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New Blackbody Radiator and Lepton Accuracy testing

A project log for Lepton 3.5 Thermal Imaging Camera

Documenting my experiments with the FLIR Lepton 3.5 thermal imaging camera.

Dan JulioDan Julio 05/11/2021 at 17:490 Comments

Trawling eBay netted a reasonably inexpensive Accuthermo FC100D TEC (Peltier thermo-electric cooler) controller mounted in an enclosure from Wells-CTI with a 12VDC 8A power supply and TEC H-bridge driver.  The FC100D uses PWM and bi-directional control of the TEC along with a NTC temperature sensor to provide precise temperature control (heating and cooling).  I used a 12V TEC sandwiched between an aluminum block (with a hole drilled in it for the NTC temperature sensor) and a CPU cooler heatsink with fan.  This setup can be controlled between 10-60°C.  The front surface of the aluminum block was coated with Black 3.0 paint to form a high-emissivity black body radiator.  Unlike the IVAC9000 this radiator can be adjusted to specific temperatures.

Temperature Readout Script

Previous testing (two log entries ago) revealed how the Lepton’s radiometric output varied for the first approximately 30 seconds after a flat-field correction.  Using the python library I wrote a simple script that triggers a FFC, waits 45 seconds and then averages approx once/second spot-meter readings over the next 90 seconds to create an average temperature reading for use in determining a Lepton’s accuracy and linearity over a temperature range.

I also removed the potentiometer from the IVAC 9000’s high-side because it caused too much temperature drift.  The IVAC9000 is now my temperature reference at 26°C and 38°C.  Using the new script produced the following results using two cameras with a distance of 15 cm between the camera and the radiator.  I made several runs with each camera at each temperature to check the repeatability of measurements after a FFC.  Camera tCam-Mini-B132's emissivity = 1.0, tCam-Mini-CB49 emissivity = 0.97.

CameraIVAC9000 temp
Run1Run2Run3Run4Run5Avg
B13226.0425.4425.4025.4225.4425.6325.47
37.9435.7036.8436.9736.9336.8136.65
CB4926.0426.3126.3526.1126.4526.3226.31
37.9438.3138.7838.7238.4638.8038.61

The longer average values show fairly low deviation between runs (each run takes over two minutes) and the average accuracy of the Lepton is pretty high (operating at room temperature).

Comparing new radiator with IVAC9000

Next I adjusted the new black body radiator to 26°C and used the same cameras to measure its output using the new script so I could get an idea of the accuracy of the FC100D and its temperature sensor.   Several runs were done again.

CameraRun1Run2Run3Run4Run5Run6Avg
B13224.9225.1525.1724.9725.3125.3525.15
CB4925.8325.5725.9525.2526.4526.1125.86

Averaging all runs showed that the new radiator runs less than 0.5°C lower than the IVAC9000 unit at 26°C.

Measuring Lepton accuracy across a series of temperatures

Finally I ran a series of readings with two cameras at 10, 20, 30, 40, 50 and 60°C and plotted both the individual averages to look at variability and then an average of averages to look at a highly averaged accuracy.

Camera

SetpointRun1Run2Run3Run4Run5Avg
CB49108.818.057.378.177.868.05
2020.3018.4319.2118.6519.5819.23
3030.4129.9830.3030.2930.4330.28
4041.6341.4841.5041.6640.8341.42
5052.3653.3752.1452.6852.5752.42
6063.0963.5463.4963.5563.5263.44
62F1108.487.138.608.578.668.29
2019.3119.2819.1819.2519.2819.26
3029.4629.0629.4729.4829.6229.42
4040.0039.8940.0239.6840.2139.96
5048.8950.6050.6550.7250.3550.24
6060.7961.0960.0460.8360.4360.64

Plotted data

I think the results are good news.  The Lepton appears, at least over this temperature range, to be linear device amenable to a two-point calibration (gain/offset).  Up next is to decide if a calibration is best applied to radiometric data once it’s been read from the Lepton or if it can be applied to the Lepton’s internal calculations using its RBFO parameters.

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