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Monday, February 8, 2016

Gloss Calibration: Are gloss tiles enough?

The new TEST/Plus Gloss instrument is available in the market. There are many Technidyne T480 instruments, old Hunter, BYK Gardner and other glossmeters. One question that is often asked, do I really need paper calibration standards since I have gloss tile standards?

For the 75° glossmeter to be properly calibrated, it must be capable of measuring both the gloss tiles according to the assigned values and paper standards. The paper standards are used to adjust the iris in the optics to the correct diameter.

Gloss tiles are a more uniform material and much easier to calibrate due to the highly spectral reflectance (Fig. 1).  
Fig. 1 - Specular Reflectance
Paper has spectral but also diffuse reflectance (Fig. 2). Therefore, the result is mixed reflectance.
Fig. 2 - Mixed, Specular & Diffuse Reflectance
If only a tile were used for calibration, the iris (sensor in Fig. 3) could be opened too large or closed too small and the instrument would still calibrate. However, if a non-specular sample was then measured, like paper, the instrument would be measuring more than the appropriate gloss if the iris were opened too large. It would include haze. The instrument would not measure enough of the specular (gloss) reflectance if the iris were closed too small.  In either case, the instrument would not conform to the appropriate 75° gloss standard e.g. TAPPI T480, ISO 8254-1, etc.
Fig. 3 - Gloss measurement (Ref. BYK Gardner)
It is for this reason that paper calibration methods are available for people that will measure them. If the glossmeter were only used to measure highly specular tiles, then the use of only a gloss tile would be an acceptable method for calibration.  This is the case with the 20° geometry.  Geometry, lamp position, chromatic output, focus and iris are all factors that are checked during the production of good gloss instruments. These properties are also checked during on-site preventative maintenance visits.

Forward any of you calibration question or concerns.