Massive multi-aperture configuration of accurately calibrated uncooled LWIR sensors enables 3D perception in extremely low-contrast environments, at long range and high speed.
At Elphel, we perform camera calibration with a large LWIR/RGB calibration pattern and goniometer for precise automatic rotation.
Our dual-modality 7.0 m × 3.0 m calibration target consists of five separate panels fitted together.
We use twelve 500 W halogen floodlights to illuminate the pattern printed on a 5 mm foamcore board glued to a 0.8 mm aluminum sheet. The forced airflow cools its back side.
This design provides a definite correspondence between the image registered by the visible range cameras (bottom) and the surface temperature (top), resulting in time-invariant images for both modalities.
Calibration is performed from 4 positions, accumulating 700 image sets.
The camera captures pattern images from four locations (2 meters left and right at a centerline at 4.5 distance, center at 7.4m, and 22.0m ) to have multiple views of each part of the pattern from multiple points in different areas of each sensor. This procedure allows efficient bundle adjustment of the camera extrinsic and intrinsic parameters and simultaneously measures the pattern geometry.
From each position, the camera scans the range of ±40° horizontally and ±25° vertically, so each pattern region covers each sensor’s entire Field of View (FoV).
4 RGB and 16 LWIR images are simultaneously acquired while the camera is rotating.
The large pixel-size (12μm), high lens numerical aperture (0.7) with a long focal length (14mm) result in 20m minimal focus distance.
The black-and-white pattern resembles the checkerboard one with each straight edge replaced by a combination of two arcs – this makes spatial spectrum uniform and facilitates point spread function (PSF) measurement.
The calibration image processing starts with measuring each detected pattern grid node coordinates to 0.03-0.05 pixel accuracy using 2D correlation to the locally fitted pattern model.
Multilayer TIFF image format of the intermediate results (over 8 thousand files) with pixel coordinates (x,y) correspondence to pattern UV indexes facilitates data overview.
With bundle adjustment based on Levenberg-Marquardt Algorithm (LMA), we typically process 5-10 million points and 3-5 thousand adjustable parameters, achieving RMSE under 0.05 pixels after supplementing the radial lens distortion model with non-axial corrections.
The next step after distortions calculation is measuring the space-variant Point Spread Functions (PSF) by combining partial arrays, each covering a fraction of the entire sensor FoV.
LWIR Point Spread Function
The PSF for the RGB camera includes chromatic aberrations (image is not in scale and shows sparse PSF with half-pixel resolution);
RGB Point Spread Function
The inverted deconvolution kernels allow partial correction of all the optical aberrations together.
Testing camera in the field.