Skip to content

Components of a Spectrophotometer

Light Source

  • Must be able to produce light within the range that will be used to excite the sample

Continuous Sources

  • Produce light in a wide range of wavelengths using blackbody radiation
  • Each wavelength will have a different relative intensity that must be accounted for in reading the background
  • Multiple lamps may be used to get complete coverage of the range to be studied
    • E.g. Tungsten lamp (Vis) + Deuterium lamp (UV)


Line Sources

  • Excitation of a specific element without blackbody will produce a line spectra
  • The spectra emitted must match the sample to be analysed
    • E.g. Ca lamp to analyse Ca content


  • Produce directional light within a specific range of wavelengths
  • High power
  • Very expensive to fine tune a laser to desired specifications

Sample Holder

  • Must be transparent to the light being analysed
  • Must be chemically inert so as to not react with the sample
  • Must be mechanically strong, so as to ensure a long life
    • Expensive and weak is unsustainable



  • Split light based on the varying refractive indexes of different wavelengths
  • Must be transparent in the wavelength to be used


Diffraction gratings

  • Implement many fine scratches on a surface to diffract light of different wavelengths at different angles
  • Much cheaper and ore reliable than prisms
  • The width of the grooves determines the wavelengths diffracted
    • 50\(\mu\)m apart for Far IR
    • 166nm apart for UV-Vis
  • Can be made of plastic to reduce cost further


Controlling the light source

  • The light needs to be collimated (made parallel) and focused appropriately to select for individual wavelengths
  • The Reflection grating will scatter the light, and the light will be focused again by a concave mirror
  • The exit slit will determine the selected wavelength
    • The narrower the exit slit, the greater the precision of the instrument, however the less light will be passed through the sample
    • A Larger slit will result in a higher SNR


  • Use two principles:
    1. External photoelectric effect: electrons become free from the metal surface by energy absorption obtained by streams of incident photons
    2. Internal photoelectric effect: free charge carriers are generated by absorption of incident photons in semiconductor junctions
  • Choice off photodetector depends on the wavelengths being studied


  • Light falls on a photosensitive alloy which causes electrons to be fired towards secondary electrodes called dynodes
  • This causes them to gain more energy and release more electrons (4-50 per dynode)
  • There is a cascade of dynodes so the process is amplified to create a much larger signal than a single photon would otherwise produce
  • The signal is then detected by an anode and sent to a computer to plot the results

Photodiode Array

  • Consist of an array of silicon based photodetectors
  • As light hits the substrate, electrons are released, which cause a change in current required to power the substrate. The change in current is detected and plotted
  • The array is set up in a way that each diode will detect for a specific wavelength range
  • Much cheaper to produce yet are less accurate
  • Have lower resolution that photomultipliers but can measure multiple wavelengths at the same time
  • Contains no moving parts, which makes them more durable