Patent ID: 8586929

Claim:
A method for determining the distance of an object emitting an IR signature by using a detector to detect a spectral intensity distribution of radiation emitted by the object in a predefined wavelength range, said method comprising: measuring the intensity distribution spectrum of the object in the region of an absorption structure of the atmosphere; determining in the measured intensity distribution spectrum a point having a maximum gradient on a flank of an intensity rise or fall caused by the atmospheric absorption structure; and determining a path length traveled by the radiation through the atmosphere, and therefore also the distance between the detector and the object, by comparison with known transmission data for the atmosphere, wherein the intensity distribution spectrum is detected using said detector and an electrically tunable wavelength filter; a control voltage is supplied to the wavelength filter for controlling filter properties of the filter; a detector signal of the detector is differentiated, then digitized, and referenced to an instantaneous value of the control voltage applied to the wavelength filter; maximum and minimum values in the differentiated detector signal are compared with predefinable threshold values; if the magnitudes of the maximum and minimum values exceed a threshold value, an offset Δ in the form of a temporal shift Δt is determined as the difference between occurrence of the maximum value in the differentiated detector signal and time t 0 of a zero crossing in a periodic component of the control voltage; and an associated wavelength λ(t 0 +Δt) is determined from said difference, using an associated value of the control voltage U(t 0 +Δt) and a characteristic curve of the wavelength filter.