The LRA consists of eight circular 1.27-cm diameter corner-cube retroreflectors mounted on a 5.11 cm diameter, 1.65 cm high hemispherical gold-painted platform. Each of the eight retroreflectors points in a slightly different direction, and each has a maximum useful light incidence angle of about +-20 degrees. Four retroreflectors are evenly distributed in a ring on the hemisphere 20 degrees from zenith, and four in a ring 40 degrees from zenith. They are made from Suprasil quartz, index of refraction is 1.46. The equivalent optical cross section is 10,000 to 100,000 square meters, but is still not enough to use Earth-based laser ranging. Total mass of the LRA is 20 grams, it requires no power.

Although wide angle lenses certainly aren't ideal, rotating the polarizing filter can sometimes make the effect appear more realistic. One approach is to ensure that the most pronounced polarization coincides with the image's edge or corner. This way the change in polarization will look more like a natural gradient across the sky (such as how the sky might appear during twilight).

Since polarizers reduce direct reflections, this often has the consequence of also reducing image contrast. This can make it easier to capture scenes with a broad dynamic range, such as trying to balance a bright sky with relatively unreflective land (which can even make using a graduated neutral density filter or high dynamic range less important).

*Note: polarizers can sometimes enhance the color and contrast of a rainbow by darkening background clouds, but only if the filter has been rotated just right. Furthermore, including both ends of a rainbow usually requires a wide angle lens, in which case the scene/rainbow may appear uneven.

Polarizer film

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Furthermore, sometimes reflections are essential to a photograph. Two key examples include sunsets and rainbows*; use a polarizer on either and the colorful, reflected light may disappear if the polarizer is rotated for maximum effect.

Polarizer examples

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In the example to the left, the sun was near the horizon, so the strip of sky directly overhead was most influenced by the polarizer (causing it to appear darker), whereas the upper left and lower right regions (nearer the horizon) were much less impacted. If a telephoto lens had been used to photograph just the tower, then the sky would have appeared much more even.

Polarizer Filter

However, just because the filter is capable of its maximum effect in the above directions, this doesn't necessarily mean this is where the image will appear most effected. Rotating your filter will toggle the angle (relative to the sun) that appears most polarized. The best way to get a feel for this is to rotate the filter while looking through the camera's viewfinder (or rear LCD), but you can also consult the box below for specifics on how this process works.

A polarizing filter can be an extremely powerful tool for removing reflections and isolating objects which are wet, underwater or behind a window. In the example below, a polarizer enables the photographer to select between subjects which are reflected from or are underneath the water's surface:

Since a polarizer's effect varies with angle, results can appear uneven when using a wide angle lens. Some portions of the scene might be in a direction which is directly into the sun, whereas others might be at a right angle to the sun. In that case, one side of the photo would have a strong polarizer effect, whereas the other side would not.

Examples ofpolarizersin Physics

Notes on Filter Rotation Angle. At one extreme, you can rotate your filter so that the direction of maximum polarization will be perpendicular to the direction of the sun (as shown in the above examples). In that case, the polarizing effect will be as pronounced as possible. If you then rotate your filter just a little (say 10-20°), you can shift the angle of maximum effect slightly towards or away from the sun, but in this case the polarizing effect won't be as pronounced as before. As this angle gets progressively closer to the direction into or away from the sun, the polarizing effect will appear progressively less pronounced. Finally, once the filter has been rotated a full 90° (to the other extreme), then no polarizing effect will be visible. Any more rotation than this and the polarizing effect increases again and the cycle repeats.

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In the example to the right, the effect on the stone and foliage is subtle, but the sky becomes a noticeably darker blue. Take care not to overdo this effect; unusually dark mid-day skies or overly vibrant foliage can easily make photos appear unrealistic.

The Intuitive Machines 1 Odysseus lander touched down on the Moon on February 22, but tipped over and came to rest tilted at 30 degrees to the horizontal. It is believed the LRA is still in a position where it will be able to return laser transmissions from orbiting spacecraft.

A polarizer can also remove unwanted reflections when taking a photo out of a window or other transparent barrier. Move your mouse over the example to the left to see how a polarizer eliminates the window reflections. This can be a very useful tool when photographing objects within store windows, out a moving train or within a glass case, for example.

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However, saturation isn't always increased uniformly. This all depends on whether a particular object is at an optimal angle to the sun, and whether this object is highly reflective. In general, more reflective objects will see a greater increase in saturation when using a polarizer. Clear sunny days are also much more heavily influenced by polarizers than overcast or rainy days.

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A polarizing filter will be capable of its maximum effect when one's line of sight (in red below) is perpendicular to the direction of the sun:

However, polarizers can also sometimes create an unrealistic-looking rainbow or ripple effect on windows which are uneven, have been tinted or are treated with coatings. A good example of this is something called "birefringement," which appears when taking a polarized photo through an airplane window:

What are polarizersin physics

In other situations polarizers can instead increase contrast. In the next example, the polarizer increased contrast by filtering the light reflecting off of the haze and sea spray. This effect appears most pronounced in the hills and the puffy clouds directly overhead:

What are polarizersused for

A good way to visualize this is to aim your pointer finger at the sun while holding your thumb straight up. Everywhere your thumb points when you rotate your hand (while still pointing at the sun) is where the polarizer is capable of the strongest impact.

