当入射光强度I0一定时，介质吸收光的强度Ia越大，则透过光的强度It越小。用It/I0表示光线透过介质的能力，称为透光率，以T表示，即T=It/I0。

透光率的数值为百分数，取值范围为0~100%。如果光被介质全部吸收，It=0，T=0。而若光全透，则It=I0，T=100%。透光率的倒数反映了介质对光的吸收程度。为了便于计算，取透光率倒数的对数作为吸光度，用A来表示，即A=Ig（1/T）=lg（I0/It）。

When the incident light intensity I0 is constant, the greater the intensity ia of the medium absorbing light, the smaller the intensity it of the transmitted light. It / I0 is used to represent the ability of light to pass through the medium, which is called transmittance. It is expressed in T, that is, t = it / I0.

The value of light transmittance is percentage, and the value range is 0 ~ 100%. If the light is completely absorbed by the medium, it = 0, t = 0. If the light is fully transparent, it = I0, t = 100%. The reciprocal of transmittance reflects the degree of light absorption of the medium. For the convenience of calculation, the logarithm of the reciprocal of light transmittance is taken as the absorbance, which is expressed by a, that is, a = Ig (1 / T) = LG (I0 / it).