The sniffer space

The sensors inside an e-Nose are made using diverse technologies. Depending on the type of sensor, a certain physical property is changed as a result of exposure to a gaseous chemical. During the measurement process a signal is obtained by constantly recording the value of the physical property. Since a typical signal is comprised of a few hundred measured values, a process of feature extraction is frequently required, which is the process of finding a small set of parameters that somehow represent the entire signal.

The set of features extracted from all the signals in a single measurement is called the feature vector, and if there are m features the vector can be viewed as an odorant vector in the m-dimensional sniffer space. When exposed to mixtures of chemicals, e-Noses produce a feature vector that reflects the combined effect of the mixture constituents. Yet, the feature vectors of a mixture do not noticeably differ in any aspect from those of pure chemicals, and in this sense e-Noses do not distinguish pure chemicals from mixtures.

As the brain maps the sensory space into the psychophysical space, we can think of an analog algorithm that maps odorant vectors in the sniffer space to their corresponding odorant vectors in the psychophysical space. We shall call this the mapping algorithm, and denote it by the function f; hence, d P (o; c) =f (d S (o; c)).