Attenuated total reflection （ATR） ratio is usually utilized to study the properties of surface plasmon resonance （SPR） sensors. The relationship between normalized light intensity and ATR ratio is investigated, and a modification coefficient is put forward to describe the relationship. A mathematical expression is built up for the coefficient based on Fresnel principle. The result shows that the ATR ratio, which cannot be measured directly in experiments, can be determined with the coefficient and the normalized intensity of light. The characteristic of the coefficient is also discussed.
