Bouguer looked at astronomical phenomena where the size of a detector is very small compared to the distance traveled by the light. In this case, any light that is scattered by a particle, either in the forward or backward direction, will not strike the detector. The loss of intensity to the detector will be due to both absorption and scatter. Consequently, the total loss is called attenuation (rather than absorption). A single measurement cannot separate the two, but conceptually the contribution of each can be separated in the attenuation coefficient. If I 0 is the intensity of the light at the beginning of the travel and I d is the intensity of the light detected after travel of a distance d , the fraction transmitted, T , is given by: T = I d I 0 = exp ⁡ ⁡ ( − − μ μ d ) , where μ μ is called an attenuation constant or coefficient. The amount of light transmitted is falling off exponentially with distance. Taking the natural logarithm in the above equation, we get: − − ln ⁡ ⁡ ( T ) = ln ⁡ ⁡ I 0 I d = μ μ d . For scattering media, the constant is often divided into two parts, μ μ = μ μ s + μ μ a , separating it into a scattering coefficient, μ μ s , and an absorption coefficient, μ μ a .