Open Access
Add to BibSonomyAbstract
Sun glints are formed by specular reflections of the sun from capillary waves formed by wind blowing over water. These glints are normally colorless for a high sun or take on the color of the light source, such as orange–red during sunset or sunrise. However, when the glints are highly polarized by reflection near the Brewster angle, i.e., with relatively high sun they can change from colorless to a blue appearance caused by blue light leakage through a polarizing filter oriented orthogonal to the plane of polarization of the reflected light. Measurements are shown of crossed-polarizer transmission spectra exhibiting blue and near infrared light leakage for photographic polarizing filters and polarized sunglasses. A variety of photographs is shown to confirm blue light leakage as the source of the blue glint color.
© 2017 Optical Society of America
1. INTRODUCTION
Glints on water arise when light from the sun, the moon, or man-made light source undergoes specular reflection [1,2]. As an example, groups of glints are observed in glitter patterns when the setting sun is observed across a body of wind-rippled water, as shown in Fig. 1. The dimensions of such glitter patterns and their viewing geometry can be related quantitatively to the slopes of capillary waves on the water, which even enables remote sensing of the near-surface wind speed [3–7]. Glint counts reveal the fractal nature of a wind-roughened water surface [4] and when the wind subsides the glitter pattern reduces to a small number of glints (or even a single glint) at the specular point; in the latter conditions, the undulating water surface can produce interesting fun-house-mirror reflections of the surroundings [8,9].
Specular reflection leaves the spectrum of the incident light unchanged, which results in white glints with a high sun and yellow–orange glints with low sun elevation angles [9,10]. Consequently, we became curious when we separately observed sun glints that appeared blue when observed through a polarizer. These observations were made through both camera polarizers and polarized sunglasses. In both observing methods, the blue color of the glints was visible only when the polarizer was oriented to minimize glint brightness (i.e., with the polarizer axis near vertical with respect to the horizontal surface).
We initially expected these conditions to either eliminate the glints or to reduce their brightness without notably altering their color. We initially suspected that this phenomenon might arise because of the wavelength dependence of the water refractive index and hence the Brewster angle, or perhaps because of blue light leakage in the polarizers. We quickly determined that blue-light polarizer leakage is the dominant cause of the blue glint appearance (which persists even when glints are observed through a polarizer by eye instead of a camera). Although we found no discussion of this in the literature, we did find that Können’s book [11] discussed the blue colors that can arise when viewing highly polarized objects through crossed polarizers with blue light leakage, but did not specifically address the question of blue glints on water.
In the balance of this paper, the source and magnitude of glint polarization are discussed in Section 2; possible mechanisms giving rise to the blue color of glints viewed through a polarizer are discussed in Section 3, and the conclusion is drawn that polarizer blue-light leakage is the dominant cause of the blue glints, and a variety of outdoor observations that support this conclusion are presented in Section 4.
2. POLARIZATION OF SUN GLINTS
Especially when observed at angles near the Brewster angle of approximately 53°, glints can be highly polarized [12–15]. The polarized light Fresnel reflectivity for a smooth air–water interface is plotted versus incidence angle in Fig. 2. This figure is for water with a red-light refractive index of 1.331 [16]; the corresponding curve for blue light () would lie just above this and would be almost indistinguishable from it, showing that the reflection is almost independent of visible wavelength. The corresponding degree of linear polarization [17] is shown in Fig. 3. Glints are only perfectly polarized at the Brewster angle, but remain at least 90% polarized for 45°–61° angles and 80% polarized for 41°–65°. Because the wind-driven capillary waves have a distribution of slopes that depends on wind speed and air–water temperature difference [3,5], only part of a glitter pattern is highly polarized. Real wave-slope distributions can be modeled as Gaussian with skewness and peakedness terms that become important when examining the details [3–5]. A simple Gaussian model of the wave-slope probability density function is shown in Fig. 4 for a wind speed, based on the simplest form of the Cox–Munk model for a clean water surface [5]. This represents a light breeze, similar to the conditions of the photograph in Fig. 1. The variance of the distribution increases linearly with wind speed according to [5]. From Figs. 3 and 4 we see that the glints would be at least 90% polarized for the majority of wave slopes in Fig. 4 if viewed nominally at the Brewster angle, although the actual range of angles over which glints appear blue depends also on the transmission spectrum of the polarizer used to observe the glints.
