To get original reflectance data from the satellite set the custom script accordingly. REFLECTANCE is a physical format, which requires a 32-bit TIFF float. The values ​​are from 0 to 1. See example below:{INSTANCE_ID} ...&LAYERS=B04_REFLECTANCE&FORMAT=image/tiff;depth=32f

Custom script example for B04_REFLECTANCE:

return [B04]

Custom script band input values are already in reflectance (hence the simple script).

We suggest to also check the FAQ about details of internal calculation.