Determination of an absolute radiometric value using blocked infrared sensors
Various techniques are provided for using one or more shielded (e.g., blinded, blocked, and/or obscured) infrared sensors of a thermal imaging device. In one example, a method includes capturing a signal from a shielded infrared sensor that is substantially blocked from receiving infrared radiation from a scene. The method also includes capturing a signal from an unshielded infrared sensor configured to receive the infrared radiation from the scene. The method also includes determining an average thermographic offset reference for the shielded and unshielded infrared sensors based on the captured signal of the shielded infrared sensor. The method also includes determining an absolute radiometric value for the scene based on the average thermographic offset reference and the captured signal of the unshielded infrared sensor.
1. A method comprising:
capturing a signal from a shielded infrared sensor that is blocked from receiving infrared radiation from a scene;
capturing a signal from an unshielded infrared sensor configured to receive the infrared radiation from the scene, wherein the shielded infrared sensor and the unshielded infrared sensor are part of an infrared sensor assembly;
determining a temperature dependent infrared radiation contribution from a shield structure of the infrared sensor assembly incident on the shielded infrared sensor;
determining an average thermographic offset reference based on the captured signal of the shielded infrared sensor reduced by the determined infrared radiation contribution;
determining an absolute radiometric value, corresponding to the infrared radiation received from the scene by the unshielded infrared sensor, based on the average thermographic offset reference and the captured signal of the unshielded infrared sensor; and
wherein the absolute radiometric value is determined without the use of a shutter.
2. The method of claim 1 , wherein:
the infrared sensor assembly is part of a wafer level package.
3. The method of claim 2 , wherein:
the absolute radiometric value is further based on infrared radiation received by the unshielded infrared sensor from a component of the wafer level package.
4. The method of claim 1 , wherein:
the absolute radiometric value is further based on a sensor-specific thermographic offset reference of the unshielded infrared sensor; and
the method further comprises determining the sensor-specific thermographic offset reference using a non-uniformity correction (NUC) term or a factory gain term.
5. The method of claim 1 , further comprising determining a temperature of the scene based on the absolute radiometric value.
6. The method of claim 1 , further comprising:
performing the capturing operations for a plurality of the shielded infrared sensors and a plurality of the unshielded infrared sensors, wherein the captured signals of the unshielded infrared sensors provide a thermal image of the scene comprising a plurality of pixels; and
determining an absolute radiometric value of the scene for each pixel based on:
the corresponding captured signal of the unshielded infrared sensor associated with the pixel, and
the average thermographic offset reference, wherein the average thermographic offset reference is further based on an average of the captured signals of the shielded infrared sensors.
7. The method of claim 2 , wherein:
the structure is a cap of the wafer level package.
8. The method of claim 1 , wherein the shielded and unshielded infrared sensors are microbolometers.
9. An apparatus comprising:
a shielded infrared sensor blocked from receiving infrared radiation from a scene;
an unshielded infrared sensor configured to receive the infrared radiation from the scene, wherein the shielded infrared sensor and the unshielded infrared sensor are part of an infrared sensor assembly; and
a processing device configured to:
determine a temperature dependent infrared radiation contribution from a shield structure of the infrared sensor assembly incident on the shielded infrared sensor,
determine an average thermographic offset reference based on a captured signal from the shielded infrared sensor reduced by the determined infrared radiation contribution,
determine an absolute radiometric value, corresponding to the infrared radiation received from the scene by the unshielded infrared sensor, based on the average thermographic offset reference and a captured signal from the unshielded infrared sensor, and
wherein the absolute radiometric value is determined without the use of a shutter.
10. The apparatus of claim 9 , wherein:
the infrared sensor assembly is part of a wafer level package.
11. The apparatus of claim 10 , wherein:
the absolute radiometric value is further based on infrared radiation received by the unshielded infrared sensor from a component of the wafer level package.
12. The apparatus of claim 9 , wherein:
the absolute radiometric value is further based on a sensor-specific thermographic offset reference of the unshielded infrared sensor; and
the processing device is further adapted to determine the sensor-specific thermographic offset reference using a non-uniformity correction (NUC) term or a factory gain term.
13. The apparatus of claim 9 , wherein the processing device is further adapted to determine a temperature of the scene based on the absolute radiometric value.
14. The apparatus of claim 9 , further comprising:
a plurality of the shielded infrared sensors;
a plurality of the unshielded infrared sensors, wherein captured signals of the unshielded infrared sensors provide a thermal image of the scene comprising a plurality of pixels; and
wherein the processing device is further adapted to determine an absolute radiometric value of the scene for each pixel based on:
the corresponding captured signal of the unshielded infrared sensor associated with the pixel, and
the average thermographic offset reference, wherein the average thermographic offset reference is further based on an average of the captured signals of the shielded infrared sensors.
15. The apparatus of claim 10 , wherein:
the structure is a cap of the wafer level package.
16. The apparatus of claim 9 , wherein the shielded and unshielded infrared sensors are microbolometers, and wherein the processing device is a logic device, microcontroller, processor, or application specific integrated circuit (ASIC).