Polarizing filter

A polarization filter (also briefly polarizing filter ) is a polarizer for light , the on dichroism based, so complementarily polarized light is absorbed , rather it as polarizing beam splitter to reflect . This makes it suitable for suppressing rays of light that vibrate in the “wrong” plane. This is used in photography, among other things, to suppress or emphasize reflections on non-metallic surfaces. It is also possible to use polarization filters directly next to a light source in order to obtain light that oscillates in the same way. It should be noted that the polarization filter can block a considerable amount of light and absorbs this amount of light by converting it into heat.

Camera polarizing filters in different orientations in front of an LCD monitor that emits linearly polarized light. In the inclined position the transmission is significantly reduced, in the transverse position it is almost zero.

historical development

Some scientists assume that already seafaring Vikings used a polarization filter ( sunstone ) to determine the direction to the sun in the cloudy sky.

As early as the 19th century, i.e. in the early days of photography, it was recognized that disruptive glossy reflections, for example when taking oil paintings or objects behind glass, could be suppressed by using polarizing media.

In the absence of alternatives, either Nicol prisms or tourmaline crystals were used initially .

The discovery of the strongly polarizing effect of artificially produced crystals from sulfuric acid iodine-quinine (" Herapathit ") by the English doctor William Bird Herapath (1820–1868) in 1851 initially led to the resulting crystals due to the small size of a maximum of 1 cm² no practical application. It was not until 1926 that A. Zimmer in Paris succeeded in producing flat crystal leaves of 2–3 cm that could be better used for optical purposes.

Ferdinand Bernauer then succeeded in 1935 in producing large, large-area, but only a fraction of a millimeter thick, monocrystalline surface filters from sulfuric acid quinine that could also be used for photographic purposes. For practical use, these crystals were mounted between two glass plates. The company Carl Zeiss took over the production, initially in memory of Herapath under the name Herapathit-Filter , but then from 1936 under the name Bernotar .

In contrast to this single-crystal filter, the multi- crystal filter ("Polaroid filter") developed by Edwin Herbert Land almost simultaneously with the Kodak company . A large number of tiny, aligned herapathite crystal needles are embedded in a colloid .

From 1939 onwards, large-area dichroic film filters made of cellulose-dye complexes ("cellipolar") were also available.

Linear polarizing filters

Linearly polarizing optical filters are usually made from macromolecular foils using the method developed by Edwin Herbert Land, which are plastically stretched in one direction. This stretching aligns the molecules in parallel. Diffused iodine attaches itself to these chains and provides charge carriers that are mobile in the direction of the chain molecules, which leads to the absorption of the parallel electric field component. These polarization filters, known as H filters, have become known under the Polaroid brand , like the films previously developed by Land with embedded herapathite crystals (called J-sheets ) .

Ideal linear polarizers are described by Malus' law.

Circular polarizing filters

Typical circular polarization filters such as those used in photography or 3D technology , like circular polarizers, generally consist of two optical elements connected directly in series: a linear polarization filter and a λ / 4 retardation layer or plate . - With circular polarization filters, these are always firmly connected to one another. First, (unpolarized) light passes through a linear polarization filter which, depending on the angle of rotation, allows a preferred direction of polarization to pass through - the other component is reflected or absorbed. The linearly polarized light then falls on the second element, the λ / 4 retardation layer, the optical axis of which is rotated by + 45 ° or −45 ° with respect to the linear polarization filter. As a result of the 45 ° rotation, the linearly polarized light can in turn be interpreted as the superposition of two linearly polarized light beams of the same phase perpendicular to one another. The λ / 4 retardation layer now causes a phase shift of δ = π / 2 of the two linearly polarized (partial) beams. The result is circularly polarized light that, depending on the rotation of the linear polarization element and the λ / 4 retardation layer, rotates left or right-hand. The quality of the circular polarization depends heavily on the efficiency of the linear polarization element and the exact alignment of the λ / 4 retardation layer, since otherwise components of other polarization would get to the λ / 4 retardation layer and produce a rotation different from 45 ° elliptically polarized light.

“Real” circular polarization filters generate circularly polarized light directly from unpolarized light, in that chiral molecules absorb the component with opposite chirality, see circular dichroism .

Another way of generating circularly polarized light is the Fresnel parallelepiped . Its function is not based on birefringence or circular dichroism, but on the total reflection of initially 45 ° linearly polarized light in a glass body with a special geometry. It is usually not called a circular polarizing filter.

