Polarization: The Invisible Property of Light

Polarization: The Invisible Property of Light Dr. Richard Young Optronic Laboratories, Inc.

MIL-STD-3009 (MIL-L-85762A) • A.4.6 Photometer polarization error. • The polarization error shall be no greater than 1%. • A.4.6.1 Photometer polarization error verification. • The polarization error shall be checked by placing a linear polarizer in the optical path between the standard lamp and the photometer and then measuring the luminance. The polarizer shall be rotated 45 and another measurement shall be made. The polarizer shall be rotated another 45 and another measurement shall be made. The photometer shall be considered as having passed the polarization error test if the difference between the three measurements is lower than or equal to the percent error specified in A.4.6. Throughout the test the alignment of the standard lamp shall not be changed. The transmission of the linear polarizer shall be greater than or equal to 20%, and the transmission of two pieces of the polarizer material, when oriented so that the direction of polarization of the two pieces are at right angles, shall be less than or equal to 0.1%.

The Vikings the weather was thick and stormy . . . The king looked about and saw no blue sky . . . then the king took the sunstone* and held it up, and then he saw where [the Sun] beamed from the stone • Hrafns Saga (possibly composed by priest Tómas Þórarinsson from Selárdalr d.1253). * Probably crystal cordierite, common in Norway

Sky polarization Sky at sunset via a polarizing filter kindly provided by Forrest Mims, South-Central Texas Observatory

Glare from water

Aquatic Life A Cuttlefish Normal view False color Red = horizontal, Blue = vertical polarization

Aquatic Life ctenophore plankton Unpolarized light Crossed Polarizers Combined image

Light is a wave Transverse Wave Longitudinal Wave Direction of travel

Polarizers

Polarizer “rules” 1. A polarizer will transmit 50% of an unpolarized light beam. This applies to ALL orientations. 2. If a second polarizer is aligned in the same orientation as the first, it has no effect. I0 ½I0 ½I0

Polarizer “rules” 3. Malus’ Law: if the angle between two polarizers is , then the effect of the combination (ignoring absorption) is cos2() cos2(45˚) = ½ I0 ½I0 ¼I0

Polarizer “rules” 4. The order of the polarizers doesn’t matter if only two are used. I0 ½I0 ¼I0

Polarizer “rules” 5. Crossed polarizers (at 90˚) transmit no light cos2(90˚) = 0 I0 ½I0 No light

Polarizer “rules” 6. The order matters if 3 or more polarizers are used. I0 ½I0 No light No light

Polarizer “rules” 6. The order matters if 3 or more polarizers are used. I0 ½I0 ¼I0 ⅛I0 It is clear from this that polarizers do not simply absorb light.

Liquid crystals Liquid crystals in displays use this principle to transmit maximum light when “on”. The alignment of the polarizer “stack” changes with voltage.

Polarization by reflection Fresnel Equations:

Polarization by reflection Reflected light completely polarized

Chirality (Optical isomers) Tartaric Acid Crystals Each rotates the plane of polarized light in different directions

Dichroism Polarization-dependent absorption (generally highly wavelength dependent so colors can be separated).

Birefringence (double refraction) Polarization-dependent refraction. One slow and one fast ray means they take different paths through the material, e.g. calcite

Birefringence (double refraction) o-ray e-ray

Quarter wave plate depolarizer The retardance of one polarization relative to the other makes the waves emerge out of phase. In the case of a quarter wave plate, by 90°. Circular polarized light Linearly polarized light at 45°

Retardance Retardance is wavelength dependent, allowing real color indications of strain etc. Plastic curve-drawing tool photographed between crossed polarizers.

Measuring polarization polarizer Detector analyzer

Partial polarization Degree of polarization

Measuring polarization

Measuring polarization OL 620-NVS telescope 550 nm

Measuring polarization OL 620-NVS telescope 900 nm

Measuring polarization OL 620-NVS telescope

Measuring polarization Commercial camera lens 550 nm

Measuring polarization Alternate telescope design (NOT an Optronic Labs product) 550 nm

Measuring polarization Monochromator system without depolarizer 550 nm

MIL-STD-3009 (MIL-L-85762A) • A.4.6 Photometer polarization error. • The polarization error shall be no greater than 1%. • A.4.6.1 Photometer polarization error verification. • The polarization error shall be checked by placing a linear polarizer in the optical path between the standard lamp and the photometer and then measuring the luminance. The polarizer shall be rotated 45 and another measurement shall be made. The polarizer shall be rotated another 45 and another measurement shall be made. The photometer shall be considered as having passed the polarization error test if the differencebetween the three measurements is lower than or equal to the percent error specified in A.4.6. Throughout the test the alignment of the standard lamp shall not be changed. The transmission of the linear polarizer shall be greater than or equal to 20%, and the transmission of two pieces of the polarizer material, when oriented so that the direction of polarization of the two pieces are at right angles, shall be less than or equal to 0.1%.

MIL-STD-3009 measurements

Conclusions • Although humans do not see polarization of light, its effects are wide ranging. • Spectroradiometers and telescopes can have large polarization errors unless corrected. • These polarization errors can lead to incorrect luminance and color values. • MIL-STD-3009 says how polarization error must be measured. • But the method is open to interpretation and may give inconsistent results.

Acknowledgements Thanks to: Juan Carlos Gonzales Steven Sparks (Optronic Laboratories employees) For some of the polarization measurements.

Polarization: The Invisible Property of Light