Spectral Analysis

The Electromagnetic Spectrum

A diagram of electromagnetic wavelengths and their types is shown below. Notice how the visible part of the spectrum is only a small part of the total spectrum.

Wavelenghs and types

Electromagnetic spectrum

Types of Spectra

A spectrum is the brightness of light for each wavelength. Many substances exhibit a characteristic pattern of bright and dark parts of the spectrum. There are several types of spectra:

  • Transmission: light emitted by electron transmissions. Typically comprised of bright lines against a dark background. (Works best for hot, glowing objects or lights)
  • Absorption: light absorbed by a fluid such as a gas or liquid. Typically comprised of reduced wavelength areas of brightness. (Works best for fluids, particularly in infrared)
  • Reflectance: light reflected by an object or area that is typically darker for some wavelengths. (Often best approach for observing solid objects, or ground/water from above; e.g. for geology)

Spectral Options

Need to determine what wavelengths we need to observe: visible, Infrared, UV?

  • Diffraction grating (transmission)
  • Diffraction grating (reflection)
  • Piece of CD (inexpensive, but quality may be insufficient)
  • Prism
  • Tunable filter
  • Multiple detector/filter combinations
  • iPhone spectrometer? (but would need an iPhone for each sensor!)

 

Water Spectrum (NIST)

Intensity versus wavelength

absorption spectrum for water (NIST)

 

Reference

Spectral grating tutorial (ThorLabs)