Reflectivity and Reflectance Meters

The principle of Reflectivity came into its own with the need for a fast method for identifying gemstones with a refractive index over the limits of a standard refractometer.

• According to the French physicist Fresnel, reflectivity is the ratio of the intensity of the reflected ray to that of the incident ray.
• The light reflected from a surface is a measure of that surface’s reflectivity or lustre.
• Reflectivity is dependent mainly on the refractive index of the gemstone along with its structure and transparency.
• Reflectivity meters are calibrated in terms of comparative reflectivity and not in absolute terms, i.e. they indicate the differences in reflectivity between polished stones.
• Reflectivity meters are generally calibrated using diamond as a standard.
• Reflectivity meters consist of a light emitting diode (LED) which is an infra red source (approximate 930nm) which acts as the incident light, a detector to sense the amount of light reflected from the sample and a meter which displays it.

Important factors to be remembered while using these meters are as follows:

• Stone should be absolutely clean and dust free.
• Polish of the stone should not contain any scratches.
• An opaque cap must be placed over the sample to prevent external light from entering the instrument.
• A single sample should be examined a number of times in different directions so as to eliminate misleading results.
• It is useful for separating diamond from its simulants.
• There are a number of reflectivity meters in the market – namely the Jewelers Eye, Jemeter, Lustremeter etc.