Spherical Cap Blackbody for IR Sensor Calibration
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Solution Overview
Problem
Calibrating infrared image sensors with a large field of view using commercially available flat blackbody surfaces is time-consuming and prone to adjustment issues due to shadowing effects from the image sensor itself, especially when trying to achieve uniform luminance across the entire field of view.
Innovation Solution
A device featuring a heatable metal surface coated with a highly emissive material, designed as the inner surface of a spherical segment with a spherical cap, allowing the infrared sensor to be calibrated from the center, providing uniform luminance independent of the viewing angle through a spherical cap structure that minimizes shadowing effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a flat blackbody surface is used for calibration, then the device structure is simple, but shadowing effects from the image sensor cause non-uniform luminance for large field of view sensors
Solution Approach 1:
The patent applies a spherical cap geometry for the blackbody radiating surface. This curved surface ensures that all detector elements in the wide-field sensor receive radiation at approximately normal incidence angles, eliminating the cos⁴θ falloff that plagues flat surface calibrations. The spherical cap with radius R and opening angle 2α is specifically designed so that the entrance pupil of the sensor lies at the center of curvature, ensuring uniform luminance across the entire field of view.
2Manufacturing precision
If section-by-section calibration is performed on flat blackbody surfaces, then shadowing effects are reduced, but calibration time increases significantly
Solution Approach 1:
The spherical cap surface can be conceptually divided into multiple zones corresponding to different detector elements, but all zones are calibrated simultaneously in a single setting. The geometry ensures that each zone receives uniform radiation from the blackbody surface without requiring sequential adjustment, thus achieving both uniformity and efficiency.
Solution Approach 2:
The spherical cap calibration device is designed to accommodate sensors with various field of view angles by adjusting the opening angle 2α. A single device configuration can calibrate multiple sensor types, eliminating the need for multiple calibration setups or section-by-section procedures.
3Object-affected harmful factors
If the entrance pupil is positioned in front of the front lens for wide-angle optics, then shadowing effects are minimized, but the entrance pupil is actually inside the optics and cannot be accessed
Solution Approach 1:
The patent positions the entrance pupil at the center of curvature of the spherical cap in three-dimensional space. This geometric arrangement allows the virtual entrance pupil location to be accessed from the outside through the aperture, even though physically it lies within the optical system. The spherical cap surface is positioned at a distance R from the entrance pupil, allowing radiation to reach the pupil without being blocked by the sensor housing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient radiometric calibration of IR image sensors with a large field of view in a single setting by maintaining uniform luminance and minimizing power variation across the field of view, reducing the impact of shadowing effects compared to flat blackbody surfaces.
Implementation Method 1
a heatable metal surface serves as the radiating surface I, which is coated with a highly emissive material
Implementation Method 2
The inside I of the spherical segment 1 is coated with the highly emissive material... an emissivity close to 1, according to the invention >0.9, is achieved
Implementation Method 3
The ball segment 1 is advantageously made of copper... Means 2 for tempering the spherical segment are soldered onto the outside of the spherical segment
Implementation Method 4
A thermocouple 3 is provided on the outside of the spherical segment 1 to regulate the temperature of the radiating inner surface
Data Source
Figure 1
AI summary
The invention relates to a device for the radiometric calibration of infra-red measuring devices (6) in the form of a heated metal surface that is used as a radiant surface (I) and is coated with a high-emission material. Said device comprises a spherical segment (1), the interior (I) of which is coated with the high-emission material and the opening (A) of the spherical segment is used as an aperture for an infra-red device (6) that can be brought into the centre of the aperture.