New daily light integral maps for the United States
Logan, J., and J. E. Faust.  2017.  Conference of the American Society for Horticultural Science, Sep 19-22, Waikoloa, HI.

Abstract:
The National Renewable Energy Laboratory developed the National Solar Radiation Database to make solar radiation data available 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 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 new DLI maps are based on an updated database that includes data from 1991 to 2010. The new database provides higher resolution data modeled from satellite images of cloud cover. The data are presented in pixels with one pixel representing each km2 area of land across the United States. The database provides global horizontal irradiance (unit) data that are converted to DLI (mol m-2 d-1) using the conversion factor of 0.007265 mol(PAR)/Wh(full spectrum) which assumes that 45% of the solar radiation is in the PAR region and 4.48 Ámol/J is the conversion from radiometric to quantum units. The new maps provide geographically more precise data reflecting more recent weather patterns.