Table of Contents
- Overview of NeXus classes
- NeXus Base Classes
- NXaperture
- NXattenuator
- NXbeam
- NXbeam_stop
- NXbending_magnet
- NXcapillary
- NXcharacterization
- NXcollection
- NXcollimator
- NXcrystal
- NXdata
- NXdetector
- NXdetector_group
- NXdisk_chopper
- NXentry
- NXenvironment
- NXevent_data
- NXfermi_chopper
- NXfilter
- NXflipper
- NXgeometry
- NXguide
- NXinsertion_device
- NXinstrument
- NXlog
- NXmirror
- NXmoderator
- NXmonitor
- NXmonochromator
- NXnote
- NXobject
- NXorientation
- NXparameters
- NXpolarizer
- NXpositioner
- NXprocess
- NXroot
- NXsample
- NXsensor
- NXshape
- NXsource
- NXsubentry
- NXtranslation
- NXuser
- NXvelocity_selector
- NXxraylens
- NeXus Application Classes
- NeXus Contributed Classes
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Information is stored in a NeXus data file by grouping together similar parts. For example, information about the sample could include a descriptive name, the temperature, and other items. NeXus specifies the contents of these groupings using classes. In some parts of this manual, these classes might be called group type or some similar term. In this section, the NeXus classes are described in detail. Each class is specified using the NeXus Definition Language (NXDL). The rules and structure of NXDL are described in a separate chapter.
There are three types of NeXus class file: base classes, application definitions, and contributed definitions. Base class definitions define the complete set of terms that might be used in an instance of that class. Application definitions define the minimum set of terms that must be used in an instance of that class. Contributed definitions include propositions from the community for NeXus base classes or application definitions, as well as other NXDL files for long-term archival by NeXus.
Each of the NeXus classes is described in two basic ways. First, a short list of descriptive information is provided as a header, then a condensed listing of the basic structure, then a table providing documentation for the various components of the NeXus class.
- category
-
The category of NXDL, either:
base(base class)application(application definition)contributed(contributed definition) - NXDL source
-
Name of the NeXus class and a URL to the source listing in the NeXus subversion repository.
- version
-
A string that documents this particular version of this NXDL.
- SVN Id
-
Subversion repository checkout identification, stripped of the surrounding dollar signs. (The Id is blank on files copied direct from the repository that are not checked out by a subversion client.)
- NeXus Definition Language
-
The NeXus Definition Language (NXDL) (described in NXDL: NeXus Definition Language) is used to describe the components in the NeXus Base Classes, as well as application and contributed definitions. The intent of NXDL is to provide a rules-based method for defining a NeXus data file that is specific to either an instrument (where NeXus has been for years) or an area of scientific technique or analysis. NXDL replaces the meta-DTD method used previously to define the NeXus base classes.
- extends class
-
NeXus class extended by this class. Most NeXus base classes only extend the base class definition (NXDL).
- other classes included
-
List (including URLs) of other classes used to define this class.
- symbol list
-
List of the
symbols(if present) that define mnemonics that represent the length of each dimension in a vector or array. - documentation
-
Description of the NeXus class. DocBook markup (formatting is allowed).
A compact listing of the basic structure
(groups, fields, dimensions, attributes, and links)
is prepared for each NXDL specification. Indentation shows
nested structure. Attributes are prepended with the @
symbol while links use the characters -->
to represent the path to the intended source of the information.
The table has columns to describe the basic information about each field or group in the class. An example of the varieties of specifications are given in the following table using items found in various NeXus base classes.
Table 3.1. Example Tabular representation of a NeXus class
| Name | Type | Units | Description (and Occurrences) |
|---|---|---|---|
program_name | NX_CHAR |
Name of program used to generate this file | |
@version | NX_CHAR |
Program version number Occurences: 1 : default | |
@configuration | NX_CHAR |
configuration of the program | |
thumbnail | NXnote |
A small image that is representative of the entry. An example of this is a 640x480 JPEG image automatically produced by a low resolution plot of the NXdata. | |
@mime_type | NX_CHAR | expected: mime_type="image/*" | |
NXgeometry
| describe the geometry of this class | ||
distance | NX_FLOAT | NX_LENGTH |
Distance from sample |
mode |
"Single Bunch" | "Multi Bunch" |
source operating mode | |
target_material |
Ta | W | depleted_U | enriched_U | Hg | Pb | C |
Pulsed source target material |
In the above example, the fields might appear in a NeXus XML data file as
Example 3.1. Example fragment of a NeXus XML data file
<program_name version="1.0a" configuration="standard">
MaxSAS
</program_name>
<NXnote name="thumbnail" mime_type="image/*">
<!-- contents of an NXnote would appear here -->
</NXnote>
<distance units="mm">125.6</distance>
<mode> Single Bunch </mode>
<target_material>depleted_U</target_material>
The columns in the table are described as follows:
- Name (and attributes)
-
Name of the data field. Since
nameneeds to be restricted to valid program variable names, no "-" characters can be allowed. Name must satisfy both HDF and XML naming rules.NameStartChar ::= _ | a..z | A..Z NameChar ::= NameStartChar | 0..9 Name ::= NameStartChar (NameChar)* Or, as a regular expression: [_a-zA-Z][_a-zA-Z0-9]* equivalent regular expression: [_a-zA-Z][\w_]*
Attributes, identified with a leading "at" symbol (
@) and belong with the preceding field or group, are additional metadata used to define this field or group.In the example above, the
program_nameelement has two attributes:version(required) andconfiguration(optional) while thethumbnailelement has one attribute:mime_type(optional). - Type
-
Type of data to be represented by this variable. The type is one of those specified in the
NeXus Definition Language(see NXDL: NeXus Definition Language).In the case where the variable can take only one value from a known list, the list of known values is presented, such as in the
target_materialfield above:Ta | W | depleted_U | enriched_U | Hg | Pb | C. Selections with included whitespace are surrounded by quotes. See the example above for usage. - Units
-
Data units, given as character strings, must conform to the NeXus units standard. See the "NeXus units" section for details.
- Description (and Occurences)
-
A simple text description of the data field. No markup or formatting is allowed.
The absence of Occurences in the item description signifies that both
minOccursandmaxOccurshave the default values. If the number of occurences of an item are specified in the NXDL (through@minOccursand@maxOccursattributes), they will be reported in the Description column similar to the example shown above.Default values for occurences are shown in the following table. The
NXDL element typeis either a group (such as a NeXus base class), a field (that specifies the name and type of a variable), or an attribute of a field or group. The number of times an item can appear ranges betweenminOccursandmaxOccurs. A defaultminOccursof zero means the item is optional. For attributes,maxOccurscannot be greater than 1.Table 3.2. Default values for occurences
NXDL element type minOccurs maxOccurs group 0 unbounded field 0 unbounded attribute 0 1