Approved American National StandardThe IES defines illumination engineering terms in ANSI/IES RP-16, which is available here online. You can search terms by their first letter using the categories listed in the Categories window, by the first letter of the term in the alphabetical list below, or by typing the term in the search window. You can find the Introduction material for RP-16 on these pages:

Copyright, Disclaimer, and American National Standards Statement

Foreward, Nomenclature Committee members, Introduction, and Scope

Spectral Parameters

AJAX progress indicator
Search: (clear)


  1. Properly, this should be a second partial derivative since area and solid angle are independent variables; however, the symbol "d" is used due to the convenience in printing and typing. For this specific use, no possible errors or confusion are foreseen. This practice is in accordance with the International Lighting Vocabulary (CIE S 017/E 2011) and the practice of the national Institute of Standards and Technology.
  2. Values of radiation constants from NBS Special Publication 398.