Now, let’s take a look at three real life examples on how this method can be used to assess ecological light pollution in different locations and weather conditions.

 

(a) Terrestrial field site, Marburg, Germany, with cloud cover

In this experiment to understand the flight to light behavior of moths, the camera was situated at the center of six sodium vapour light sources (labelled 1 to 6 in figure below) that were arranged in a circle.   

From the figure above, light emitted comes predominantly from the six street lamps as well as skyglow that is being intensified by cloud cover. What is interesting to note here is the difference in light illuminated from the all-sky view, (a), compared to that of the vertical plane images seen in (b), (c), (e) and (f). While light from the lamps seen in the all-sky view seem to appear almost identical, when we look at the zoomed in version of that in vertical plane images, lamps 1, 2, 3 and 4 produces light reflected from the ground (reddish colour on the ground below light source) while lamp 5 and 6 do not (reddish colour only on street lamp) because of bushes and grass surrounding the street lamp (Jechow et al, 2019). 

 

(b) International Dark Sky Park, Zselic, Hungary, little cloud cover

As compared to the terrestrial field site in Germany that was relatively dominated by artificial night light, we see in the figure below that the dark sky park shows much lower levels of light illuminance. The importance of assessing light pollution from a vertical plane image is further emphasised here as sources of artificial light below the horizon is only evident when viewed in (e) and (f). 

  

(c) International Dark Sky Reserve Westhavelland, with snow cover

Aside from light sources that directly contribute to light pollution in surrounding areas, there are indirect factors such as weather conditions and type of surface that contributes to the amount of light incident at the Earth’s surface. Because changes to vegetation, soil moisture and amount of snow cover, the reflectivity of the surface and hence light that is reflected can also change spatially and temporally (Jechow et al, 2019). For example from the figure below, we see that snow, which has a relatively high albedo level, reflects a significant amount of light from the ground. 

 

Advantages of fisheye lens and vertical plane imagery 

From the three examples above, we see how using vertical plane imagery using fisheye lens imagery provides additional information to better assess ecological light pollution on top solely looking at all-sky imagery (Jechow et al, 2019). This method is not only able to present results of light illuminance in a clear and efficient manner, spatial information from light reflected from the ground and light emitted from light sources below the horizon can also be taken into account when assessing the extent of light pollution in specific locations. 

Fisheye lens imagery and its usage of vertical plane imagery is also important when we look at effects of light pollution on biodiversity. This is because many animals, such as sea turtle hatchlings rely on visual cues below or at the horizon. Additionally, because different species’ fields of vision are distinct from one another, having light information from a full-sphere perspective aids in assessing what an animal sees in specific light conditions (Jechow et al, 2019). It is also stated that conventional ways of measuring light pollution such as the lux meter may not be effective in assessing animal behavior in response to light because it measures information from a single point without spatial resolution. The ability of an animal to use terrestrial cues to navigate through the night depends more on whether the cues have sufficient contrast against the surrounding background rather than the luminance of a particular object (Jechow, 2019).

Perhaps it’s time to rethink our anthropocentric ways of measuring light pollution as measurements of light by lux are adjusted according to the luminous sensitivity of humans (Owens, 2018). However, as we have seen in the previous blog posts, many animals and insects are too impacted by light pollution. Improvements in the way we measure light and light pollution could possibly translate to better and more well targeted lighting solutions that consider the perspectives of both humans and animals.

 

Stay tuned for more!

Jean    

References:

• Jechow, A., Kyba, C., & Hölker, F. (2019). Beyond All-Sky: Assessing Ecological Light Pollution Using Multi-Spectral Full-Sphere Fisheye Lens Imaging. Journal of Imaging, 5(46), 1-17.
• Owens, A. C. S., & Lewis, S. M. (2018). The impact of artificial light at night on nocturnal insects: A review and synthesis. Ecology and Evolution, 8(22), 11337-11358.