Difference between absorbance and transmittance
Share
When defining how light interacts with optical components like bandpass filters, absorbance and transmittance are two sides of the same coin, but they measure that interaction in fundamentally different ways.
Here is the straightforward breakdown of the difference.
Transmittance
Transmittance measures how much light successfully passes through a sample or optical component.
- What it represents: It is the ratio of the transmitted light intensity (I) to the initial incident light intensity (I0).
- The Formula: T = I / I0
- The Scale: It is a linear scale, usually expressed as a percentage from 0% (completely opaque, no light passes through) to 100% (completely transparent, all light passes through).
Absorbance
Absorbance measures how much light is captured or absorbed by the sample.
- What it represents: Instead of measuring what makes it out the other side, absorbance calculates what was lost inside the material.
- The Formula: Absorbance is the negative logarithm of transmittance. A = -log 10(T) or A = log10(I0 / I).
- The Scale: It is a logarithmic scale. An absorbance of 0 means 100% of the light is transmitted. An absorbance of 1 means 10% of the light is transmitted. An absorbance of 2 means 1% is transmitted, and so on.
Key Differences at a Glance
| Feature | Transmittance (%T) | Absorbance (A) |
| Measures | Light that makes it through | Light that is stopped/absorbed |
| Scale Type | Linear | Logarithmic |
| Range | 0 to 100% | 0 to infinity (practically 0 to ~3) |
| Mathematical Link | T = 10-A | A = -log10(T) |
| Primary Use | Rating the clarity and efficiency of optical filters, lenses, and glass. | Determining the concentration of a substance (via the Beer-Lambert law). |
Because absorbance is logarithmic, it is incredibly useful when you need to quantify very dark samples or calculate the exact concentration of a material. Transmittance is generally more intuitive for understanding the baseline efficiency of an optical setup.