Ocean Optics S2000 - Manual (Page 95)

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
λλ
- D
λλ
I
λλ
= B
λλ
( )
R
λλ
- D
λλ
where B is the relative energy of the reference (calculated from the color temperature) at wavelength
λ
, 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 relative 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. To take a relative irradiance measurement using OOIBase32, our spectrometer operating software,
follow these steps:
1.
Make sure you are in scope mode, by either clicking the scope mode icon on the toolbar, or selecting
Spectrum | Scope Mode from the menu. Make sure the signal is on scale. The peak intensity of the reference
signal should be about 3500 counts. The light source must be a blackbody of known color temperature.
2. Take the reference reading by clicking the store reference spectrum icon on the toolbar or selecting
Spectrum | Store Reference from the menu. (This command merely stores a reference spectrum. You must
use the Save Reference command to permanently save the spectrum to disk.) 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 completely blocking the light path. (If possible, do not turn
off the light source.) Take the dark reading by clicking the store dark spectrum icon on the toolbar or selecting
Spectrum | Store Dark from the menu. (This command merely stores a dark spectrum. You must use the
Save Dark command to permanently save the spectrum to disk.) Storing a dark spectrum is requisite before the
software can calculate relative irradiance spectra.
4. Take a relative irradiance measurement by first positioning the fiber at the light source you wish to measure.
Then choose the irradiance mode icon on the toolbar or select Spectrum | Relative Irradiance Mode from
the menu. In the Reference Color Temperature dialog box, enter the light source’s color temperature in
Kelvin and click OK. To save the spectrum, click the save icon on the toolbar or select File | Save | Processed
from the menu.
! !
If at any time any sampling variable changes -- including integration time, averaging, boxcar smoothing,
distance from light source to sample, etc. -- you must store a new reference and dark spectrum.
A typical configuration for an irradiance experiment.
Experiment Tutorial: Relative Irradiance
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