Fig. 2. Polarization-dependent reflectivity of a flat water surface with index of for red light [16].
Fig. 3. Degree of polarization for red light reflected from a flat water surface. The glints remain 90% polarized for angles of 45°–61°.
Fig. 4. Wave-slope distribution for wind speed according to the Gaussian term of the Cox–Munk model for a clean water surface and equal air and water temperatures [5].
The frequent highly polarized state of glints has been exploited to reduce sun glints using polarizers in both the visible [18] and infrared [19] spectral regions. Sun and moon glints exist also in the thermal infrared [20–22], and in that spectral region there is a combination of reflection and emission polarization [21–28], although the bright sun typically results in the reflection polarization dominating the emission term [22].
3. OBSERVATIONS AND ORIGIN OF BLUE SUN GLINTS
On separate occasions, we both independently observed sun glints with a distinct blue appearance: one of us onboard a ferry boat while photographing glitter on the ocean through a polarizer and viewing the glitter through polarized sunglasses, the other onboard a recreational boat on a lake while wearing polarized sunglasses. Figure 5 shows glints photographed with a digital single-lens reflex (DSLR) camera with the lens covered by a polarizing filter. In this case, the camera was a Nikon D300 with an 18–200 mm lens set at 200 mm, with the polarizer rotated to achieve maximum glint brightness (i.e., the polarizer’s transmission axis would have been approximately horizontal, parallel to the water surface). The image exposure was 1/800 s at f/14 and ISO400. Figure 6 shows that when the polarizer was rotated to minimize glint brightness (i.e., with the polarizer axis approximately vertical for a horizontal surface), the glints turned blue instead of disappearing or just getting dimmer (the exposure was 1/80 s at f/5.6 and ISO400).
Fig. 5. Close-up view of sun glints on the ocean with a polarizer rotated for maximum glint brightness.
The same effect occurred for both linear and circular polarizing filters. Circular polarizing filters for cameras have a linear polarizer toward the scene and a quarter-wave plate toward the camera, so they act as a regular linear polarizer for detecting light from the scene. The circularly polarized light entering the camera has advantages for the camera autofocus and auto exposure features.
Explanations initially considered for the blue glints included the spectral variation of the water Brewster angle (ruled out because it varies only between 53.33° for blue light and 53.08° for red light), light leakage in one or more of the color filters in the camera sensor Bayer array, and near infrared (NIR) or blue light leakage through the polarizer. A camera effect and NIR leakage were immediately ruled out when the same blue glints were visible by eye when looking through polarized sunglasses. Later experiments and spectral transmission measurements of photographic polarizing filters and sunglasses showed that blue light polarizer leakage was the dominant cause of the blue glint appearance.
Figure 7 shows the results of an experiment to photograph a fluorescent ceiling light with: (a) no polarizer, (b) a single polarizer, and (c) two crossed polarizers on a Casio Exilim FH 100 camera. The crossed polarizers immediately produced a blue color, which indicated that they were transmitting blue light (the fluorescent lights generally emit continuously across the visible spectrum with a few additional peaks due to spectral lines, and we have observed the same effect even for white clouds observed through crossed polarizers). This was confirmed by measuring the transmission spectrum of photographic polarizers and polarized sunglasses using an Ocean Optics USB2000 spectrometer and a broadband light source viewed through a polarizer with a contrast ratio near for 350–580 nm and at least for 580–2600 nm (Meadowlark Optics OWL polarizer). Between the polarized light source and spectrometer input, we mounted a test polarizer that was rotated to measure crossed-polarizer transmission spectra like the one shown in Fig. 8.
Fig. 7. Office lights photographed through (a) no filter, (b) one polarizer, and (c) two crossed polarizers.
