An unwanted input frequency that arises from a source other than that to which a receiver used in heterodyne reception is tuned and that causes spurious signals to be output; the unwanted response is generated in the intermediate-frequency amplifier. The spurious signals are termed image interference. The image frequencies are those frequencies such that the difference between them and the local-oscillator frequency falls within the bandwidth of the intermediate-frequency amplifier. The characteristics of the particular receiver determine the input conditions that cause image interference. The image frequencies can be equal to the desired input signal frequency, equal to the intermediate frequency, or to twice the local-oscillator frequency.

The image ratio is the ratio of the image-frequency amplitude to the desired signal amplitude when identical outputs are produced.