Actually the values are reflectances. Modis files have conversion factors for radiances and reflectances.
The DNs are automatically converted to reflectances.
Level 1B generates two calibrated data products for the Reflective Solar bands (RSBs): the
reflectance and the Earth-exiting radiance. Writing these two products in floating-point format to
the Level 1B Earth view product files would make the file sizes prohibitively large.
Instead, Level 1B writes a 16-bit scaled integer representation of the calibrated digital signals
measured by the MODIS, from which the reflectances and radiances can be calculated from two
pairs of scale and offset terms written as attributes to the reflective band SDSs. These two pairs are
derived from the calibration parameters that are input to Level 1B as look up tables. Reflective
Solar Band radiance can also be calculated more precisely using calculated reflectances and
appropriate granule metadata; see Section 5.4.3.
So It sounds to me that the MODIS L1B MOD021 product should be a 16-bit integer that you can calculate either reflectances or radiances using formulas.
Reading a little further, it appears that the scaled integers (SI which is a 16-bit integer) are representations of dn** (which is a 32-bit floating point). So I am thinking that SNAP is actually showing me the dn**, not the SI?
Therefore I need to (1) convert dn** to SI and then (2) convert SI to radiances?
Interestingly when you inspect the information for just one band of the file (EV_1KM_RefSB_8) you get the following information.
These (I think) are the conversion factors to go from dn** to SI and I don’t see the radiance scaling/offset factors or the reflectance scaling/offset factors.
Name: EV_1KM_RefSB_8
Type: Band
Description: Earth View 1KM Reflective Solar Bands Scaled Integers
Geophysical unit: none
Geophysical data type: float32
Raw data type: uint16
Raster width: 1354 pixels
Raster height: 2030 pixels
Scaling factor: 2.609128932817839E-5
Scaling offset: -0.008270213339695576
Is log 10 scaled: false
Is no-data value used: false
No-data value: 0.0
Geophysical no-data value: -0.008270213339695576
Valid pixel expression: null
Spectral band index: 0
Wavelength: 412.0 nm
Bandwidth: 0.0 nm
Solar flux: 0.0 mW/(m^2*nm)
Product name: MOD021KM.A2018314.1800.006.2018315073121.hdf
Product type: MODIS Level 1B
Product description: MOD021KM.A2018314.1800.006.2018315073121.hdf
Product format: MODIS-L1B
Product reader: MODIS L1B Products
Product reader class: gov.nasa.gsfc.seadas.dataio.L1BModisProductReaderPlugIn
Product reader module: NASA OBPG Ocean Color Product Reader - v1.8
Product file location: C:\Users\Don\Documents\Brian\Fire\LAADS\CAMP\MOD021KM.A2018314.1800.006.2018315073121.hdf
Product scene width: 1354 pixels
Product scene height: 2030 pixels
Product start time (UTC): 10-NOV-2018 18:00:00.000000
Product end time (UTC): 10-NOV-2018 18:05:00.000000
We haven code which reads the MODIS file is taken from SeaDAS a while ago.
Maybe use SeaDAS directly, it could be that they have improved the reading meanwhile.
In SNAP are float values shown wit ha max of ~1.4.
This indicates reflectances.