Fig. 8. Cross-polarization transmittance spectrum of polarizing filter oriented orthogonal to a second broadband polarizer.
The first thing that we noticed in the crossed-polarizer transmission spectrum of Fig. 8 was the large light leakage beyond approximately 780 nm; however, because that leakage existed only for NIR wavelengths, it could not have contributed to the blue glints seen by eye. The camera sensor has a strong response to such NIR radiation, but most cameras suppress this with a NIR-blocking filter to closely mimic the human eye spectral response [29,30]. Therefore, the NIR leakage was not a likely contributor to the blue glints recorded by our cameras, although it would be a very important source of colors in a camera lacking a NIR-blocking filter because the red, green, and blue pixel filters in a typical digital color camera have spectral response curves that rise up beyond approximately 700 nm to be highly transparent to NIR radiation [29,30]. Such images are encoded with a mixture of red, green, and blue light and typically have a reddish cast because the red channel tends to have more NIR leakage than either green or blue.
The next thing we noticed was the smaller amount of blue light leakage at wavelengths below 450 nm. This is what we identified as the primary cause of the observed blue glints. The blue leak is actually a rather well-known phenomenon among the community involved with designing and producing visible-light polarizers, especially for relatively low-cost applications dominated by polyvinyl alcohol film polarizers. In fact, discussion of the blue light leakage, and a similar red leakage, is scattered throughout the display literature and corresponding patents [31–40]. The leakage arises as a result of surface treatments designed to protect the polarizer from heat, moisture, and ultraviolet radiation.
4. ADDITIONAL OUTDOOR OBSERVATIONS
Having identified the likely cause of the blue glints, we went to a small lake to reproduce the effect. The surface of this lake was quite smooth, so the experiments only worked when we observed at times with the sun very near the Brewster angle. Nevertheless, the smooth water surface provided a stable sun reflection that allowed us to carefully and repeatedly observe the sun reflection varying from bright and white (Fig. 9) to small and blue (Fig. 10). We were able to make the deepest blue color appear and disappear with very tiny rotations of the camera polarizer rotation angle.
Fig. 9. Reflection from a calm lake surface with the sun 1.2° from the Brewster angle (19 May 2016, 1529 UTC, solar elevation ) and the polarizer rotated for maximum glint brightness.
Fig. 10. Reflection from a calm lake surface with the sun 1.2° from the Brewster angle (19 May 2016, 1529 UTC, solar elevation ) and the polarizer rotated for minimum glint brightness. A violet–blue glint is visible near the bottom of the image.
Note also in Fig. 10 how small the blue glint is compared to the much larger bright, white region centered on the sun’s reflection in Fig. 9. Both photographs were taken with a tripod-mounted Nikon D800 DSLR camera and a 18–300 mm lens set at 32-mm, with a manual exposure of 1/160 s at f/8 and ISO400. Reducing the exposure time to 1/400 s produced a visibly much darker blue glint color than the partially saturated lighter blue glint in Fig. 10, but the image was deemed too dark to be seen reliably in print. The photographs in these two figures both have a 58.3° horizontal field of view, and the blue glint diameter in Fig. 10 was measured to be approximately 0.55°, in close agreement with the angular diameter of the Sun as seen from Earth. Conversely, the much larger bright region in Fig. 9 is 10 times wider because it includes a reflection of both the Sun and the bright region of forward-scattered light in the partly cloudy sky. Mie scattering calculations showed that there is essentially zero polarization in the forward-scattered light for realistic cloud particles or aerosols at scattering angles below approximately 30°. We also note how well the underwater surface features can be seen in Fig. 10.
A similar set of photographs were taken at 300-mm focal length to emphasize the blue glint on the smooth water surface. Figure 11 was recorded with the polarizer adjusted for maximum glint brightness (i.e., horizontal polarizer axis and horizontal water surface), and Fig. 12 was recorded with the polarizer adjusted for minimum glint brightness (i.e., vertical polarizer axis and horizontal water surface). This zoomed-in view allowed an exposure (1/400 s at f/8 and ISO400) that nicely captured the deep blue color of the crossed-polarizer sun reflection.
