AQ-00275-000, Rev. 3
22
where:
E
s
is the energy of the sample beam,
E
r
is the energy of the reference beam,
ρ
s
is the reflectance factor of the sample at the sample reflectance port
ρ
r
is the reflectance factor of the reference port, and
κ
s
and
κ
r
are the efficiency with which the energy reflected from the sample and reference
beams is captured by the sphere and converted into a signal by the detector.
The sphere efficiency factors (
κ
) for a given sample are a function of many variables including the
spatial distribution of the energy reflected from the sample, the reflectance of the sphere wall, the
geometry of the sphere (location of ports, baffles, etc.) and the efficiency of the detector itself.
However, proper measurement procedures make it possible to greatly reduce or eliminate the effect
of such factors on transmittance and reflectance factor measurements.
In transmittance measurement, the transmittance (T
s
) of a non-scattering sample placed in the path
of the sample beam affects the amount of energy entering the sphere and reaching the sample
reflectance port. The value S
r
of the ratio recorded by the instrument is:
The value D
T
displayed by the instrument, however, is S
T
divided by the value recorded in the
Uncorrected Baseline:
Therefore, in a transmittance measurement of a non-scattering sample, the value displayed by the
instrument is simply equal to the transmittance of the sample.
In reflectance measurement, the standard positioned at the sample reflectance port during the
Uncorrected Background measurement is replaced by another sample of unknown reflectance
ρ
u
.
The value S
r
of the ratio recorded by the instrument is:
B
E
s
ρ
s
κ
s
E
r
ρ
r
κ
r
----------------
=
Eq. 2
S
T
T
s
E
s
ρ
s
κ
s
E
r
ρ
r
κ
r
----------------------
=
Eq. 3a
D
T
S
T
B
------
T
s
E
s
ρ
s
κ
s
E
r
ρ
r
κ
r
E
r
ρ
r
κ
r
E
s
ρ
s
κ
s
---------------------------------------
T
s
=
=
=
Eq. 3b