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SpectRx NIR Technology
irradiance (E) The power collected by the primary (q.v.) of the 491
2
radiometer (q.v.) per unit area. Units are expressed in W/cm .

point source A source of infinitesimal dimension that radiates
in all directions.

primary In a telescope, the “primary” is defined as the fi rst
focusing mirror that converges incoming radiation into the optical
system.
radiance (L) The power radiated from an object per solid angle
2
unit per unit area. Units are expressed in W/cm sr.
radiometer An instrument allowing the measurement of radi-
ometric quantities such as E and L, under a given throughput
(q.v.). Measurement involves comparing an unknown quantity to
a known reference or standard.
Sabrie’s Index (SI) The total transmitted or refl ected spectral
energy of an object receiving light radiation from an independent
light source.
spectral intensity (Iσ) The power emitted by a point source (q.v.)

into a solid angle within a specified small spectral interval centered
–1
on frequency σ. Units are expressed in W/sr cm . The apparent
spectral intensity is the measurement of the spectral intensity of a
source without taking atmospheric transmission into account.
spectral irradiance (Eσ) The power reaching the primary (q.v.) of

the radiometer (q.v.) per unit area within a specified small spectral
interval centered on frequency σ. Units are expressed in W/cm
2
–1
cm .
spectral radiance (Lσ) The power radiated from an object per

unit solid angle, per unit area within a specified small spectral
interval centered on frequency σ. Units are expressed in W/cm
2
–1
sr cm . This unit is used by the spectroradiometer for calibrating
the spectrum.
spectroradiometer An instrument allowing the measurement of
radiometric quantities such as Eσ and Lσ, under a given through-
put (q.v.). Measurement involves comparing an unknown quantity
to a known reference or standard.
throughput (Θ) The factor quantifying the geometric aspect of
power transfer between two objects.
wave number (σ) The optical frequency of light expressed in
cm .
–1

10.4 Formula and Mathematical Relations
π × D 2
• The area of an optical element A = where D is the
diameter of the optical element. 4