Note that laser altimeters are not designed to be used to range to the LRAs, they have very small footprints and would only "hit" an LRA on occasional passes. Lidars with wider footprints are more efficient for actual ranging from orbiters. Also note that if the range to an orbiter can be precisely determined (e.g. from a larger LRA on the orbiter that can be ranged to from Earth), an accurate distance from Earth to the LRA on the lunar surface can be obtained. Measurements from multiple LRAs at different points on the Moon can be used to create an accurate geodetic network.

However, less glare/contrast is sometimes undesirable. In the example below, the artistic intent was to (literally) highlight the curving road by portraying it in stark contrast to its surroundings. Using a polarizer actually detracted from this goal:

How dopolarizerswork physics

On the other hand, in most other situations a decrease in glare is desirable and generally creates a more pleasing photo. For instance, in the above example the light doesn't appear as harsh and reflective on the rocks to the far right.

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In the above example, note how the polarizer was unable to remove reflections entirely (although it did a pretty good job). This isn't ever possible, but fortunately polarizers are usually able to make reflections imperceptible unless they're relatively intense. Unfortunately the one exception is with metal surfaces — which often also happens to be the brightest and least desirable type of reflection.

The Laser Retroreflector Array (LRA) is designed to use reflected laser light from orbiting spacecraft laser (typically a laser altimeter or light detection and ranging - lidar) to precisely determine the location of the lander, as a fiducial marker, and the distance to that point on the lunar surface with respect to the orbiter. The retroreflectors reflect any light striking them directly back to the source. They can be tracked by an orbiting laser altimeter or lidar from a few hundred kilometers.

The above panorama would have appeared uneven with a polarizer, and the rainbow could have even disappeared at some positions. Photo from Arches National Park - Utah, USA.

Polarizer material

In general, using a polarizer on clouds and skies will almost always increase contrast, but if the subject itself is highly relfective then a polarizer will instead likely decrease cotrast.

Polarizers are placed in front of your camera lens, and work by filtering out sunlight which has been directly reflected toward the camera at specific angles. This is beneficial because the remaining light is often more diffuse and colorful, but it also requires a longer exposure time (since light has been discarded). The angle that is filtered is controlled by rotating the polarizer itself, and the strength of this effect can be controlled by changing the camera's line of sight relative to the sun.

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Polarizing filters can increase color saturation and decrease reflections — and are one of the only lens filters which cannot be replicated using digital photo editing. They are an indispensable tool that should be in every photographer's camera bag. However, developing an intuition for how a polarizer might impact a photo often requires extensive experimentation. This tutorial aims to accelerate that process by demonstrating how and why polarizing filters can help — and in some cases harm — different types of scenes.

The Laser Retroreflector Array (LRA) is designed to use reflected laser light from orbiting spacecraft laser (typically a laser altimeter or light detection and ranging - lidar) to precisely determine the location of the lander, as a fiducial marker, and the distance to that point on the lunar surface with respect to the orbiter. The retroreflectors reflect any light striking them directly back to the source. They can be tracked by an orbiting laser altimeter or lidar from a few hundred kilometers. The LRA consists of eight circular 1.27-cm diameter corner-cube retroreflectors mounted on a 5.11 cm diameter, 1.65 cm high hemispherical gold-painted platform. Each of the eight retroreflectors points in a slightly different direction, and each has a maximum useful light incidence angle of about +-20 degrees. Four retroreflectors are evenly distributed in a ring on the hemisphere 20 degrees from zenith, and four in a ring 40 degrees from zenith. They are made from Suprasil quartz, index of refraction is 1.46. The equivalent optical cross section is 10,000 to 100,000 square meters, but is still not enough to use Earth-based laser ranging. Total mass of the LRA is 20 grams, it requires no power. Note that laser altimeters are not designed to be used to range to the LRAs, they have very small footprints and would only "hit" an LRA on occasional passes. Lidars with wider footprints are more efficient for actual ranging from orbiters. Also note that if the range to an orbiter can be precisely determined (e.g. from a larger LRA on the orbiter that can be ranged to from Earth), an accurate distance from Earth to the LRA on the lunar surface can be obtained. Measurements from multiple LRAs at different points on the Moon can be used to create an accurate geodetic network. The Intuitive Machines 1 Odysseus lander touched down on the Moon on February 22, but tipped over and came to rest tilted at 30 degrees to the horizontal. It is believed the LRA is still in a position where it will be able to return laser transmissions from orbiting spacecraft. Image credit: NASA-GSFC

When direct reflections are reduced, a greater fraction of the subject's light is of the diffuse variety — resulting in a more colorful representation. Foliage will be rendered with a brighter green, skies will have a deeper blue and flowers will appear more intense.