ray. Matte surfaces, on the other hand, show a diffuse reflectance function, which means that a ray of light coming from the sources gets reflected at a random direction.

The purpose of spectrophotometers as specified in the ISO 13655 is to measure the diffuse component of the reflectance. Therefore, 45/0 devices, such as the Barbieri Spectro LFP qb, illuminate the surface under study at 45 degrees and detect the light reflected at normal direction. This permits to neglect the gloss peak, since this is detected only when the viewing direction is specular to the illuminating one.
However, sometimes one need to measure the color of wet ink, which could generate specular peaks in the direction of the detector. If the light of this peaks is acquired by detector, it can greatly distort the measured values.

For this reason, the M3 measurement condition was introduced. The M3 setup includes two linear polarizers orthogonal to each other, one in front of the light source and the other before the detector. In fact, while the rays of light included in the specular peak are polarized in the same direction, the diffuse ones have a random polarization. 

Therefore, the filter in front of the detector allows almost only diffuse light beams to pass, getting rid of the superficial gloss (see Fig. 2,3,4).

Fig. 2: Chart printed on glossy material

Fig. 3: Surface without polarizationfilter

Fig. 4: Surface with polarization filter

Empirically, the M3 condition has demonstrated to improve the precision of measurements in various cases, for example when one is printing glossy textured surfaces, or when bronzing is involved, or when one wishes to measure the colour while the ink is still wet. However, it should be used with caution: in fact, without the effect of specular reflection, the measured lightness of dark colors is usually lower than with M0, M1 and M2 measurement conditions, therefore the resulting gamut appears wider. This effect could distort the measured hue of the sample and provide quantities that don’t correspond to real viewing conditions. Therefore, the M3 condition should be adopted only when a glossy irregular surface needs to be measured.

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