Light radiates from its source randomly and equally in electromagnetic waves. Light polarization most often occurs because of reflection. Polarizing filters are used in photography and science to reduce the amount of glare produced by reflection. This filtering is used to give sharper definition of objects and deeper color rendition. Modern camera systems require a circular polarizing filter to allow proper auto-focus and auto-exposure operations.
What Is Polarized Light?
Light is electromagnetic energy that travels in waves. Normal light waves travel randomly from their source. Light waves radiate in a spherical pattern from the point of origin. Light rays become polarized when they reflect off of shiny surfaces such as chrome, a mirror or the surface of water. Polarized waves of light are no longer random. They now oscillate (vibrate) in only one plane or, with circular polarization, in two associated out-of-phase planes.
Linear Polarization
Light becomes linearly polarized when the electric component of the electromagnetic light energy is oriented to oscillate in a single plane. This occurs when light waves reflect off a surface. Reflected waves of light are oriented at an angle corresponding to the reflecting object. A vertical wall will cause the light wave to oscillate vertically. The surface of a lake or polished floor will result in light waves that oscillate horizontally. The light wave is represented as a sine wave oscillating equally on each side of the line of travel (electric field vector) of the light energy.
Circular Polarization
Circular polarization occurs when two linearly polarized light waves of equal amplitude travel perpendicular to each other while 90 degrees out of phase. One wave precedes the other as it moves along the electric field vector. Because of the phase difference, the two electromagnetic components cause the light waves to rotate around the electric field vector. When viewed end-on, a point on the electromagnetic wave follows a circular path, giving this form of polarization its name.
How a Circular Polarizing Filter Works
A circular polarizing filter has two elements. The first element is a linearly polarizing filter. This reduces or blocks scattered light from the sky entering at 90 degrees and light polarized by reflection from water, glass and other shiny surfaces. The second element is a quarter-wave plate. This element serves to convert the linearly polarized light from the first filter into circularly polarized light. Mount the right-sized filter on the camera lens. Then, looking through the view finder, rotate the lens element until the clearest image is seen.
Uses of a Circular Polarizing Filter
A circular polarizing filter is necessary to reduce glare and sharpen pictures when using modern (post-1970s) single-lens reflex (SLR) or digital SLR (DSLR) cameras. These cameras split the beam of light from the lens through the semi-silvered mirror. This mirror acts as a linear polarizer for light going to the auto-focus (AF) and auto-exposure (AE) sensors. This linearly polarized light is necessary for the proper operation of the AF and AE systems. Use of a linear polarizing lens filter causes light cancellation (blanking) or underexposure of photos because little or no light reaches the sensors. Use a circular polarizing filter to reduce glare and assure proper operation of AE and AF systems.
Tags: light waves, linearly polarized, electric field, electric field vector, field vector