While it is important to look at the impacts of light pollution, attention should also be paid to how light pollution is being measured as this affects the quality of data collected and hence influences the effectiveness of solutions to reduce the impact of light pollution. In this two part series we will first look at existing measures and assess its effectiveness, concluding with a new method of assessing ecological light pollution called fisheye lens imaging and vertical plane imaging. The next part will put this method to the test in three real world scenarios.
Current ways to measure light pollution
In assessing light pollution, the question that we should first ask ourselves is: are we measuring light right? One of the more common ways to measure light pollution is through satellite remote sensing. This method is particularly useful when assessing the effects of artificial light at night (ALAN) at large spatial scales (Jechow et al, 2019). There are also existing measures to measure sky glow using photoelectric photometers. However, satellite measurements of light pollution only account for light that is emitted or reflected upwards from the Earth’s surface. This means that light that is emitted at shallow angles, horizontally, or downward from certain sites will not be accounted for (Jechow et al, 2019).
While there are some ground-based measurements that could give a more comprehensive information about light pollution compared to remote sensing, ALAN researches often only make use of horizontal illuminance to assess the effects of light pollution. Hence, even with ground-based measurements, this is insufficient to provide an accurate representation on the light field as it lacks spatial and spectral data (Jechow et al, 2019). So, how else can we bridge this gap to ensure that light pollution can be measured from a more holistic and spatially complete angle?
Digital camera and fisheye lens
By using a commercial digital camera and a fisheye lens to retrieve vertical plane multi-spectral (RBG) images that encapsulate the full solid angle, this affordable and efficient method might just do the trick. Unlike conventional all-sky photometry where the primary imaging plane is horizontal (figure 1a), fisheye lens photometry attached to digital cameras uses 2 vertical plane images placed at opposite directions (figure 1b) to ensure that there is full sphere 4pi light field information (Jechow et al, 2019). According to Jechow (2019), with respect to ecological light pollution, scalar illuminance (right of figure 2) is a better representation for total light incident at a specific location compared to conventional horizontal illuminance seen in the horizontal plane as mentioned earlier. Images that are captured are then translated into illuminance maps that can be used to assess ecological light pollution.
The use of vertical plane images is important in accounting for light that is emitted below the horizon (including the ground) which are often left unaccounted for in all-sky imaging (figure 1a). However, many animals that are affected by ecological light pollution often have viewing visions toward the horizon or directed toward the ground (Jechow et al, 2019).
Click on the next post for part II to find out how fisheye lens imagery works in assessing ecological light pollution.
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.