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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.

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polarization the invisible property of light

Polarization: The Invisible Property of Light

Dr. Richard Young

Optronic Laboratories, Inc.

mil std 3009 mil l 85762a
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 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 polarization

Sky at sunset via a polarizing filter

kindly provided by Forrest Mims,

South-Central Texas Observatory

aquatic life
Aquatic Life

A Cuttlefish

Normal view

False color

Red = horizontal, Blue = vertical

polarization

aquatic life7
Aquatic Life

ctenophore plankton

Unpolarized light

Crossed Polarizers

Combined image

light is a wave
Light is a wave

Transverse Wave

Longitudinal Wave

Direction of travel

polarizer rules
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 rules11
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 rules12
Polarizer “rules”

4. The order of the polarizers doesn’t matter if only two are used.

I0

½I0

¼I0

polarizer rules13
Polarizer “rules”

5. Crossed polarizers (at 90˚) transmit no light

cos2(90˚) = 0

I0

½I0

No light

polarizer rules14
Polarizer “rules”

6. The order matters if 3 or more polarizers are used.

I0

½I0

No light

No light

polarizer rules15
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

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
Polarization by reflection

Fresnel Equations:

polarization by reflection19
Polarization by reflection

Reflected light completely polarized

chirality optical isomers
Chirality (Optical isomers)

Tartaric Acid Crystals

Each rotates the plane of polarized light in different directions

dichroism
Dichroism

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

birefringence double refraction
Birefringence (double refraction)

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

quarter wave plate depolarizer
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

Retardance is wavelength dependent, allowing real color indications of strain etc.

Plastic curve-drawing tool photographed between crossed polarizers.

measuring polarization
Measuring polarization

polarizer

Detector

analyzer

partial polarization
Partial polarization

Degree of polarization

measuring polarization29
Measuring polarization

OL 620-NVS telescope

550 nm

measuring polarization30
Measuring polarization

OL 620-NVS telescope

900 nm

measuring polarization31
Measuring polarization

OL 620-NVS telescope

measuring polarization32
Measuring polarization

Commercial camera lens

550 nm

measuring polarization33
Measuring polarization

Alternate telescope design (NOT an Optronic Labs product)

550 nm

measuring polarization34
Measuring polarization

Monochromator system without depolarizer

550 nm

mil std 3009 mil l 85762a35
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%.
conclusions
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
Acknowledgements

Thanks to:

Juan Carlos Gonzales

Steven Sparks

(Optronic Laboratories employees)

For some of the polarization measurements.