There are many different non-uniformity corrections and radiometric calibration techniques available and each technique has its own benefits and drawbacks. The major shortcomings of these calibration techniques are the limited use of preset exposure integration times and their failure to compensate for altering ambient temperature. These issues make these calibration techniques almost unfeasible in real field campaigns. This article discusses a new image correction and calibration technique devised by Telops.
Telops Calibration Technique
Telops’ new image correction and calibration technique closely match the camera physics to apply correction in the same reference domain where the effect to be eliminated appears. It relies on the characterization of fluxes [DL/µs units] rather than on the observation of the deviations of the digital level as a function of blackbody temperature or in-band radiance. The characterization of fluxes signifies the sensitivity of the camera reading corresponding to the exposure time.
Telops’ calibration of a typical camera is presented in Figures 1a/b/c. Figure 1a illustrates the characteristic deviation of the measured digital counts as a function of the exposure time, revealing the increment of the detected electron flux (slope) with blackbody temperature. Nevertheless, the intercept remains constant as it depends largely on the offset level of the readout circuit. Figure 1b shows the increment in the slope (DL/µs) corresponding to the measured blackbody temperature, revealing strong similarity between curves relative to different pixels at the first instance. Figure 1c depicts the impact of rising ambient temperature, which adds a specific flux, independent from the scene temperature. This offset is due to the camera self-emission.
Figure 1a. Change in digital counts as a function of exposure time for increasing blackbody temperatures, for a given pixel.
Figure 1b. Slopes (fluxes) as a function of blackbody temperature for a few randomly selected pixels.
Figure 1c. The expected increase in slope (flux) when the ambient temperature is increased.
Without any limitations, the Telops calibration approach may be applied to temperature or in-band radiance. The method basically carries out nonlinear interpolation for the digital level and flux domains for each pixel. It provides an integrated NUC and radiometric calibration, parameterized independently for each pixel.
Key Features of Telops Calibration Technique
The Telops calibration technique processes the data in a unique way, thus presenting unique features that are impossible with other calibration methods. The following are the key features of the Telops Calibration Technique:
- Global and permanent calibration technique, meaning that calibration parameters are acquired in factory
- Does not require any external blackbody references, thus eliminating the requirement to perform any challenging blackbody measurements in field conditions to ensure a highly precise infrared calibration
- Implemented in Telops cameras to deliver key benefits in real-time
- Valid for any ambient temperature
- Yield quantitative estimates (in-band radiance or radiometric temperature)
- Applicable to contrasted scenes
- Full compensation of analog data acquisition non-linearity
- Valid for any exposure time compatible with a given FPA
- The predetermination of the exposure time is not required
Key Advantages of Telops Calibration Technique
The key advantages of the Telops Calibration Technique are discussed in this section. After the FPA reaches its operating temperature, the infrared camera provides calibrated quantitative images devoid of any laborious post-processing, thanks to the Telops Real-Time Processing hardware. During the whole acquisition, the camera maintains an accurate measurement even if there is a change in the exposure time by a factor of 3.
After determining a full calibration dataset in the factory, users are able to generate a different data set for the support of a different lens, thereby allowing them to use the advanced performances of the Telops Calibration Technique for their own lenses. This task can be performed with fewer blackbody acquisitions under controlled conditions, compared to the requirement of a very large number of blackbody acquisitions under very restrictive conditions as in the case of multi-point approaches.
Inherent compensation for optics self-emission is another advantage of the Telops Calibration Technique, ensuring valid NUC and calibration over a broad range of ambient temperatures automatically. The Telops Calibration Technique enables operating the infrared camera in Automatic Exposure Control (AEC) mode. In this mode, the camera changes its integration time to the scene radiance level dynamically and autonomously, thus yielding homogenous and calibrated images in real-time (Figure 2).
Figure 2. AEC example showing the weak changes in radiometric temperature for a single pixel when the camera is looking at a stable blackbody source set at 25°C.
Properly calibrated infrared camera data is essential for users to extract valid data from the measurements of the camera. For that purpose, the Telops Calibration Technique ascertains clear and precise images and automatically offsets the very diverse sources of effects modifying the acquired data without any user involvement. The Telops Calibration Technique provides decision quality infrared data and ensures user-friendly camera operation.
This information has been sourced, reviewed and adapted from materials provided by Telops Inc.
For more information on this source, please visit Telops Inc.