ASTM E131-00a
(Terminology)Standard Terminology Relating to Molecular Spectroscopy
Standard Terminology Relating to Molecular Spectroscopy
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Designation: E 131 – 00a
Standard Terminology Relating to
1,2
Molecular Spectroscopy
This standard is issued under the fixed designation E 131; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
DISCUSSION—The equivalent IUPAC term is “molar absorption coef-
ficient.”
absorbance, A— the logarithm to the base 10 of the
reciprocal of the transmittance, (T).
acceptance angle, n—for an optical fiber, the maximum angle,
A 5 log ~1/T!52log T (1) measured from the longitudinal axis or centerline of the fiber
10 10
to an incident ray, within which the ray will be accepted for
DISCUSSION—In practice the observed transmittance must be substi-
transmission along the fiber by total internal reflection.
tuted for T. Absorbance expresses the excess absorption over that of a
specified reference or standard. It is implied that compensation has
DISCUSSION—If the incidence angle exceeds the acceptance angle,
been effected for reflectance losses, solvent absorption losses, and
optical power in the incident ray will be coupled into leaky modes or
refractive effects, if present, and that attenuation by scattering is small
rays, or lost by scattering, diffusion, or absorption in the cladding. For
compared with attenuation by absorption. Apparent deviations from the
a cladded step-index fiber in the air, the sine of the acceptance angle is
absorption laws (see absorptivity) are due to inability to measure
given by the square root of the difference of the squares of the refractive
exactly the true transmittance or to know the exact concentration of an
indexes of the fiber core and the cladding, that is, by the relation as
absorbing substance.
follows:
2 2
absorption band—a region of the absorption spectrum in
sin A 5 n 2 n (2)
=
1 2
which the absorbance passes through a maximum.
absorption coefficient, a—a measure of absorption of radiant where A is the acceptance angle and n and n are the refractive indexes
1 2
of the core and cladding, respectively. If the refractive index is a function
energy from an incident beam as it traverses an absorbing
−ab
of distance from the center of the core, as in the case of graded index
medium according to Bouguer’s law, P/P = e .
o
fibers, then the acceptance angle depends on the distance from the core
DISCUSSION—In IRS, a is a measure of the rate of absorption of
center. The acceptance angle is maximum at the center, and zero at the
energy from the evanescent wave.
core-cladding boundary. At any radius, r, the sine of the acceptance angle
of a graded index fiber is defined in compliance with that of a step-index
absorption parameter, a—the relative reflection loss per
fiber as follows:
reflection that results from the absorption of radiant energy
2 2
sin A 5 n 2 n (3)
=
at a reflecting surface: a=1− R, and R = the reflected r 1 2
fraction of incident radiant power.
where A is the acceptance angle at a point on the entrance face at a
r
absorption spectrum—a plot, or other representation, of
distance, r, from the center, n is the refractive index of the core at a
r
absorbance, or any function of absorbance, against wave-
radius, r, and n is the refractive index of the cladding. In air, sin A and
2
length, or any function of wavelength.
sin A are the numerical apertures. Unless otherwise stated, acceptance
r
absorptivity, a— the absorbance divided by the product of the
angles and numerical apertures for fiber optics are those for the center of
the endface of the fiber, that is, where the refractive index, and hence the
concentration of the substance and the sample pathlength,
numerical aperture, is the highest.
a = A/bc. The units of b and c shall be specified.
accuracy—the closeness of agreement between an observed
DISCUSSION—1—The recommended unit for b is the centimetre. The
recommended unit for c is kilogram per cubic metre. Equivalent units value and an accepted reference value (See Terminology
3 3
3
are g/dm , g/L, or mg/cm .
E 456) .
DISCUSSION—2—The equivalent IUPAC term is “specific absorption
DISCUSSION—The term accuracy, when applied to a set of observed
coefficient.”
values, will be a combination of a random component and a common
absorptivity, molar, e—the product of the absorptivity, a, and systematic error or bias component. Since in routine use, random
components and bias components cannot be completely separated, the
the molecular weight of the substance.
reported “accuracy” must be interpreted as a combination of these two
components.
1
This terminology is under the jurisdiction of ASTM Committee E-13 on
active fiber optic chemical sensor, n— a fiber optic chemical
Molecular Spectroscopy and
...
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