Application in photography

Polarization filters are used in different ways in photography:

• Unwanted reflections from smooth, non-metallic surfaces (e.g. water, glass) can be suppressed. Light with perpendicular polarization is reflected noticeably more strongly on non-metallic surfaces, especially if the exit angle to the surface is about 30 ° to 40 °, i.e. close to the Brewster angle . If the polarization filter is appropriately aligned, the reflected light waves are suppressed so that the unpolarized background is not over-shined by the reflections . So it is e.g. B. possible to hide annoying reflections on window panes or water surfaces.
• The green rendering of leaves and grasses is improved because the polarization filter partially suppresses disruptive (blue) reflections from the sky.
• The blue of a cloudless sky is partially polarized. With the help of a polarization filter, a large part of the bright sky can be held back, so that the sky in the photo appears darker and therefore stronger in its color. White clouds stand out more clearly against the blue sky. This effect occurs particularly strongly at an angle of 90 ° to the sun, less or not at all at other angle values. This is most evident when photographing with an extremely wide-angle lens : often the sky is almost black on one side of the photo and light blue on the other.
• When photographing a rainbow , a polarizing filter has the following effects in both of its extreme positions: Since the color lines are polarized light, they are suppressed with suitable polarization - no rainbow is visible. If you turn the polarizing filter 90 ° out of this position, the rainbow is almost completely let through, the randomly polarized light from the clouds around it is swallowed up to a little more than half. Relative to the surroundings, the rainbow seems so much stronger.
• Unwanted reflections on metallic surfaces can be suppressed when using artificial light by using polarization filters on the camera and the lighting fixtures. Since the cost of the expensive large-format filter foils for the headlights is very high, this method is not used to any significant extent. Alternatively, you can work with a flash and polarization filters ("cross-pole flashes"). Since the polarization filter is used here at the light source, however, there is a great risk that the polarization filter will overheat if sufficient heat dissipation is not ensured.

• 

  Photograph: above without polarizing filter, below with

• 

  Photo (camera flash) without and with the circular polarization filter actually intended for monitors: metallic reflections disappear

• 

  An image taken exactly 90 ° to the sun with a polarizing filter and wide-angle lens : the sky is darkest in the middle, and it becomes lighter towards both sides

• 

  If the filter is used sparingly, the colors appear halfway natural

Special features in photography

In contrast to older cameras without autofocus , indoor exposure metering etc. Like., But also digital compact cameras without semitransparent mirrors, in which one can also fall back on simple linear polarization filters, linear polarization filters with subsequent circular polarization by a so-called λ / 4 retardation plate must be used in modern analog and digital SLR cameras . This is because linearly polarized light can lead to incorrect measurement results in some components of such cameras (e.g. the autofocus or the indoor exposure metering) and thus to malfunctions of the autofocus, for example. For this reason, circular polarization filters (CPL) have prevailed on the market today .

Due to their asymmetrical structure, the effect of circular polarization filters on linearly polarized light (such as reflections) can only be seen if you look through it from the side with the λ / 4 plate (with camera filters this is the side with the lens thread ). In the “wrong” direction, on the other hand, the λ / 4 plate generates an elliptical or circular polarization from the linear one, which can only be partially suppressed by the subsequent polarization filter.

If you arrange two linear polarization filters one behind the other and twist them against each other (at 90 ° to each other: “crossed”, “crossed pole”), you get the effect of a gray filter that can be darkened continuously . If you want to use the effect on current cameras, this is done in this arrangement:

• The filter that is screwed on on the motif side (front) must either be a linear one or a circular one used upside down.
• The camera side (rear) should be a circularly polarizing filter so that the polarization of the front filter does not affect the exposure measurement.

Many common filters no longer have a great blocking effect in the blue area. If these are used crossed, a bluish-tinted image is obtained with only moderate darkening.

A polarizing filter reduces the amount of incident light by around two f-stops.

Enhancement of colors and contrasts

In the following example, the motif was first photographed without a polarizing filter and immediately afterwards with a polarizing filter. The colors of the sky and the sea surface appear more saturated through the polarizing filter and the contrast increases. The filter also makes the leaves appear more colorful, more flat (less spatial) and more matt, because the filter absorbs the sheen of the leaves, which provides the viewer with information about the three-dimensional shape and surface texture of the leaves.

Without polarization filter	With polarization filter

Avoidance of reflections

The following example shows how a linear polarization filter affects the visibility of reflections on non-metallic surfaces, e.g. B. paint, glass and water. Polarization effects on metallic surfaces are much less pronounced.

Without polarization filter

The filter is in the plane of polarization of the reflections

The filter is perpendicular to the plane of polarization of the reflections

Examples of the effect of a polarizing filter:

• Left: Motif without polarizing filter
• Middle: The particularly noticeable reflections of the main subject (car) are highlighted, since the polarization plane of the filter is the same as the polarization plane of the dominant reflections. The other orientation of the window panes on the right in the picture causes a polarization that allows the polarizing filter to absorb the reflex. The light that is reflected by the leaves, for example, has many planes of polarization and appears unpolarized in its entirety. There the polarizing filter acts like a neutral density filter .
• Right: The reflections of the main subject are strongly attenuated; you can see through the windshield of the car. The camera's automatic exposure correction has increased the brightness of the background.

On closer inspection, it can still be seen here that the reflex of the bonnet only appears strongly darkened in a central spot, but not on its front edge, where the reflex occurs outside the Brewster's angle and not on the left rear corner. Sky reflections are also gradually retained at the corner adjoining here and the right half of the windshield in the image.

Light that is polarized parallel to the polarization filter direction remains bright: In particular, the bright, narrow strip of sky on the fender sheet metal on the left and the brownish reflection on the windshield and paint that comes from the yellow house wall on the right.

