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!)



Spectra show the intensity of light as a function of wavelength. Below is a spectral diagram for light transmitted though water in the infrared range. The high areas are wavelengths for which water is nearly transparent, while the low areas represent wavelengths for which water absorbs most of the light.

Intensity versus wavelength

absorption spectrum for water (NIST)


Spectral grating tutorial (ThorLabs)