The attenuation experienced by a hypothetical perfectly collimated beam of light. Represented as c, it is equal to a (absorption coefficient) + b (scattering coefficient). It is measured by a transmissometer, which ideally receives only the unscattered light that traverses the path from its source to its receiver.
Because every practical transmissometer has imperfect beam collimation and a finite field of view, some scattered light is collected by the receiver, causing c to be underestimated. For example, HOBI Labs' c-ßeta has divergence and FOV of less than 0.5° (half-angle), resulting in c values that are slightly higher (and more accurate) than other transmissometers with wider divergence angles.