Daily Light Integral: A research review and high-resolution maps of the United States
J. E. Faust, and J. Logan.  2018.  HortScience, 53: 1250-1257.

Abstract:
The National Renewable Energy Laboratory developed the National Solar Radiation Database to provide accessible solar radiation data to the research community for various uses. Previously, we created a series of monthly daily light integral (DLI) maps to provide a tool for horticulturists to estimate the potential growth and flowering responses for various plants throughout the year. The original DLI maps were based on solar radiation data from 239 sites recorded from 1961 to 1990. The DLI maps presented in this article were created from an updated database that included data from 1998 to 2009. This database provides higher resolution data modeled from satellite images of cloud cover. The data are presented in pixels with each pixel representing 100 km(2) of land across the lower 48 United States and Hawaii, whereas the Alaska data are 1600 km(2) pixels. The database provided global horizontal irradiance data that were converted to DLI (mol.m(-2).d(-1)) using the conversion factor of 0.007265 mol (400-700 nm).Wh(-1) (400-2700 nm), which assumes that 45% of the solar radiation is in the photosynthetically active radiation (PAR, 400-700 nm) region and 4.48 mu mol.J(-1) is the conversion from radiometric to quantum units. The updated DLI maps provide more geographically precise data reflecting recent weather patterns. We present a comprehensive review of recent research exploring the growth and flowering responses of horticultural crops to DLI.