Transmittance
The ratio of radiant power transmitted through a sample to radiant power incident on it.
When a sample appears to absorb light, the instrument first observes a change in transmitted light relative to the reference. Transmittance is the most direct ratio between these two radiant-power measurements.
Transmittance T is defined as radiant power P after passing through the sample divided by incident or reference radiant power P₀. T = 1 means the two are equal; a smaller T means less light reaches the detector. Absorbance is related by A = −log₁₀(T).
Displayed percent transmittance is simply T multiplied by 100%. Because absorbance uses a logarithmic scale, the same absolute change in transmittance does not correspond to the same absorbance change in different signal regions. Particular care is needed at low transmittance.
T = P/P₀；A = −log₁₀(T)
Transmittance is a dimensionless ratio; percent transmittance is 100T%.Loss of light does not necessarily mean absorption alone
Total transmission may include the effects of absorption, interface reflections and scattering. Molecular absorption in a clear solution is closer to the ideal model, whereas turbid cell suspensions, particulate samples or bubbles in a liquid column deflect some light away from the detection path. Transmittance still decreases, but the entire change cannot be attributed to absorption by target molecules.
A Blank establishes reference conditions and should match the solvent, buffer, container or measurement surface as closely as possible. Differences in matrix, liquid level or cleanliness between Blank and sample enter P/P₀ directly.
Why low transmittance is harder to measure
A highly absorbing sample leaves very little intended transmitted light. Source fluctuations, detector noise and stray light become larger fractions of the total signal. A displayed number does not necessarily still meet the method's accuracy and linearity requirements.
For low-transmittance samples, dilution or a shorter path is usually more meaningful than indiscriminately increasing integration time. Also check the full spectrum for plateaus, assess sample scattering and confirm that the measurement region has been verified with reference materials or method validation.
Transmittance describes what the optical system receives; absorbance is a mathematical expression of that ratio. Both require interpretation in the context of sample condition and measurement conditions.