Polarized vision for pilots and hawk watchers

June 19 2008 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

Polarized sunglasses have a remarkable ability to selectively darken a bright, blue-sky background.  Under some conditions, a blue-sky background will be transformed into a shade of dark blue that appears almost black.  This sky-darkening effect causes light-colored surfaces on birds, aircraft, clouds, and other objects to leap into brilliant, striking contrast against the dark background.  This makes these objects vastly easier to detect when viewed through polarized glasses than when viewed through unpolarized lenses or with the naked eye.

 

This sky-darkening effect is most pronounced in a wide belt of sky that is oriented at a ninety-degree angle to the sun.  (Imagine that the sun is at the "north pole" of the sky, and think of this belt as being the sky's "equator".)  This belt is where the light from the sky naturally has the strongest polarization.  This polarization is strongest in clear, thin (high-altitude) air without a lot of dust and haze.

 

The sky-darkening effect only occurs when the observer's head is tilted at certain angle in relation to the sun.  In clear, thin, air, this sky-darkening effect can be so strong that dark-colored objects will be difficult to spot against the dark background of the sky.  But if the observer tilts his head to a slightly different orientation, the sky brightens to its normal intensity, against which most objects tend to be seen as dark silhouettes, just as they would with the unaided eye.  With a slight tilt of the head, the observer can easily switch back and forth between these two viewing modes, allowing him to efficiently scan for both light-colored objects and dark objects.  It is rather striking to see an aircraft, bird, or cloud stand out as brilliant, shining feature against a dark backdrop, and then with a slight movement of the head, to reverse the contrast so that the background of the sky becomes brighter than the aircraft, bird, or cloud. 

 

Polarized glasses also make distant dust and smoke seem to disappear, allowing the viewer to see distant mountains, clouds, etc. that would otherwise be obscured.  This effect is strongest at the two points where the polarized belt of sky described above, located at a ninety-degree angle to the sun, intersects the horizon. 

 

Brown unpolarized sunglasses also selectively darken a blue-sky background, though to a much lesser extent than would an untinted, polarized lens under the most ideal conditions.  Again, this causes light-colored surfaces on birds, aircraft, clouds, and other objects--unless they happen to be blue, in which case they too are darkened--to stand out in enhanced contrast against the darker background of the sky.  In theory, dark-colored objects should be slightly harder to spot against this darkened sky than they would be with the naked eye, but in actual practice, a typical brown sunglass tint seems to "tone down" the overall brightness of the sky just enough to increase the viewer's eye comfort and allow him to spot dark silhouettes more easily than he would with the naked eye.  So the visibility of both light-colored objects and dark-colored objects is enhanced.

 

Brown sunglasses also make distant dust and smoke seem to disappear, allowing the viewer to see distant mountains, clouds, etc. that would otherwise be obscured.

 

Brown, polarized sunglasses combine these effects and dramatically enhance a viewer's ability to detect distant, light-colored objects against the sky.  On the whole, the observers ability to detect dark-colored objects is also enhanced, but to a lesser degree. 

 

I've found brown, polarized sunglasses to be extremely helpful for keeping RC model airplanes and gliders in view at great distances on clear, sunny days.  When I tilt my head in the direction that maximizes the sky-darkening effect, the white or light-colored portions of a model leap into brilliant contrast against the dark background.  If a model happens to have an open wing structure, and is covered in a bright, translucent color, it will become strikingly brilliant when viewed in this manner, especially if it happens to be orange or red.  (These colors are selectively favored by the brown sunglasses.)  As noted above, the glasses are not a handicap for scanning for dark-colored models--a slight re-orientation of the viewer's head increases the intensity of the sky and brings out the silhouettes of dark objects, while keeping things "toned down" just enough to enhance the viewer's eye comfort.  Depending on the orientation of the model, the coloring of the model, and the characteristics of the background, one or the other of these two viewing modes will bring the model into sharp, clear view. 

 

Brown, polarized sunglasses are also ideal for spotting high-flying birds against blue sky.  As described above, with a slight tilt of the head, the viewer can alternate between two different viewing modes.  In one mode, the sky is very dark and white-colored surfaces are highly visible.  Translucent, back lit surfaces such as light-colored tail and wing feathers are strikingly brilliant in this viewing mode, and all the more so if they happen to include brown or rufous tones.  The other viewing mode is essentially the same as what one would see with the naked eye, except that the overall picture is "toned down" a bit and is less harsh on the eyes.  In this mode, the sky is quite bright and most color contrasts are lost and all distant birds--be they primarily light or dark--tend to be first seen as dark silhouettes against the sky, just as they would with the unaided eye.  Of course, an observer needs to alternate between the two viewing modes, because some birds such as Turkey Vultures and adult Golden Eagles have no light surfaces. 