Fig. 11. Close-up photograph of the sun reflection on a calm lake surface with the sun 1.2° from the Brewster angle (same time as Fig. 10) and the polarizer rotated for maximum glint brightness.
Fig. 12. Close-up photograph of the sun reflection on a calm lake surface with the sun 1.2° from the Brewster angle (same time as Fig. 10) and the polarizer rotated for minimum glint brightness.
We also found that a particular pair of polarizing sunglasses (Outlaw by Balzer) had notably more blue light leakage than the photographic polarizing filters or other polarizing sunglasses. A crossed-polarizer transmission spectrum for these glasses is shown in Fig. 13. While wearing these glasses, designed for outdoor sports applications, such as for fishermen to suppress disturbing reflections from water, an observer can readily see blue glints from not only water, but also cars and any other relatively smooth surface (other polarized sunglasses have lower blue-light leakage, similar to Fig. 8, which allows blue reflections to be seen over a smaller range of angles). Figure 14 shows blue sun reflections on a pool of water at the Alhambra in Granada, Spain, photographed with a smartphone camera viewing the scene through this particular pair of sunglasses oriented for minimum glint brightness. For these images, the solar elevation angle was 34.3° or 2.9° from the Brewster angle where Fig. 3 shows the degree of linear polarization is still above 99%.
Fig. 13. Transmission spectrum of Balzer sunglasses with a particularly strong blue light leakage that allows the observer to easily see blue glints on water, cars, and even patches of ice on the road.
Fig. 14. Close-up photograph of a single sun glint on a calm water surface at the Alhambra in Granada, Spain, photographed with a smartphone camera viewing the scene through the sunglasses whose transmission is plotted in Fig. 13, oriented for minimum glint brightness (Samsung Galaxy S4, automatic setting).
We have observed similar blue glints on water in situations such as looking down from Hoover Dam to the Colorado River and viewing rivers and pools within Yellowstone National Park. Especially when wearing the sunglasses corresponding to Fig. 13, we now easily see and notice blue glints on polished outside surfaces, such as marble floors, and car windshields or metallic painted surfaces. The various orientations of windshields and car surfaces allow blue glints to appear frequently because the Brewster angle condition is achieved for many different sun positions. In these cases, the best contrast occurs for glints on windshields because the car interior is usually dark.
5. CONCLUSION
Blue glints can be seen when viewing water or other smooth, reflective surfaces through a polarizer oriented orthogonal to the plane of polarization of the strongly polarized reflected light. For a horizontal body of water, these glints can be seen by simply viewing the surface with polarized sunglasses, which have vertical polarizer axes to minimize horizontally polarized reflections. The blue color arises from reduced polarizer extinction for blue light (i.e., blue light leakage). To readily see these blue glints, the sun angle must be very near the Brewster angle, which for water is approximately 53° measured from the surface normal, corresponding to a solar elevation angle of 37°.
Funding
Air Force Office of Scientific Research (AFOSR) (FA9550-14-1-0140); National Science Foundation (NSF) (1638758); Montana Research and Economic Development Initiative (51040-MUSRI2015-01).
Acknowledgment
We express appreciation to Dr. Javier Hernández-Andrés and the University of Granada local organizing committee for arranging the visit to the Alhambra as part of the 2016 Light and Color in Nature Conference in Granada, Spain.
