Calculate optical density (OD) using multiple methods. Choose your calculation mode — enter transmittance, incident and transmitted intensities, or use the Beer-Lambert Law with molar absorptivity, concentration, and path length. Get back optical density, absorbance, and transmittance instantly.
Optical Density (OD)
--
Absorbance (A)
--
Transmittance (T)
--
Transmittance (%T)
--
Transmitted Intensity (I)
--
Optical density is a dimensionless measure of how much a medium attenuates light passing through it. It is defined as OD = –log₁₀(T), where T is transmittance. Higher OD values indicate greater light attenuation — an OD of 1 means 10% transmission, while OD of 2 means 1% transmission.
In most practical contexts, optical density and absorbance are numerically equal, both defined as –log₁₀(I/I₀). However, absorbance technically refers to pure light absorption, while optical density can also include contributions from scattering and reflection. For solutions measured in a spectrophotometer, the two terms are used interchangeably.
Use the formula OD = –log₁₀(T), where T is transmittance expressed as a decimal (0 to 1). For example, if 25% of light passes through, T = 0.25 and OD = –log₁₀(0.25) ≈ 0.602. If you have transmittance as a percentage, divide by 100 first.
The Beer-Lambert Law states that OD = ε × c × l, where ε is the molar absorptivity (L·mol⁻¹·cm⁻¹), c is the molar concentration (mol/L), and l is the path length (cm). It describes how strongly a chemical species absorbs light at a given wavelength and is widely used in spectroscopy to determine concentrations.
OD 1 means 10% transmittance (90% of light is blocked), OD 2 means 1% transmittance, and OD 3 means 0.1% transmittance. In laser safety, OD 3 or higher filters are often required for high-powered laser systems to protect eyes from damage.
In laser safety, OD ratings on protective eyewear and filters specify how much laser power is attenuated. A filter with OD 4 reduces the transmitted power by a factor of 10,000. The required OD is calculated based on the laser's power, wavelength, and the maximum permissible exposure (MPE) for human eyes.
Theoretically yes, but practically measuring OD beyond 5–6 is difficult due to stray light and detector noise limitations in standard spectrophotometers. For laser safety filters, ratings up to OD 7 or higher are specified and tested using specialized equipment.
Optical density is dimensionless — it has no units. It is simply the negative base-10 logarithm of the transmittance ratio (I/I₀), so units in the numerator and denominator cancel out. This makes it convenient to compare attenuation across different media and path lengths.