Difference Between Photometry and Spectrophotometry

Photometry vs Spectrophotometry
 

Photometry and spectrophotometry are two important applications of light measurements. These two methods have various applications in fields such as chemistry, physics, optics and astronomy. It is vital to have a solid understanding in these concepts in order to excel in such fields. This article presents the definitions, applications, examples, similarities and finally the differences between photometry and spectrophotometry.

What is Spectrophotometry?

To understand spectrophotometry, one must first understand the concept of spectrum, especially the absorption spectrum. The light is a form of electromagnetic waves. There are other forms of EM waves such as X-Rays, Microwaves, Radio waves, Infrared and Ultraviolet rays. The energy of these waves is dependent on the wavelength or the frequency of the wave. High frequency waves have high amounts of energies, and low frequency waves have low amounts of energies. The light waves are made up of small packets of waves or energy known as photons. For a monochromatic ray, the energy of a photon is fixed. The electromagnetic spectrum is the plot of the intensity versus the frequency of the photons. When a beam of waves having a whole range of wavelengths is passed through liquid or gas, the bonds or electrons in these materials absorb certain photons from the beam. It is due to the quantum mechanical effect that only photons with certain energies get absorbed. This can be understood using the energy level diagrams of atoms and molecules. Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. For the visible region, the perfect white light contains all the wavelengths within the region. Assume white light is sent through a solution absorbing photons with a wavelength of 570 nm. This means the red photons of the spectrum is now reduced. This will cause a blank or reduced intensity at the 570 nm mark of the plot of intensity versus wavelength. The intensity of the passed light as a proportion to the projected light can be plotted for some known concentrations, and the resultant intensity from the unknown sample can be used to determine the concentration of the solution.

What is Photometry?

The term “photo” means light and the term “metry” refers to measurement. Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. In photometry, the standard is the human eye. The sensitivity of the human eye to different colors is different. This has to be considered in photometry. Therefore, amplification methods are used so that the effect from each color would be same as that of the eye. Since the human eye is only sensitive to visible light, photometry only falls in that range.