The histogram output lets us look a little at the Lepton 3.5's internal operation. I put the sensor just above a flat black metal surface (emissivity approaching 1.0) that has a reasonably constant temperature (~21.9°C at the time of measurement).
The default operation is Radiometry enabled - TLinear output so that every pixel value from the Lepton is an absolute temperature with resolution of 0.01 K. After letting the sensor run for about 30 minutes the output looks like this.
There appears to be a temperature gradient across the surface of the sensor. Perhaps the outer areas are prone to absorb radiant heat being generated by the Lepton and the ESP32 module on the other side of the board. I won't claim that this setup is incredibly accurate but the temperature measured by the thermistor on the surface is different than the temperature reported by the Lepton by more than the +/- 5°C accuracy claimed in the spec sheet. At some point I'd like to make a temperature controlled black-body radiator for some more precise accuracy measurements. This might allow additional calibration factors to be applied on a per-sensor/camera basis for improved accuracy.
The software's data processing algorithm scales the radiometric data to 8-bits to simplify display through the various look-up tables. This accounts for some of the quantized look of this histogram data as the 120 possible radiometric values for a 1.2°C delta between minimum and maximum pixel temperatures is being displayed in a 256 point field. However it appears the Lepton's internal algorithm may be slightly reducing the resolution as we only see 60 bin values.
The placement of the ESP32 behind the Lepton maybe a source of error in this design if it causes a thermal gradient to appear across the camera. I'm not sure this is the case however because over the course of many images I have not seen a constant gradient and usually the coldest part of the image is more-or-less centered.
Interesting, also, to see what effect AGC has on the output data. This image was taken only seconds after the one above.