When do I use Specular Exclusion in Color Measurement?

FAQ: ASTM D6290 recommends that I measure my plastic pellets for yellowness using the specular exclude mode. What is this?

 Specular reflectance is not color. It is the shine or gloss that reflects from a very smooth surface. It is a geometric attribute related to light scattering off an opaque surface and independent of light absorption by the material which creates the perception of color.

Reflectance measurements in the specular excluded mode can be expressed as RSEX, SPEX or SEX in short forms.

Measurements made on an instrument with a directional 45/0 or 0/45 geometry are always specular excluded by definition of the geometry.

With sphere instruments with a diffuse d/8 sphere geometry, the user has a choice of measuring in the RSIN reflectance specular included or RSEX reflectance specular excluded modes.

Plastic pellets are typically measured in reflectance in a 50 mm cell on HunterLab sphere instruments.
On HunterLab sphere instruments, plastic pellets and other non-solid materials are typically measured behind the optically clear window of a 50 mm cell in RSEX reflectance specular excluded mode.

When measuring opaque or translucent samples that are not solid such as powders, pellets, granules or liquids, it is always necessary to contain them behind an optically clear window for color measurement.

The specular reflectance from the window is about 4% of the total signal – a small amount when measuring light colors but significant when measuring dark colors. Also the specular reflectance or gloss reflecting from the front window of the sample container is not related to the color of the sample. These two points make a good case for making a color measurement of non-solid samples in the RSEX Reflectance Specular Excluded mode on a diffuse d/8 sphere instrument.

FAQ: “If I measure our plastic pellets in a cell in Reflectance Specular Excluded mode, is it still required to apply Glass Correction to the measurement?”

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After excluding the specular reflectance, this just leaves the absorbance of the window (typically 0.3% for thin plastic; up to 0.5% for thicker glass) uncompensated for in an RSEX measurement.

The case of glass correction is less compelling than specular exclusion as the bias effect of the window absorbance is very small (often less than measurement variation), spectrally neutral and typically consistent from one container to another.

There are two ways to deal with the window absorption.

  • One approach is to implement a glass correction factor to the reflectance of the every sample measurement to compensate for the window absorbance. This can be done optically by standardizing the instrument with a container window in front of the instrument standards, or applying a mathematical formula (some assumptions required on surface interactions) to the data after the reflectance measurement is taken.
  • An alternative is to treat the window as a condition of measurement defined in your SOP (Standard Operating Procedure) by your choice of container. The color of opaque or translucent, non-solid powder, pellet, granule or liquid samples are always going to have to be measured in some form of container with a clear window.

Typical industrial color measurement practice is to:

  • Make the color measurement of an opaque or translucent, non-solid sample in the RSEX reflectance specular excluded mode to exclude the specular shine or gloss of the cell window from the color measurement of the sample.
  • Treat the slight absorbance of the glass or plastic window as an uncompensated condition of measurement defined by the cell in the measurement method.

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