 

During a raptor migration count at the Grand Canyon--where the high-altitude, haze-free air causes the natural polarization of the sky, and the sky-darkening effect of polarized glasses, to be very strong-- I soon had the entire counting team converted to using polarized glasses.  On days when birds were flying high overhead, it was much more efficient to spot the birds by scanning the sky with polarized glasses, without the aid of binoculars, than any other way.  Birds directly overhead, presenting a full planform view to the observer, could be spotted when several thousand feet above the observer.  Under blue-sky conditions it would be very challenging to spot a high proportion of these high-flying birds without the aid of polarized glasses, with or without the aid of binoculars.

 

My personal preference is to use brown, polarized sunglasses for scanning without binoculars, and to remove the glasses when viewing through binoculars.

 

Polarized glasses are also ideal for airborne pilots of full-scale aircraft.  Again, both viewing modes should exploited--one orientation of the viewers head will darken the sky and cause light-colored surfaces to leap out in contrast.  (Note that most aircraft are white.)  In this viewing mode, distant clouds near the viewer's horizon that would normally be lost in the haze also leap into view, making it possible to spot distant thunderstorms or approaching weather fronts long before they would be visible to the unaided eye.  Another orientation of the viewer's head will deliver a view that is much like the view that would be seen with the naked eye, except that the overall brightness is toned down for comfort.  This viewing mode is ideal for spotting dark objects against the brighter sky.

 

As noted above, brown glasses are superior to grey glasses for darkening the non-polarized parts of the blue sky, which greatly enhances a pilot's ability to detect light-colored aircraft, birds, clouds, etc..  Pilots are often advised to use grey rather than brown sunglasses, to preserve "purity of colors".  In my opinion this is poor advice.  Grey lenses do not selective darken the blue sky, or improve the visual contrast between the blue sky and light-colored objects.

 

The advantages of brown, polarized glasses for soaring pilots cannot be overstated.  Soaring birds are much more easily seen, as are the white surfaces of other sailplanes or hang gliders.  The sky-darkening effect delivered both by the polarization and by the brown tint brings distant cumulus clouds near the horizon into clear view, when they would otherwise be lost in the hazy distance.  (Even at high altitudes on very clear days, there is enough normally enough haze to obscure distant clouds that are very close to the viewer's horizon.)  Even more importantly, brown polarized glasses darken the sky enough to allow the viewer to see the faint, milky, hazy patches that precede the formation of nearby cumulus clouds.  The brown, polarized glasses also help the viewer to spot dust devil columns, even high above the ground, where they are rather dispersed.

 

Polarized glasses do have one unfortunate disadvantage for pilots.  LCD screens (e.g. GPS's, variometers, etc.) are easily seen at some viewing angles but become opaque at other viewing angles. In my own experience, it is usually not inconvenient to mount a GPS or vario in a position where it is easy to see through polarized glasses, and I always prefer to wear polarized glasses even when I am using a GPS and/or vario with an LCD screen. Some particular LCD screens like the one on the "Ball Graphics Comp" vario are unusually sensitive to the angle of viewing and are therefore inconvenient to use with polarized glasses. Pilots of aircraft with "glass cockpits" or comprehensive LCD displays will probably not wish to wear polarized glasses.

 

Since a camera often needs to be held in a wide range of positions, polarized glasses can be a bit bothersome during in-flight photography with a digital camera with a LCD screen. In this situation I often pull down the glasses a bit to look over the top of the frame, or use the viewfinder. Don't buy a digital camera without a viewfinder if you plan on using it in flight while wearing polarized glasses!

 

Some canopies or windscreens can create visual interference patterns when viewed through polarized glasses.  In my own experience this is usually limited to very faint, translucent rainbow patterns near the corners of the windscreen or canopy that in no way inhibit visibility, but in some particular aircraft the problem could be much more severe.

 

Pilots need glasses that cover nearly all of the field of view, without blocking areas from view or leaving large areas unshaded. Many of the current styles are wrapped to give a good wide field of view in the horizontal dimension, but have lenses that are very small in the vertical dimension. This lets too much light in above and below the lenses. The problem is compounded when the glasses sit too low on the face. Glasses that are too small in the vertical dimension and sit too low on the face leave much of the upward field of view uncovered, which may be fine for folks who wear baseball caps pulled low over their eyes at all times, but not for pilots. Glasses that leave much of the upward field of view uncovered are particularly annoying for hang glider pilots, who fly from a prone position and therefore tend to look slightly upward most of the time. Glasses with nose pieces tend to sit higher than glasses with a simple molded bridge over the nose, such as some plastic frames have. Also, the Rayban "Warrior" RB 3342 is one frame that gives particularly good coverage of the whole visual field, including the upward portion. The traditional "aviator" style is also good in this regard.

 

Highly "wrapped" sunglass styles that give a good, wide field of view are not always compatible with moderate to strong prescription lenses. My understanding is that Oakley can cut prescription lenses to fit most of their frames. Their "M-frame" series seems particularly good for covering the whole field of view. The Rayban "Warrior" RB 3342 noted above has a moderate enough wrap that it can be fit with moderately strong prescription lenses by most glasses shops such as Lenscrafters. Frames with smaller lenses can be easily fit with a strong prescription, but leave a lot of the field of view uncovered.