Input data format for Face Detect IR

What sort of preprocessing has to be done on temperature data (floating point image) to produce the 8bit grayscale image used as input for face detect ir model (i.e. how to map temperature to grayscale values?). Was there any specific temperature range for training images the model was trained on? We have already tried different mappings and got different results but none resulted in consistent detections.

Many thanks,
Jan