REFERENCES
1. D. K. Lynch, D. S. Dearborn, and J. A. Lock, “Glitter and glints on water,” Appl. Opt. 50, F39–F49 (2011). [CrossRef]
2. J. A. Shaw, “Glittering light on water,” Opt. Photon. News 10(3), 43–45 (1999). [CrossRef]
3. J. A. Shaw and J. H. Churnside, “Scanning-laser glint measurements of sea-surface slope statistics,” Appl. Opt. 36, 4202–4213 (1997). [CrossRef]
4. J. A. Shaw and J. H. Churnside, “Fractal laser glints from the ocean surface,” J. Opt. Soc. Am. A 14, 1144–1150 (1997). [CrossRef]
5. C. Cox and W. Munk, “Measurement of the roughness of the sea surface from photographs of the sun’s glitter,” J. Opt. Soc. Am. 44, 838–850 (1954). [CrossRef]
6. N. Levanon, “Determination of the sea surface slope distribution and wind velocity using sun glitter viewed from a synchronous satellite,” J. Phys. Oceanogr. 1, 214–220 (1971). [CrossRef]
7. L. Wald and J.-M. Monget, “Sea surface winds from sun glitter observations,” J. Geophys. Res. 88, 2547–2555 (1983). [CrossRef]
8. E. J. Walsh, M. L. Banner, J. H. Churnside, J. A. Shaw, D. C. Vandemark, C. W. Wright, J. B. Jensen, and S. Lee, “Visual demonstration of three-scale sea-surface roughness under light wind conditions,” IEEE Trans. Geosci. Rem. Sens. 43, 1751–1762 (2005). [CrossRef]
9. J. A. Shaw, Optics in the Air: Observing Optical Phenomena through Airplane Windows (SPIE, 2017).
10. M. Vollmer and S. D. Gedzelman, “Colours of the sun and moon: the role of the optical air mass,” Eur. J. Phys. 27, 299–309 (2006). [CrossRef]
11. G. P. Können, Polarized Light in Nature (Cambridge University, 1985).
12. J. A. Guinn Jr., G. N. Plass, and G. W. Kattawar, “Sunlight glitter on a wind-ruffled sea: further studies,” Appl. Opt. 18, 842–849 (1979). [CrossRef]
13. M. Ottaviani, K. Stamnes, J. Koskulics, H. Eide, S. R. Long, W. Su, and W. Wiscombe, “Light reflection from water waves: suitable setup for a polarimetric investigation under controlled laboratory conditions,” J. Atmos. Ocean. Technol. 25, 715–728 (2008). [CrossRef]
14. M. Ottaviani, C. Merck, S. Long, J. Koskulics, K. Stamnes, W. Su, and W. Wiscombe, “Time-resolved polarimetry over water waves: relating glints and surface statistics,” Appl. Opt. 47, 1638–1648 (2008). [CrossRef]
15. C. D. Mobley, “Polarized reflectance and transmittance of properties of windblown sea surfaces,” Appl. Opt. 54, 4828–4849 (2015). [CrossRef]
16. G. M. Hale and M. R. Querry, “Optical constants of water in the 200-nm to 200-μm wavelength region,” Appl. Opt. 12, 555–563 (1973). [CrossRef]
17. J. S. Tyo, D. L. Goldstein, D. B. Chenault, and J. A. Shaw, “Review of passive imaging polarimetry for remote sensing applications,” Appl. Opt. 45, 5453–5469 (2006). [CrossRef]
18. G. Wang, J. Wang, Z. Zhang, and B. Cui, “Performance of eliminating sun glints reflected off wave surface by polarization filtering under influence of waves,” Optik 127, 3143–3149 (2016). [CrossRef]
19. J. J. Beard, “Reduction of solar glints from the sea with a linear polarizer,” Technical Report ADA036152 (Defense Technical Information Center, 1976), https://archive.org/details/DTIC_ADA036152.
