What is transmittance?

Transmittance is the ratio of radiant power transmitted through a sample to reference radiant power at a specified wavelength, commonly written T=P/P₀. P₀ is not a fixed constant independent of the experiment; it is the incident or baseline signal defined by the reference measurement. Changes in the Blank, container or optical path may change the reference used in subsequent calculations.

Stronger absorption by the target substance usually reduces transmitted light at the detector and thus lowers transmittance. But signal loss is not due only to absorption: sample scattering, interface reflections, container contamination, bubbles and optical misalignment can all reduce light traveling in the intended direction. Transmittance is first a measured ratio, not the fraction of light absorbed by the target substance.

If a transmittance spectrum shifts overall or fluctuates abnormally, first compare Blank and sample spectral shapes and check container orientation, liquid level, bubbles, particles and reference stability before deciding whether absorption has really changed. At low transmittance, even a small amount of additional background light can become a larger relative contribution, causing underestimated high absorbance or loss of linearity.

Transmittance alone does not directly provide concentration or purity. Conversion to absorbance and subsequent concentration assessment require specified wavelength, reference conditions and effective path length, and confirmation that the sample satisfies the chosen quantitative relationship. Values from different arrangements or containers likewise cannot be compared directly until experimental conditions are aligned.