30
Relative Irradiance Experiments
Irradiance is the amount of energy at each wavelength from a radiant sample. In relative terms, it is the
fraction of energy from the sample compared to the energy collected from a lamp with a blackbody
energy distribution, normalized to 1 at the energy maximum. Relative irradiance is calculated by the
following equation:
S
λ
-
D
λ
I
λ
=
B
λ
(
R
λ
-
D
λ
)
where
B
λ
is relative energy of the reference calculated from the color temperature in Kelvin,
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 characterizing the light output of LEDs, incandescent lamps and other
radiant energy sources such as sunlight. Also included in irradiance measurements is fluorescence, in which
case the spectrometer measures the energy given off by materials that have been excited by light at a
shorter wavelength.
!
!
!
!
The components that came with the CHEM2000-UV-VIS will not allow the user to make relative
irradiance measurements. Only CHEM2000 users can perform relative irradiance measurements.
In order to make relative irradiance measurement, the reference spectrum must be made in Scope
Mode with a blackbody light source of known color temperature. This color temperature is needed
in order to calculate relative irradiance. The light source that comes with the CHEM2000-UV-VIS is
not a blackbody light source with a known color. To purchase a blackbody light source, contact
Ocean Optics.
To take a relative irradiance measurement:
1.
Select
Spectrometer | Spectrometer Configuration
from the menu. Next to
Color Temp
, make sure
the color temperature in Kelvin of the reference lamp you are going to use is entered here. The color
temperature of the tungsten halogen light source that came with your CHEM2000 has a color
temperature of 3100 Kelvin. Click
OK
.
2.
Select
Scope
under
Mode of Operation
in the software display area. Make sure the signal is on scale
by adjusting acquisition parameters. Take a reference spectrum of your reference lamp. 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 relative
irradiance spectra.
3.
While still in
Scope Mode
, take a dark spectrum by first completely blocking light from going to your
spectrometer. 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 relative
irradiance spectra.
4.
Begin a relative irradiance measurement by first positioning the fiber at the light or emission source
you wish to measure. Then select
Relative Irradiance
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.
