Using Relative Irradiance Mode with an Ocean Optics Spectrometer

Using Relative Irradiance Mode

Q. How do I use Relative Irradiance Mode with Ocean Optics Spectrometers?

A. Every Ocean Optics spectrometer has what is called an “instrument response function”, or IRF. The IRF refers to how much the spectrometer responds to light across its wavelength range. This response is far from uniform: a spectrometer will produce a different response (here defined as the number of Scope-mode counts produced for a fixed number of photons) at every pixel. The IRF is non-uniform because of the cumulative effects of optical inefficiencies in the light path. These include, but are not limited to:

- Attenuation of light in the fiber optic cable
- Absorbance of light by the mirrors (which varies with wavelength)
- Grating efficiency (the catalog has plots of these over wavelength)
- Detector response (the CCD is more sensitive to some wavelengths than others)

The IRF for each spectrometer is unique, and cannot really be measured. However, it is possible to compensate for the IRF. The two common corrections are Relative Irradiance and Absolute Irradiance calculations.

Absolute Irradiance uses a lamp of known output (in terms of spectral power) to calibrate the spectrometer’s response at every pixel. This corrects the shape and magnitude of the spectrum, and the resulting spectrum is in terms of power per nanometer. Absolute irradiance is beyond the scope of this tutorial.

Relative Irradiance uses a lamp with a known color temperature (but not necessarily known power output) to correct just the shape of the spectrum but not the magnitude (hence its “relative” identifier). Relative Irradiance allows the user to determine whether there is more light at one wavelength than another (which cannot be determined from Scope mode due to the IRF), though it does not provide any information on how much power there is in absolute terms. This can be useful in some applications, such as measuring fluorescence.

The calculation for Relative Irradiance is as follows:

IL,T = N * BL,T * (SL - DL) / (RL - DL)

where:

IL,T is the relative irradiance at wavelength L and color temperature T Kelvins
N is a normalizing term to bound IL,T into the range 0-100
SL is the sample spectrum at wavelength L
DL is the dark spectrum at wavelength L
RL is the reference spectrum at wavelength L
BL,T is the emission of a blackbody radiator at wavelength L and color temperature T Kelvins

We compute BL,T from Planck’s law:

BL,T = 2*h*c2 / (L5 * (e( h*c / L*k*T ) - 1 ))

where:

L is the wavelength in meters
T is the temperature of the blackbody in Kelvins
h is Planck’s constant (approximately 6.626 * 10-34 J*s)
k is Boltzmann’s constant (approximately 1.38 * 10-23 J/K)
c is the speed of light (approximately 3 * 108 m/s)
e is the base of the natural logarithm (approximately 2.718)

We compute N by first finding the wavelength at which the maximum output for a blackbody of a given temperature occurs. This is done using the Wein displacement law (shown greatly simplified here, with an approximate value):

Lmax = 2898 / T
Thus,
N = 100 / BLmax,T

System Setup

Connect any blackbody light source with a known color temperature at its output to the spectrometer with an optical fiber assembly.

A light source with an attenuator and shutter are desirable (i.e. HL-2000-FHSA). It is important to ensure that the detector is not saturated; therefore, attenuation of the signal may be necessary.
Sources like the LS-1 have a filter slot in which the metal piece from a floppy disk can be placed to partially block the light path and attenuate the amount of light.

When storing a dark spectrum, the light path must be completely blocked. If you do not have a shutter on your source, do not turn off the source to store the dark spectrum. This would then create the need to warm the source until stable output has been achieved once again.

Using the SpectraSuite Relative Irradiance Wizard

1. Open SpectraSuite and close any open graphs by clicking on the “x” toward the right side of the graph’s tab. Go to the main menu under File / New / Relative Irradiance Measurement

2. The “Select spectral source wizard” window is displayed. If you have more than one Ocean Optics spectrometer connected to the Universal Serial Bus (USB), these instruments will be displayed in the listbox. You have the opportunity to select the desired spectrometer.
If only one spectrometer is connected, it will be displayed in the listbox and selected for you.
After the selection is made, click the “Next” button.

3. The “Set acquisition parameters wizard” window is displayed. Set the integration time so that the “Last peak value” is close to the “Recommended peak value.” This can be done by clicking on the “Set automatically…” button or manually using the numeric up down control.

The spectrometer saturates at 2 to the power of the number of bits of the A/D converter. The recommended peak value is 85% of the number of counts where the detector is saturated. This allows for a strong signal, yet not too high as to saturate the detector. If you reach the spectrometer’s minimum integration time and are still saturating the detector, use an attenuator, a smaller core diameter fiber, or partially block the light path.

You can optionally use “Scans to Average” to increase the signal to noise ratio and “Boxcar Width” to smooth the curve.

4. Once the acquisition parameters have been set, click the “Next” button.

5. The “Store reference spectrum” wizard window is displayed. Click the “yellow light bulb” button to store the reference spectrum.

6. Once the reference spectrum has been stored, a reference spectrum preview is displayed. If the reference is unsatisfactory, you may click the button again until you have a good reference spectrum. SpectraSuite stores the last reference spectrum in its memory. Click the “Next” button.

7. The “Store dark spectrum wizard” window is displayed. Click the “dark light bulb” button to store the reference spectrum.

8. Once the dark spectrum has been stored, a dark spectrum preview is displayed. The dark spectrum should be a relatively flat line. If the dark is unsatisfactory, you may click the button again until you have a good reference spectrum.

SpectraSuite stores the last dark spectrum in its memory. Click the “Next” button.

9. The “Store Color Temperature wizard” window is displayed. Enter the color temperature of your blackbody source. Click the “Finish” button.

10. You will be returned to the main SpectraSuite window. The y-axis is now in Relative Irradiance units.

Measuring Relative Irradiance outside the wizard

1. In SpectraSuite, go to the Main Menu under Processing / Set Color Temperature. Ensure that the color temperature is set to the color temperature of your blackbody light source. The Ocean Optics LS-1 Tungsten Halogen Light Source with the standard bulb has a color temperature of 2800K.

2. Start an acquisition in Scope Mode. Set the integration time to ensure that you have a strong signal, but not one that is saturating the detector. The spectrometer saturates at 2 to the power of the number of bits of the A/D converter. The recommended peak value is 85% of the number of counts where the detector is saturated. This allows for a strong signal, yet not too high as to saturate the detector. If you reach the spectrometer’s minimum integration time and are still saturating the detector, use an attenuator, a smaller core diameter fiber, or partially block the light path. You can optionally use “Scans to Average” to increase the signal to noise ratio and “Boxcar Width” to smooth the curve.

3. Click the “Store Reference Spectrum” icon to store the reference spectrum.

4. Block the light path so that no light is getting into the detector.

5. Click the “Store Dark Spectrum” icon to store the dark spectrum. Unblock the light path.

6. Click on the “I” icon for Relative Irradiance Mode. Note that the y-axis is now in Relative Irradiance units. You are now in Relative Irradiance Mode.