The threshold is in terms of the data values in your images. If the data is a clinical CT, then it would be HU. But your data appears to be a scan of a printed film, so the units are probably not calibrated.
The threshold is in terms of the data values in your images. If the data is a clinical CT, then it would be HU. But your data appears to be a scan of a printed film, so the units are probably not calibrated.