People frequently inquire as to the sensitivity of Ocean Optics spectrometers -- particularly, sensitivity as a function of wavelength. The issue typically comes up in the context of the user wanting to convert the amplitude of the raw data spectra -- or the "Scope" mode, as it's denoted in OOIBase32™ Spectrometer Operating Software -- into some meaningful energy spectra. 

To understand the answer, be aware that it is not practical to apply correction factors for the various phenomena that affect the amplitude. We provide a more useful alternative: a NIST-traceable radiant standard (the LS-1-CAL), which can be used to normalize the spectra to energy terms. In our OOIBase32 operating software, this normalized data can be processed in the "I" (Irradiance) mode as relative energy (scaled 0 to 1). In our OOIIrrad Irradiance Measurement Software, the data can be processed in absolute terms (calculated in mW/cm²/nm or in lumens or lux per unit area).

For experiments investigating transmission or reflection, the data are normalized to the spectra of a physical standard such as transmission in air or reflection of a diffuse white standard.

These are some of the factors affecting spectrometer system amplitude:

• CCD detector response. A typical response curve of an unmodified silicon detector is available from our supplier, although it provides only part of the story. We add a coating to the CCD to spoil an optical cavity formed by the SiO2 layers in the structure. This eliminates large oscillations in amplitude that vary with wavelength. For UV response, we add a phosphor coating. The data in the manufacturer's detector specifications is at best an approximation of the response you can anticipate from the detector in your system.
  
• Fiber attenuation. Attenuation is fairly flat in the Visible but increases dramatically in the UV region. In the NIR, there are water absorption bands at 750 nm and 900 nm that affect fiber attenuation, as shown in these spectral attenuation curves.
  
• Grating efficiency. All ruled or holographically etched gratings optimize first-order spectra at certain wavelength regions, depending on the blaze wavelength of the grating and other factors. We offer 14 gratings; each has characteristics that define its efficiency. Use these grating charts to compare grating efficiencies. 
  
• Collection optics. Sampling optics can have spectral signatures themselves. A good example would be the collimating lenses used in our cuvette holder assemblies. These are simple lenses with chromatic aberrations, which vary with their focus. You can see the magnitude of these chromatic effects in these UV cuvette holder transmission curves.
  
• Sources and samples. Light sources and samples also have their own spectral response. If the light itself is the sample, this spectral response is what you're trying to measure. If you are using the light to measure a sample -- such as in transmission and reflection experiments -- then the spectra of the light source must be considered. An example of such a spectra is available in the description of our LS-1-Tungsten Halogen Light Source.
  
• Other factors. Some characteristics of the design and electronics of the CCD array also can affect sensitivity. For example, the detector's voltage signal includes an offset consisting of dark-current signal and an amplifier zero set point that we call "Dark." This value varies from pixel to pixel, so it must be subtracted for each CCD element. Also, there is some variation in responsivity among pixels, so that data normalization must be done on a pixel-by-pixel basis as well. (This normalization removes so-called "fixed pattern noise.")
  
  The only practical way to account for all of these factors is to do a calibration experiment and "normalize" the data by comparing the sample spectra to a suitable reference spectra:
  
  1. %Transmission(i) or %Reflection(i)  = [S(i)-D(i)]/[R(i)-D(i)], where S is the sample intensity at pixel (i) (CCD pixel number from 0 to 2047), D is the dark intensity at pixel (i), and R is the reference intensity at pixel (i);      
  
  2. Absorbance(i) = -log[T(i)]; or
  
  3. Energy I(i) = B(i)[T(i)], where B is the spectra of the radiant standard.

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Operating Principles