29
Reflection Experiments
Reflection is the return of radiation by a surface, without a change in wavelength. The reflection may be:
•
Specular, in which the angle of incidence is equal to the angle of reflection.
•
Diffuse, in which the angle of incidence is not equal to the angle of reflection.
Every surface returns both specular and diffuse reflections. Some surfaces may return mostly specular
reflection, others more diffuse reflection. The glossier the surface, the more specular the reflection.
Reflection is expressed as a percentage (
%R
λ
) relative to the reflection from a standard substance (such as
our WS-1 white reference for a diffuse reflection measurement):
S
λ
-
D
λ
%R
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 measuring the reflection properties of mirrors, anti-reflection coatings, and
measuring the visual properties of the color in paints, graphic arts, plastics, and food products.
!
!
!
!
The components that came with the CHEM2000 and CHEM2000-UV-VIS will not allow the user to
make reflection measurements. A reflection probe is necessary. We offer several components for
reflection measurements such as a variety of reflection probes, a reflection probe holder, diffuse
reflectance standards and an integrating sphere. Contact Ocean Optics for more information.
To take a reflection 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 reflection 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
reflection spectra.
3.
Begin a reflection 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. The mathematics required to calculate reflection measurements are identical to those
necessary to compute a transmission spectrum. 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.
