# What is Optical Density?

Jessica Reed

In the most general sense of the term, optical density measures how much light an object absorbs and how much of the light passes through the object. In the science and engineering world, optical density is used to determine the types of materials that make up an object. Engineers and scientists can use optical density to find out more information about the properties of an object, like the components and properties of oil, or in the medical field to examine bacteria and proteins found in a cell.

Scientists working on medical experiments may use optical density to study cells. A cell is suspended and a beam of light passed through it. Based on how much of the light passes through, the scientist can determine certain organisms like bacteria that are growing within the cell. High populations of bacteria change the optical density of an object and keep light from passing through it. Cells with fewer bacteria may allow more light to pass through.

To see the results of these tests, a spectrophotometer is used. This device measures how much light is reflected off an object or passed through an object in terms of wavelengths. Once the cell or other test material is placed inside the spectrophotometer, it passes a specific beam of light onto the sample and the readout expresses the results. The scientist can then determine certain properties about the cell in question, such as the amount of bacteria residing in it.

What the spectrophotometer is reading isn't just the absorption of the light, but also the scattering of the light. When considering optical density, it's important to remember that light may also scatter when it hits an object. The more bacteria present in a sample, the more the light will scatter when the spectrophotometer tries to pass a beam through it. Simple properties such as this allow scientists to study materials and determine more about what they're made of.

Mathematical formulas are used to calculate optical density, also known as absorbance. The mathematical formula divides the light's intensity before passing through the sample by the intensity after passing through the sample. It then inserts this result as the exponent of logarithm with a base of 10. After calculating the logarithm with this number plugged in, the answer is the optical density of the light at a specific wavelength.