Method and apparatus for determining the distance to an object emitting an IR signature

1. 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.

2. The method as claimed in claim 1 , wherein:

the distance between the detector and the object is determined at two different points in time; and

relative speed between the detector and the object is calculated from a temporal shift in the position.

3. The method as claimed in claim 1 , wherein the control voltage has a periodically varying voltage component ΔU and a predefinable fixed offset voltage component U 0 .

4. The method as claimed in claim 1 , wherein

a variable proportional to the offset Δ is used as a controlled variable for tracking the magnitude of the control voltage, with the offset Δ being regulated to zero.

5. The method as claimed in claim 1 , wherein:

a time interval is selected symmetrically around the time t 0 ; and

the sum of the products of each digitized value of the differentiated detector signal with the associated value of the periodic component of the control signal is calculated within the interval of time.

6. The method as claimed in claim 1 , wherein the predefined wavelength range is in the vicinity of atmospheric absorption lines that are caused by gases whose concentration fluctuates only slightly with climatic conditions.

7. The method as claimed in claim 6 , wherein the predefined wavelength range is in the mid-infrared on the short-wave and long-wave sides of the CO 2 absorption line, at approximately 4.25 μm.

8. An apparatus comprising:

input optics with an electrically tunable wavelength filter having a voltage-dependent filter characteristic curve, and a passively imaging detector for detecting an entire two-dimensional image field of an object to be investigated;

a first circuit for processing a detector signal generated by the passively imaging detector, and

a second circuit for generating a periodically varying control voltage for the wavelength filter;

wherein, the first circuit comprises a differentiating element, an analog/digital converter and a signal analysis processor,

wherein the detector and electrically tunable wavelength filter are configured to measure an intensity distribution spectrum of the object in a region of an absorption structure of the atmosphere;

wherein a control voltage is supplied to the wavelength filter for controlling filter properties of the filter;

wherein a detector signal of the detector is differentiated by the differentiating element, then digitized by the analog/digital converter, and referenced to an instantaneous value of the control voltage applied to the wavelength filter by the signal analysis processor;

wherein the signal analysis processor is configured to

compare maximum and minimum values in the differentiated detector signal with predefinable threshold values;

determine an offset Δ in the form of a temporal shift Δt if the magnitudes of the maximum and minimum values exceed a threshold value, wherein the offset Δ 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;

determine an associated wavelength λ(t 0 +Δt) from said difference using an associated value of the control voltage U(t 0 +Δt) and a characteristic curve of the wavelength filter;

determine 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

determine 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.

9. The apparatus as claimed in claim 8 , wherein the second circuit comprises a clock generator, a function generator for generating a periodically varying voltage in response to input of clock signals from the clock generator and a summation element for summing the periodically varying voltage generated by the function generator with the voltage from a constant voltage source to produce the control voltage.

10. The apparatus as claimed in claim 9 , wherein the first circuit and the second circuit form a closed control loop together with the constant voltage source.