Polarised Light Source
Polarised Light Source
An illuminator with a polarising filter, the foundation of the crossed-polar examination
A polarised light source is an illuminator equipped with a polarising filter that selects a single plane of vibration from the unpolarised output of an ordinary lamp or LED. In gemmology it is the foundation accessory for crossed-polar examination: when paired with a second polarising filter, the analyser, oriented at ninety degrees, the resulting crossed-polar configuration is the standard tool for revealing strain, twinning, and anomalous double refraction in cut and rough gemstones.
How polarisation works
Light from an ordinary source vibrates in all planes perpendicular to its direction of travel. A polarising filter transmits only the component vibrating in one chosen plane and absorbs the rest. The transmitted beam is then plane-polarised, and its subsequent interaction with a gemstone depends on the optical character of the stone. An isotropic material — diamond, spinel, glass, garnet — does not change the polarisation, and a second crossed analyser extinguishes the beam to darkness. An anisotropic material — corundum, beryl, tourmaline, quartz — splits the polarised beam into two component rays travelling at different velocities, and the rotated polarisations recombine through the analyser to give visible transmission.
What it reveals
Under crossed polars, isotropic stones remain dark while anisotropic stones brighten and dim through extinction four times in a single 360-degree rotation. Strain, whether residual from cooling or induced by heat treatment, breaks isotropic symmetry locally and produces patches of anomalous double refraction visible as brightenings against an otherwise dark field. Tatami strain in flame-fusion synthetic corundum, snake strain in glass, and cross-hatched strain in synthetic spinel are diagnostic patterns long catalogued in the gemmological reference.
In practice
The polarised light source is sold as a stand-alone polariscope, as a built-in feature of higher-end gemmological microscopes, and as a snap-on accessory for converting a standard microscope to crossed-polar work. LED-based units have largely replaced the older incandescent designs, offering cooler operation and longer service life. Many laboratories prefer a cool, diffused source to avoid heating the immersion fluid during prolonged observation.