20. A. W. Cooper, E. C. Crittenden Jr., E. A. Milne, P. L. Walker, E. Moss, and D. Gregoris, “Mid and far infrared measurements of sun glint from the sea surface,” Proc. SPIE 1749, 176–185 (1992). [CrossRef]
21. C. R. Zeisse, C. P. McGrath, M. Littfin, and H. G. Hughes, “Infrared radiance of the wind-ruffled sea,” J. Opt. Soc. Am. A 16, 1439–1452 (1999). [CrossRef]
22. J. A. Shaw, “Degree of linear polarization in spectral radiances for water-viewing infrared radiometers,” Appl. Opt. 38, 3157–3165 (1999). [CrossRef]
23. J. A. Shaw, M. R. Descour, D. S. Sabatke, J. P. Garcia, and E. L. Dereniak, “Measurements of midwave and longwave infrared polarization from water,” Proc. SPIE 3754, 118–125 (1999). [CrossRef]
24. F. J. Iannarilli, J. A. Shaw, S. H. Jones, and H. E. Scott, “Snapshot LWIR hyperspectral polarimetric imager for ocean surface sensing,” Proc. SPIE 4133, 270–283 (2000). [CrossRef]
25. J. A. Shaw, “Polarimetric measurements of long-wave infrared radiance from water,” Appl. Opt. 40, 5985–5990 (2001). [CrossRef]
26. J. A. Shaw, “Infrared polarization in the natural earth environment,” Proc. SPIE 4819, 129–138 (2002). [CrossRef]
27. J. A. Shaw, “The effect of instrument polarization sensitivity on sea surface remote sensing with infrared spectroradiometers,” J. Atmos. Ocean. Technol. 19, 820–827 (2002). [CrossRef]
28. J. A. Shaw, “A survey of infrared polarization in the outdoors,” Proc. SPIE 6660, 666006 (2007). [CrossRef]
29. K. Mangold, J. A. Shaw, and M. Vollmer, “The physics of near-infrared photography,” Eur. J. Phys. 34, S51–S71 (2013). [CrossRef]
30. M. Vollmer and J. A. Shaw, “Atmospheric optics in the near infrared,” Appl. Opt. (this issue).
31. W. J. Gunning and J. Foschaar, “Improvement in the transmission of iodine-polyvinyl alcohol polarizers,” Appl. Opt. 22, 3229–3231 (1983). [CrossRef]
32. L. Y. Ignatov, P. I. Lazarev, V. V. Nazarov, and N. A. Ovchinnikova, “Thin crystal film polarizers and retarders,” Proc. SPIE 4658, 79–90 (2002). [CrossRef]
33. Y. Bobrov, O. Kuchenkova, M. Kouznetsov, P. Lazarev, A. Manko, V. Nazarov, N. Ovchinnikova, M. Paukshto, P. Protsenko, and S. Remizov, “LCD applications of thin-crystal-film polarizers,” J. Soc. Inf. Disp. 12, 125–133 (2004). [CrossRef]
34. S. Yoon, G. H. Yang, P. Nayek, H. Jeong, S. H. Lee, S. H. Hong, H. J. Lee, and S.-T. Shin, “Study on the light leakage mechanism of a blue phase liquid crystal cell with oblique interfaces,” J. Phys. D 45, 105304 (2012). [CrossRef]
35. N. W. Schuler, “Iodine stained light polarizer,” U.S. patent 4,166,871 A (4 September , 1979), https://www.google.com/patents/US4166871.
36. C. M. Berke, “Polarizer: dichroic dye in oriented polyacrylic acid/chitosan complex sheet,” U.S. patent 4,440,541 A (3 April , 1984), https://www.google.com/patents/US4440541.
37. J. J. Cael, R. L. Jones, R.-C. Liang, G. B. Trapani, and T.-F. Yeh, “Synthetic UV bleached polarizer and method for the manufacture thereof,” U.S. patent 5,925,289 A (20 July , 1999), https://www.google.com/patents/US5925289.
38. J. J. Cael, J. W. Balich, P. V. Nagarkar, K. M. Vogel, and H. Sahouani, “Enhanced K-type polarizer,” U.S. patent 6,814,899 B2 (9 November , 2004), https://www.google.com/patents/US6814899.
39. M. V. Paukshto and L. D. Silverstein, “Color correcting polarizer,” U.S. patent 7,144,608 B2 (5 December , 2006), https://www.google.com/patents/US7144608.
40. Y. Egi, T. Ishitani, and T. Nishi, “Display device,” U.S. patent 9,164,313 B2 (20 October , 2015), https://www.google.com/patents/US9164313.
