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Transmission Experiments
Transmission is the percentage of energy passing through a system relative to the amount that passes
through the reference. Transmission Mode is also used to show the portion of light
reflected
from a sample.
Transmission and reflection measurements require the same mathematical calculation. The transmission is
expressed as a percentage (
%T
λ
) relative to a standard substance (such as air). The software calculates
%T
λ
(or
%R
λ
) by the following equation:
S
λ
-
D
λ
%T
λ
=
R
λ
-
D
λ
x 100%
where
S
is the sample intensity at wavelength
λ
,
D
is the dark intensity at wavelength
λ
,
R
is the
reference intensity at wavelength
λ
.
Common applications include measurement of transmission of light through solutions, optical filters,
optical coatings, and other optical elements such as lenses and fibers.
!
!
!
!
For transmission of light through solutions, we offer a transmission dip probe with screw-on,
removable tips in 2-mm, 5-mm or 10-mm path lengths. Contact Ocean Optics for more information.
To take a transmission measurement:
1.
Select
Scope
under
Mode of Operation
in the software display area. Make sure the signal is on scale.
Adjust acquisition parameters so that the peak intensity of the reference signal is about 3500 counts.
Take a reference spectrum by first making sure nothing is blocking the light path going to your
spectrometer. The analyte you want to measure must be absent while taking a reference spectrum. Take
the reference reading by clicking the
Reference
button in the software display area. (This command
merely stores a reference spectrum. To save a spectrum, you must select
File | Save Spectral Values
from the menu.) Storing a reference spectrum is requisite before the software can calculate
transmission spectra.
2.
While still in
Scope Mode
, take a dark spectrum by first completely blocking the light path going to your
spectrometer. (If possible, do not turn off the light source. If you must turn off your light source to store a
dark spectrum, make sure to allow enough time for the lamp to warm up before continuing your
experiment.) Take the dark reading by clicking the
Dark
button in the software display area. (This
command merely stores a dark spectrum. To save a spectrum, you must select
File | Save Spectral
Values
from the menu.) Storing a dark spectrum is requisite before the software can calculate
transmission spectra.
3.
Begin a transmission measurement by first making sure the sample is in place and nothing is blocking
the light going to your sample. Then select
Transmission
under
Mode of Operation
in the software
display area. Click on the
Scan
button in the display area to take a scan. If
Single
is selected, only one
scan will be taken. If
Continuous
is selected, the spectrometer will continuously take scans until you
click on the
Stop
button. To save the spectrum, select
File | Save Spectral Values
from the menu.
!
!
!
!
If at any time any sampling variable changes -- including integration period, averaging, boxcar
smoothing, distance from light source to sample, etc. -- you must store a new reference and
dark spectrum.
