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Components of a Spectrophotometer

While component types and devices vary from brand to brand, the core principle of how a spectrophotometer works stays largely the same. Listed below are some of the key components that make measuring transmittance possible.

The light is emitted from a light source and travels through an oval-shaped lens called a collimator. The light travels through a prism-shaped monochromator and is separated into its component wavelengths. The light reaches a wavelength selector or a slit, and only one specific wavelength of light is let through. The light with an intensity of I 0 then travels through a cuvette filled with a sample solution. The light that passed through the cuvette has an intensity I t. A photocell or detector measures the intensity of this light, and the results are displayed on a digital display or meter.

Figure 1: Components of a spectrophotometer: Light emitted from the source passes through the slit, letting only one specific wavelength through. This light partially passes through the sample placed in a cuvette, and the detector measures the intensity on the other side of the cuvette.

Core components

  • Light source: An internal lamp that is the origination point for measurement light.
  • Collimator: Can be a mirror or a lens, depending on instrument configuration. Bends light so that photons travel parallel to each other.
  • Prism: Some instruments use a diffraction grating rather than a prism, but the goal of the component is the same. Prisms and diffraction gratings refract light to separate out the different wavelengths. Rotating the prism changes the wavelengths that fall over the hole of the slit card, allowing the user to select a wavelength for use.
  • Slit or Aperture: In some instruments, the size of the hole through which light passes can be controlled ('slit width' value). Only light that falls over the hole will be permitted to fall on the sample. This allows a specific section of wavelengths to be selected. Research quality instruments often allow you to control the width of the aperture to widen the band of wavelengths permitted to fall on the sample.
  • Sample holder: It's where the cuvette goes!
  • Detector: Most often, these are photovoltaic cells. Such components convert the intensity of light falling upon them to a change in voltage in a circuit, thus converting a light signal to an electrical signal that can be interpreted by software.

Together, the collimator, prism and slit components form a unit called a monochromator.