ENLIGHTEN™ Manual: Spectroscopic Techniques


4.3.1 Scope Mode

Scope Mode is the simplest and most straightforward spectroscopic mode, showing the spectrum as a graph of intensity (counts) versus wavelength (nm).

4.3.2 Raman Mode

Switching to Raman Mode changes the X-axis to wavenumbers (cm-1 or “inverse centimeters,” computed as Raman shifts from the configured laser excitation wavelength).

4.3.3 Reflectance and Transmission Mode

Transmission and reflectance are mathematically identical, although they represent physically different properties. Transmission represents the amount of light which successfully passes through a material without being absorbed, reflected, or scattered. Reflectance represents the amount of light which is reflected off a material, being neither absorbed nor reflected.

Both techniques require a reference spectrum to be collected. For transmission, that would be a direct spectrum of the illuminating light source, with the transmitting sample removed from the optical path. For reflectance, that could be likewise a direct spectrum of the illuminant, or perhaps a diffuse reflection thereof taken with a reflectance standard like Spectralon® or an appropriate synthetic resin (Delrin® or Teflon™).

Transmission (T) and reflectance(R) are both computed using the same equation:

Spectroscopic equations

Technically you can generate a reflectance or transmission measurement without having access to a good optical dark measurement, for instance if your optical path does not include a mechanical shutter yet your lamp loses thermal stability if powered off. However, the quality of your measurement will be improved if you can take a true optical dark measurement.

Note that dark spectra may change slightly over time and temperature. It is desirable to “dark correct” a spectrum using a dark which was taken as close to the spectrum as possible, so that a similar thermal environment will be in effect. Therefore, ENLIGHTEN™ will track the dark spectrum used with your “reference” measurement separate from the dark measurement used with your “sample” measurement, so that the “best available” (most temporally proximate) dark will be used with each component spectrum.

To be clear, if you take the following collections:

t0 record dark
t1 record reference
t2 record sample
t3 record sample
t4 record sample
t5 record dark
t6 record sample
t7 compute transmission

…ENLIGHTEN™ will compute transmission using the spectra collected at the following timestamps:


That is because the dark collected at t0 was the most-recent when the reference was saved at t1, while the dark taken t5 at was the most-recent when the sample was recorded at t6.

4.3.4 Absorbance Mode

Changing to absorbance mode switches the graph Y-axis to Absorbance Units (AU), and the X-axis to wavelength (nm).

Like reflectance/transmission mode, absorbance requires a reference spectra to be taken before processed spectra can be generated. See the discussion on reference and dark timing in that section.

Absorbance is computed using Beer’s Law as:

Beer's Law Taking Dark and Reference Spectra

When performing operations that combine multiple spectra, which includes dark subtraction and any reference-based techniques such as transmission, reflectance and absorbance, accuracy will be improved by applying the same post-processing operations to all spectra involved.

That is to say, if you are using 10-scan averaging and a boxcar of 2 on your measurements, you should use the same 10-scan averaging and 2-halfwidth boxcar when you record dark and reference spectra. This ensures you are subtracting “apples from apples” as it were.

4.3.5 Other Modes

Future versions of ENLIGHTEN™ will include Color Processing modes.