Polarimetry
As mentioned in our article on polarization, light travels as an electromagnetic wave. This wave vibrates in a direction that is perpendicular to its direction of travel. In other words, if the wave is traveling towards you, then as it travels toward you it will vibrate horizontally, vertically, or any angle in between. In addition, the wave may vibrate in a rotating pattern as it travels towards you (think of a corkscrew). All of these different behaviors go to make up the light's polarization state. Polarimetry is the measurement of the polarization characteristics of light either as it is emitted from a light source (ex. a light bulb or a laser) or as it is reflected off of or transmitted through a material (water, glass, etc.). Polarimetry is composed of many components and values including DoLP (Degree of Linear Polarization), Jones vectors, Stokes parameters, and the Mueller matrix and the devices which measure these values are called polarimeters.
Light travels through or reflects off of a material by interacting with the electrons contained in the atoms of the material. When linearly polarized light travels through some materials, it is rotated to the left. When it travels through other materials, it is rotated to the right. Some materials don't rotate the light at all. It is through polarimetry that such rotation can be determined. The amount of rotation or lack thereof can tell us important things about the material making polarimetry an important measurement tool.
Polarimetry: Imaging Polarimeters
An imaging polarimeter, as the name implies, measures the polarization state of an object as a function of object coordinate (pixel coordinate). The polarized light can be either emitted or reflected from the object. To measure the polarization state at each pixel, the imaging polarimeter must capture multiple measurements of the polarization state emitted or reflected from the pixel. For example if the fraction of horizontally polarized light vs total light must be known, then a minimum of two measurements are required, one of the intensity of the horizontally polarized light, and one of the intensity of the vertically polarized light. Or, alternatively, one measurement must be made of horizontally polarized light and one of total intensity. To completely characterize the polarization state of the pixel, a minimum of four measurements of polarized intensity must be made.
Several different polarimetry methods have been employed to measure the polarization content of an image. The following briefly describes a number of approaches for imaging polarimetry.
Polarimetry: Pixelated Polarizer Imaging Polarimeters
In this approach a checkerboard array of polarizers are overlaid onto a focal plane array (FPA). The transmission axis orientation for polarizer pixels are varied in a regular fashion. For example, in a 2x2 array of pixels, the orientations could be 0, 45, 90, and 135 degrees. Thus each pixel would be measuring the fraction of light polarized in the corresponding polarizers orientation.
Each 2x2 array can be considered a super-pixel. The spatial variation of the scene would have to be less than the corresponding projected area of the super-pixel or a cross-coupling between intensity variation and polarization content would occur. This is the downside to this approach. The spatial resolution of the FPA is traded for determining the polarization content of the scene. For many applications, this is acceptable. The advantages of this approach are speed, and size. All of the polarization information is captured in a single frame, thus minimizing artifacts due to misregistration between multiple images of polarization. The impact to the size of the sensor is negligible, because only one component, added to the surface of the focal plane, is required.
Polarimetry: Division of Aperture Imaging Polarimeters
n this approach, optics are used to duplicate the image of a scene of multiple quadrants of the focal plane array (FPA). The irradiance of the aperture is subdivided spatially optically. The irradiance of each sub-aperture is imaged onto its corresponding quadrant of the focal plane. Thus the image of the scene is replicated multiple times on the FPA. Polarization optics are included in each sub-aperture to accept a particular polarization state. For example, if four sub-apertures are used, then polarizers at 0, 45, 90, and 135 degrees can be used to measure those polarization states. To determine the fraction of horizontally polarized light over vertically polarized light, then the images from quadrants corresponding to 0 and 90 degrees are subtracted. To determine the fraction of 45 degree polarized light over 135 depolarized light, then images from quadrants corresponding to 45 and 135 degrees are subtracted.
The advantage of this method over pixilated polarizer is that there is no restriction to the spatial frequency of the scene. Another advantage is that it captures all the polarization in a single image. The disadvantage is that it requires very complicated optics to replicate multiple images on a single FPA.
The Division of Aperture polarimeter can be made into a single pixel polarimeter as well. Unlike the optics for the imaging Division of Aperture polarimeter, the optics for this method are relatively simple, owing to the single point field of view. The Division of Aperture method is very advantageous for the single pixel polarimeter because it can be made very small, with no moving parts.
Rotating Polarization Element Polarimeters
A popular method for realizing a polarimeter is the rotating polarization element polarimeter. This is also called a sequential measurement in time polarimeter. For this method, a sensor takes a measurement with a polarization element in the optics, usually placed in a pupil. After the first measurement is taken, the polarization element is rotated to a new position, and another measurements is taken. This step is repeated until a sufficient number of measurements are taken to determine the desired metric of polarization. This method has been implemented for single pixel polarimeters as well as imaging polarimeters. The advantage of this method is that it is relatively simple to build. The disadvantage is that it takes multiple measurements sequentially in time. If there is movement in the sensor or the scene between the measurements, then the image motion will show up as spurious polarization signature.
