Anthropogenic light at night alters natural ecosystem function by disrupting the natural rhythms of day and night, the driving force behind many biological functions. While the sources of light pollution stem from the built environment, the scattering of anthropogenic light in the atmosphere can be detected hundreds of kilometers from its source. The spatial extent of this scattering, called skyglow, can significantly degrade the photic environment in otherwise pristine or protected natural places such as national parks. The 1916 Organic Act created the National Park Service (NPS) with a mandate to preserve the resources of national parks for future generations. The NPS considers the natural photic environment an integral natural, cultural, historical resource, thus is committed to its protection. To understand the impact of anthropogenic light on park ecosystems and visitor use, the NPS has developed camera systems to precisely measure its intensity and spatial extent from both direct and indirect sources, and to quantify its impact on the natural environment. Twenty years of measurements in over 130 park units has revealed a wide range of exposure to light pollution, from pristine conditions to severely degraded. The sources of light pollution detected in parks are predominantly from sky glow generated by towns and cities, some as far as 200 kilometers away. Here I present techniques used by the NPS to measure the photic environment, measurement results, examples of ecological impacts, and steps taken to preserve this critical resource.