Polarized Light Photography (PL)

Polarized light photography (PL) is a useful method for documentation of works of art and archaeology. PL photography eliminates surface reflections and professional photographers use it for high quality documentation of varnished art.

 


Applications in Art examination

PL photography  eliminates glare in photos, in particular, disturbing reflections from varnish. Light from the lamps can be reflected or diffused from a painting. Reflected light gives glare while diffused light provides colors and texture. When the light is reflected it keeps its polarization while diffused light loses it. So, when polarized light illuminates a painting, the glare  is still a polarized beam while diffused light vibrates in any direction.

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Experimental Set up

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Light from a bulb or an LED lamp is non-polarized. Light waves vibrate in any direction while propagating from the light source into space. A single particle of light can be imagined as a wave which propagates from its source into space. A beam of light from the sun, an halogen or an LED lamp, is composed of multiple light waves which vibrate in any directions.

 

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To polarize light we put in front of it a polarizing filter which allows through just the waves vibrating in one specific direction.

A classical experiment with polarized light is that done with a calcite crystal and a polarizing filter.

 

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Reflected light keeps its polarization while diffused light vibrate in any direction.

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The final step is to add a polarizing filter on our camera and rotate it 90 degree so to extinguish all reflected light, i.e. disturbing glare.

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Circular filter on camera at 0 degree (Up figure) and 90 degree (Down figure). The Polarized light photograph shows colors more saturated and glare extinguished. See detail of coin.

For manual focus it can be used a linear polarizing filter. A circular polarizing filter is necessary for auto focus. Indeed, the camera focusing system doesn’t work properly with polarized light. The circular polarizing filter resolves this problem. It is essentially a liner polarizing filter that diffuse the polarized light just before feeding it to the camera.

A video on linear polarizing filter and circular polarizing filter: