What is the formula for percent transmittance in spectrophotometry?

• A. TL/IL x 100
• B. IL/TL x 100
• C. (TL-IL)/IL x 100
• D. TL+IL

Answer: (A)

In spectrophotometry, percent transmittance tells you what fraction of the incident light actually passes through the sample. It is defined as the transmitted light intensity divided by the incident light intensity, times 100. Using TL for transmitted light and IL for incident light, that is T% = (TL/IL) × 100. This makes intuitive sense: if no light is absorbed, TL equals IL and you get 100%; if the sample absorbs all the light, TL is zero and you get 0%.

The other expressions don’t reflect a true fraction of light that gets through. IL/TL would invert the ratio and can give values that don’t properly represent transmittance. (TL − IL)/IL would yield zero when transmission equals incident, which isn’t correct for percent transmittance. Adding TL and IL isn’t a ratio at all and doesn’t express the transmitted fraction.