The valve action of intensity receptor has the following properties which are essential to an encoder. (1) The response amplitude corresponds to the stimulus intensity in a one-to-one manner. (2) The dynamic ranges of the stimulus intensity and the response amplitude are respectively set and able to move in parallel with their axes. (3) The probability density distribution of the stimulus intensity may be transformed into that of the response amplitude by a stimulus-response relation. Concerning stimulus-response relations, three empirical formulae, i.e. logarithmic, power and tanh log functions, are well known. These formulae can be arranged for the forms to express the above three properties. Then, power and tanh log functions are converted to a logarithmic function as the exponents of them approach zero. From a viewpoint of information theory, the stimulus-response transformation may play a role to improve the probabilistic nature of stimuli in order to make an efficient code for a given channel of sensory fibre.