A spectrum is the distribution of electromagnetic radiation that has been reflected, emitted, or transmitted from ground objects because of their inherent physical characteristics. Remote sensing measurements provide a solid theoretical basis for applications such as target detection and identification fields considering that different materials possess distinct abilities for reflection, emission, and scattering mechanisms over different wavelength ranges. To support such studies, a spectral library is necessary for quantitative remote sensing modeling, land surface parameter inversion, and ecological environment monitoring. For nearly half a century, domestic and international research institutes have designed various spectral libraries by assembling spectral data of typical materials. These spectral libraries include a long-term, reliable, accumulative observation for a certain period and indispensable standard ancillary data. Such spectral data are essential resources for ground cover classification and target identification.
This study reviews the history of the development of spectral libraries by analyzing and comparing each spectral library with those of different wavelength ranges, types, ancillary data, and resources. The applications of two kinds of spectral libraries for different disciplines have been summarized. In this study, the general spectral library refers to a database with more comprehensive and abundant ground objects, including the United States Geological Survey Spectral Library, Advanced Spaceborne Thermal Emission and Reflection Radiometer Spectral Library, and Spectral Database System of Typical Objects in China. Meanwhile, the professional spectral libraries that were established focus mainly on the specific disciplines.
Comparison results reveal that spectrum data are relatively insufficient in the microwave band because previous research focused more on visible and near-infrared bands during the last century, especially on geological and ecological applications. This observation can also account for the over whelming quantity of minerals, rocks, and crops spectral data in existing data bases. However, most of the available spectra data that exist still exclude canopy or urban grassland spectra, although Nonetheless, vegetation spectra data have been considerable. Based on the research demand investigation for spectral library, the study should mainly focus on data management, quality control standard in measurements, and data utilization efficiency.
The establishment and full sharing of a comprehensive spectral library are crucial steps for domestic research. The waveband and the spatial and temporal gradients of the ground objects present the issues for accurate classification and identification applications. Thus, the full-wave band, spatial scales, or phenological information are significant in the completion of future spectral libraries.