58 Fundamentals of Sensors for Engineering and Science
is the emissivity, and A
o
= 2π~c
2
o
and A
1
= ~c
o
/k
B
are constants equal to
374.15 MW µm
4
/m
2
and 14 388 µm K, respectively. The units of I
λ,T
are
W/(m
2
µm). Planck’s constant, ~, equals 6.6256×10
−34
Js and Boltzmann’s
constant k
B
, equals 1.3805 × 10
−23
J/K. When = 1, the body is termed
a blackbody or ideal radiator. When < 1, the body is termed a non-ideal
radiator.
When a body is in thermal equilibrium with its surroundings, it emits the
same amount of energy that it absorbs. In this case, the amount of emitted
energy as compared with the total energy, which also includes reflected and
transmitted energies, is characterized by its emissivity. Surfaces such as
paper, brick, smooth glass, and water have