Light at Night (LAN) Measurement Methodologies

We conduct LAN measurements using several types of complementary instruments, some of our own design and manufacture. Basic photometric instrumentation includes single channel luminance meters, single and multi-channel photometers, scientific grade CCD all-sky cameras, and high dynamic range imaging cameras. We have added the innovation of dynamic motion to some of our photometers by incorporating Global Positioning System (GPS) capability along with automatic data logging; these systems have been operated from a variety of moving vehicles, including both manned and unmanned aircraft. These tools are often augmented with satellite data. We operate these systems in four basic modes, briefly defined as follows.

Ground Static Surveys (GSS)

Ground Static Surveys involve using fixed photometric or imaging sensors at ground level to record ambient light intensity usually, but not always, at high temporal frequency, typically once every few minutes. These data are particularly useful for understanding how local light intensity is modulated by weather and human activity, however, it is limited in its ability to characterize light over large areas, or to discriminate specific lights impacting the site. It is useful to bear in mind that most astronomical observatory monitoring of LAN is in the form of the GSS.

sbm meter
Ground Mobile Surveys (GMS)

Ground Mobile Surveys use the same photometric detectors developed for the GSS, but they are mounted on vehicles and driven over prescribed courses, while continuously recording ambient light intensity. The photometers are equipped with Global Positioning System receivers (GPS) that allow us to record not only light intensity, but also position and time. These systems work most efficiently in more heavily populated areas where there are systematic networks of roadways over which to move the photometers. GMS are most useful for characterizing LAN on time frames of days or weeks, and over moderate sized communities.

mobile sbm
Airborne Surveys (ABS)

We conduct Airborne Surveys using several different light detectors. These include up and down looking photometers of our own design and manufacture, as well as High Dynamic Range Imager (HDRI) 2-D CCD cameras. We use both manned and unmanned aircraft to carry these detectors over areas of interest. The ABS programs are best suited to larger areas than the GMS (typically city size or larger areas), for which high temporal frequency is not a requirement. They are uniquely well suited to high spatial and contrast resolution data collection.

airborne unit
Satellite Data Surveys (SDS)

There are small numbers of polar orbit satellites that are accumulating various types of LAN observations. We have developed software and methods for mining and exploiting these databases. The data tend to be inhomogeneous, and require great care in their use and application. They are often of low contrast resolution, and have lower spatial resolution than our HDRI ABS programs. Nonetheless, these data, if knowledgably and properly processed, can provide a very useful complement to the other methods we have developed.