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Ocean Optics Web Book

Light and Radiometry


Page updated: Apr 11, 2017
Principal author: Curtis Mobley

We adopt the International System of Units, commonly called SI (for Système International) units. This system rests on seven base units and two supplementary units, which are shown in Table 1. With the exception of the candela, which is needed only for the Level 2 discussion of photometry, we presume that the reader is familiar with these SI units from basic physics and chemistry. All other quantities are derivable from these units.

Table 1: SI base units.
Physical quantity Base Unit Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
temperature kelvin K
amount of substance mole mol
luminous intensity candela cd
  Supplementary units  
plane angle radian rad
solid angle steradian sr

Our choice of nomenclature and symbols generally follows the recommendations of the Committee on Radiant Energy in the Sea of the International Association of Physical Sciences of the Ocean (IAPSO; see Morel and Smith (1982)). This is the nomenclature most widely used today in optical oceanography. However, neither the SI units nor the recommended IAPSO notation are entirely satisfactory. In particular, they are sometimes inconvenient for our measurements and mathematical manipulations; consequently we occasionally shall make minor deviations from the IAPSO recommendations. Several derived units that we shall need are shown in Table 2.

Table 2: Derived units useful in radiative transfer studies.
Physical quantity Derived Unit Symbol Definition
wavelength of light nanometer nm $ \rm {10^{-9} \, m}$
energy joule J $ \rm {1 \, kg \, m^2 \, s^{-2}}$
power watt W $ {\rm {1 \, kg \, m}^2 \, s^{-3}}$
number of photons einstein einst 1 mol of photons
( $ \rm {= 6.023\cdot10^{23}}$ photons)