Polarizing Filters by Reflection

Light striking a smooth surface is reflected from it at an angle equal to and opposite from the angle of incidence.

Nonmetallic Surfaces

  • Some surfaces reflect a very high percentage of the light that strikes them; others reflect some and absorb some.

  • With dielectric (nonconducting) materials, a portion of the light that is reflected is polarized. The plane of polarization is parallel to the reflecting surface, and is strongest at a single reflectance angle particular to each material. Polarization diminishes rapidly as the angle changes. The angle of maximum polarization varies according to the nature of the material reflecting the light, but for most substances it is between 30 and 40 degrees. Reflected light so polarized can be absorbed by a properly oriented polarizing filter. Thus, reflections or glare on many surfaces can be partially or completely eliminated.

  • The controlling factor in eliminating reflections, if the effect is to be maximized with a polarizing filter, is the necessity of using the reflection angle of the material involved as the angle of view of the camera. Thus, if the angle of strongest polarized reflection from the water is 37 degrees, then the camera angle must also be 37 degrees, or the control of the polarized reflection will only be partial. Such a camera angle is often not desirable, and occasionally not even practical. So this mode of control is not perfect.

Metallic Surfaces

  • Although the vast majority of materials likely to be encountered in general photography have the property of being able to plane polarize light by reflection, however, metals do not, for reasons not well understood. Because the reflection of an un-polarized incident light beam from a smooth metallic surface does not produce plane polarization, polarizers cannot eliminate such reflections.

  • All is not lost, however. If the incident beam is already plane polarized when it strikes the metallic surface, as is the light from various portions of the sky, it will remain plane polarized after reflection, and the strength of the reflection can be altered by a polarizing filter in the normal manner. For, even as a metal surface will not plane polarize a beam by reflection, so it will not depolarize it.

Opposing Surfaces

  • There are occasions when two surfaces, oriented at an angle to one another, both reflect light toward the camera lens at approximately the polarizing angle. Because the planes of polarization of the two reflections are parallel to the reflecting surfaces, only one at a time can be maximally absorbed by a polarizing filter. If the angle between the two surfaces is 90 degrees, the filter will be least effective for one reflection just when it will be most effective for the other, and vice versa. The best overall effect can only be partial.

More about Camera Filter

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