As you can see, the science of polarimetry adds a great deal to our quality of life. If you have an application where polarimetry is required, feel free to call. For more information on polarimetry, you may also refer to our pages on polarization and polarimeters
As mentioned in our article on polarization, light travels as an electromagnetic wave. This wave vibrates in a direction that is perpendicular to its direction of travel. In other words, if the wave is traveling towards you, then as it travels toward you it will vibrate horizontally, vertically, or any angle in between. In addition, the wave may vibrate in a rotating pattern as it travels towards you (think of a corkscrew). All of these different behaviors go to make up the light's polarization state. Polarimetry is the measurement of the polarization characteristics of light either as it is emitted from a light source (ex. a light bulb or a laser) or as it is reflected off of or transmitted through a material (water, glass, etc.). Polarimetry is composed of many components and values including DoLP (Degree of Linear Polarization), Jones vectors, Stokes parameters, and the Mueller matrix and the devices which measure these values are called polarimeters.
Light travels through or reflects off of a material by interacting with the electrons contained in the atoms of the material. When linearly polarized light travels through some materials, it is rotated to the left. When it travels through other materials, it is rotated to the right. Some materials don't rotate the light at all. It is through polarimetry that such rotation can be determined. The amount of rotation or lack thereof can tell us important things about the material making polarimetry an important measurement tool.
Polarimetry: Imaging Polarimeters
An imaging polarimeter, as the name implies, measures the polarization state of an object as a function of object coordinate (pixel coordinate). The polarized light can be either emitted or reflected from the object. To measure the polarization state at each pixel, the imaging polarimeter must capture multiple measurements of the polarization state emitted or reflected from the pixel. For example if the fraction of horizontally polarized light vs total light must be known, then a minimum of two measurements are required, one of the intensity of the horizontally polarized light, and one of the intensity of the vertically polarized light. Or, alternatively, one measurement must be made of horizontally polarized light and one of total intensity. To completely characterize the polarization state of the pixel, a minimum of four measurements of polarized intensity must be made.
Several different polarimetry methods have been employed to measure the polarization content of an image. The following briefly describes a number of approaches for imaging polarimetry.
Polarimetry: Pixelated Polarizer Imaging Polarimeters
In this approach a checkerboard array of polarizers are overlaid onto a focal plane array (FPA). The transmission axis orientation for polarizer pixels are varied in a regular fashion. For example, in a 2x2 array of pixels, the orientations could be 0, 45, 90, and 135 degrees. Thus each pixel would be measuring the fraction of light polarized in the corresponding polarizers orientation.
Each 2x2 array can be considered a super-pixel. The spatial variation of the scene would have to be less than the corresponding projected area of the super-pixel or a cross-coupling between intensity variation and polarization content would occur. This is the downside to this approach. The spatial resolution of the FPA is traded for determining the polarization content of the scene. For many applications, this is acceptable. The advantages of this approach are speed, and size. All of the polarization information is captured in a single frame, thus minimizing artifacts due to misregistration between multiple images of polarization. The impact to the size of the sensor is negligible, because only one component, added to the surface of the focal plane, is required.
Polarimetry: Division of Aperture Imaging Polarimeters
n this approach, optics are used to duplicate the image of a scene of multiple quadrants of the focal plane array (FPA). The irradiance of the aperture is subdivided spatially optically. The irradiance of each sub-aperture is imaged onto its corresponding quadrant of the focal plane. Thus the image of the scene is replicated multiple times on the FPA. Polarization optics are included in each sub-aperture to accept a particular polarization state. For example, if four sub-apertures are used, then polarizers at 0, 45, 90, and 135 degrees can be used to measure those polarization states. To determine the fraction of horizontally polarized light over vertically polarized light, then the images from quadrants corresponding to 0 and 90 degrees are subtracted. To determine the fraction of 45 degree polarized light over 135 depolarized light, then images from quadrants corresponding to 45 and 135 degrees are subtracted.
The advantage of this method over pixilated polarizer is that there is no restriction to the spatial frequency of the scene. Another advantage is that it captures all the polarization in a single image. The disadvantage is that it requires very complicated optics to replicate multiple images on a single FPA.
The Division of Aperture polarimeter can be made into a single pixel polarimeter as well. Unlike the optics for the imaging Division of Aperture polarimeter, the optics for this method are relatively simple, owing to the single point field of view. The Division of Aperture method is very advantageous for the single pixel polarimeter because it can be made very small, with no moving parts.
Rotating Polarization Element Polarimeters
A popular method for realizing a polarimeter is the rotating polarization element polarimeter. This is also called a sequential measurement in time polarimeter. For this method, a sensor takes a measurement with a polarization element in the optics, usually placed in a pupil. After the first measurement is taken, the polarization element is rotated to a new position, and another measurements is taken. This step is repeated until a sufficient number of measurements are taken to determine the desired metric of polarization. This method has been implemented for single pixel polarimeters as well as imaging polarimeters. The advantage of this method is that it is relatively simple to build. The disadvantage is that it takes multiple measurements sequentially in time. If there is movement in the sensor or the scene between the measurements, then the image motion will show up as spurious polarization signature.
As you can see, the science of polarimetry adds a great deal to our quality of life. If you have an application where polarimetry is required, feel free to call. For more information on polarimetry, you may also refer to our pages on polarization and polarimeters