If you compare the wall of the house in the pictures (right picture area), the increase in brightness through the automatic exposure correction in the middle picture does not quite reach the level of the unfiltered picture (left); In the right picture, however, the exposure (based on the house wall in the unfiltered picture) is overcorrected.

Polarization filter with special color effects

Color changes are not desired with the usual polarization filters. A so-called warming polarizer takes out part of the blue light, so it works like a combination of a polarization filter and a light yellow filter. Red, yellow and green colors are emphasized and convey a warmer image impression.

There are special polarizing filters that emphasize polarized light in certain colors. Depending on the position of the filter and the direction of polarization, these are different contrasting colors. The resulting images experience significant color changes. There are such polarization filters for green and red as well as for yellow and blue. The yellow-and-blue filter is also marketed as the Gold-N-Blue filter. The area of ​​application is mostly in landscape photography. On cloudy days you can use it to create artificial blue and yellow colors in a gray sky or body of water. The red-green filter is rarely used in professional photography, as the results appear very artificial and are rarely aesthetically pleasing. The automatic white balance in digital cameras will not work properly with such filters.

Replacement by filters in digital post-processing

In many image processing programs, the function of the polarization filter to change colors or contrasts can in principle also be carried out manually or through digital filters. Since the image processing program i. A. If there is no information about the polarization of the incident light, the function of the polarization filter can at best be approximated.

Reflections cannot be removed retrospectively by digital filters, as the information in the image is lost when the saturation value is reached. A reflection can only be retouched manually.

General uses

Demonstration of mechanical stress during deformation

Tensions in glass

• In the stress optics , mechanical stress (stresses and stress peaks) in technical components were made visible by reproducing the components made of Plexiglas and placing them between polarization filters. The tensions lead to lines with different colors, which indicate the level of tension due to their density. In the meantime, the method has been replaced by the computational determination of the stresses using the finite element method , which is not limited to two dimensions. If a transparent plastic lens is deformed between two crossed polarization filters, the plane of polarization is rotated depending on the force. Colored structures can be seen in the light shining through.
• Polarizing filters are used in scientific instruments, e.g. B. polarization microscopes , used to make structures in thin sections stand out more clearly. Two polarizing filters are used in polarimeters to measure the optical activity of organic substances .
• One of the methods for projecting 3D films uses polarization filters to feed the two images, recorded from two different points and projected on top of one another, to the right and left eyes.
• In the 1950s, polarization filters were used as "sky compass" for determining the position of the sun in cloudy skies in polar seas for navigation , where the magnetic compass is not very helpful either. A corresponding use of a " sun stone " by Vikings is assumed as early as the 9th to 11th centuries.
• Linear polarization filters are indispensable components of liquid crystal screens .
• Polarization filters - with a vertical direction of polarization - are also used for sunglasses , which are then called polar glasses or “fishing glasses”. This opens a dark window for anglers - only near Brewster's Winkel - through the light reflection of the sky and sun, so that they can look into the flat water. Boaters also benefit - especially when the sun is at a medium altitude - as long as they don't tilt their heads sideways. On foot and by bike, the structure of the pavement appears clearer only in the area of ​​two to three steps in front of you in backlight, because it is less reflective, you can see the bottom of a puddle of water. However, such glasses are counterproductive for the sideways view into a shop window along the way, they emphasize this reflex on the - vertical - pane of glass. The reflex from their own engine hood is softened for motorists, but hardly at all that produced by distant car headlights on a wet road at night, because the reflex angle is too shallow.

Web links

Commons : polarizing filter  - collection of images

• Cross-pole flashes: polarizing filters and polarizing filter foils in practical use with flash units
• Polarizing Filters - Myth and Practice

Individual evidence

1. ↑ Fritz Meisnitzer: photography made easy. A guide for beginners and advanced users. Book Guild Gutenberg, 1973, ISBN 3-7632-1689-8 , pp. 264-265.
2. ↑ Ramón Hegedüs et al .: Could Vikings have navigated under foggy and cloudy conditions by skylight polarization? On the atmospheric optical prerequisites of polarimetric Viking navigation under foggy and cloudy skies. In: Proc. R. Soc. A. Volume 463, No. 2080, 2007, pp. 1081-1095, doi: 10.1098 / rspa.2007.1811 .
3. ^ Martin Grabau: Polarized Light Enters the World of Everyday Life . In: Journal of Applied Physics, Volume 9, April 1938, No. 4, p. 217.
4. ↑ Wolfgang Baier: Source representations for the history of photography . 2nd edition, Schirmer / Mosel, Munich 1980, ISBN 3-921375-60-6 , p. 323 ff.
5. ^ State of polarization of light (PDF file; 298 kB). Faculty of Physics and Astronomy, Friedrich Schiller University Jena, p. 3.
6. ↑ F. Pedrotti, L. Pedrotti, W. Bausch, Hartmut Schmidt: Optics for engineers: Fundamentals . Edition: 3rd, edit. u. updated. Springer, Berlin 2005, ISBN 3-540-22813-6 , pp. 413 ff .
7. ↑ Article on polarizing filters on matthiashaltenhof.de