3.2.1.2. NXDL: Data Types and Units¶
Data Types allowed in NXDL specifications¶
Data types for use in NXDL
describe the expected type of data for a NeXus field. These terms are very
broad. More specific terms are used in actual NeXus data files that describe
size and array dimensions. In addition to the types in the following table, the
NAPI
type is defined when one wishes to permit a field
with any of these data types.
ISO8601:  ISO8601 date/time stamp 

NX_BINARY:  any representation of binary data  if text, line terminator is [CR][LF] 

NX_BOOLEAN:  true/false value ( true  1  false  0 ) 

NX_CHAR:  any string representation All strings are to be encoded in UTF8. Includes fixedlength strings, variablelength strings, and string arrays. Some file writers write strings as a string array of rank 1 and length 1. Clients should be prepared to handle such strings. 

NX_DATE_TIME:  alias for the ISO8601 date/time stamp 

NX_FLOAT:  any representation of a floating point number 

NX_INT:  any representation of an integer number 

NX_NUMBER:  any valid NeXus number representation 

NX_POSINT:  any representation of a positive integer number (greater than zero) 

NX_UINT:  any representation of an unsigned integer number (includes zero) 

Unit Categories allowed in NXDL specifications¶
Unit categories in NXDL specifications describe the expected type of units for a NeXus field. They should describe valid units consistent with the NeXus units section. The values for unit categories are restricted (by an enumeration) to the following table.
NX_ANGLE:  units of angle, example: m 

NX_ANY:  units for things like logs that aren’t picky on units 

NX_AREA:  units of area, example: m2 or barns 

NX_CHARGE:  units of electrical charge, example: c 

NX_CROSS_SECTION:  

units of area, example: barns (alias of NX_AREA) 
NX_CURRENT:  units of electrical current, example: A 

NX_DIMENSIONLESS:  

units for fields where the units cancel out, example: “” or mm/mm (NOTE: not the same as NX_UNITLESS) 
NX_EMITTANCE:  units of emittance (length * angle ) of a
radiation source, example: nm*rad 

NX_ENERGY:  units of energy, example: J or keV 

NX_FLUX:  units of flux, example: s1 cm2 

NX_FREQUENCY:  units of frequency, example: Hz 

NX_LENGTH:  units of length, example: m 

NX_MASS:  units of length, example: g 

NX_MASS_DENSITY:  

units of mass density, example: g cm3 
NX_MOLECULAR_WEIGHT:  

units of molecular weight, example: g mol1 
NX_PERIOD:  units of time, period of pulsed source, example: microseconds (alias to NX_TIME) 

NX_PER_AREA:  units of length, example: m 

NX_PER_LENGTH:  units of length, example: m 

NX_POWER:  units of power, example: W 

NX_PRESSURE:  units of pressure, example: Pa 

NX_PULSES:  units of clock pulses (alias to NX_NUMBER) 

NX_SCATTERING_LENGTH_DENSITY:  

units of scattering length density, example: cm2 
NX_SOLID_ANGLE:  units of solid angle, example: sr  steradian 

NX_TEMPERATURE:  units of temperature, example: K 

NX_TIME:  units of time, example: s 

NX_TIME_OF_FLIGHT:  

units of (neutron) time of flight, example: s (alias to NX_TIME) 
NX_TRANSFORMATION:  

units of the specified transformation could be any of these: There will be one or more transformations defined by one or more fields
for each transformation. The units type

NX_UNITLESS:  for fields that don’t have a unit (e.g. hkl) so that they don’t inherit the wrong units (NOTE: not the same as NX_DIMENSIONLESS) 

NX_VOLTAGE:  units of voltage, example: V 

NX_VOLUME:  units of volume, example: m3 

NX_WAVELENGTH:  units of wavelength, example: angstrom 

NX_WAVENUMBER:  units of wavenumber or Q, example: angstrom1 or nm1 
