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This documentation is for OMERO 5.2. This version is now in maintenance mode and will only be updated in the event of critical bugs or security concerns. OMERO 5.3 is expected in the first quarter of 2017.


A number of properties in the OME model are physical measurements which inherently have a unit associated with them. Earlier versions of OME defined a default unit for the measurement. Now users, clients, and acquisition systems can specify the unit themselves rather than converting their internal unit to the OME default.

Supported units

Each of the supported units contain a number of values from the International System of Units (SI) in the most common prefixes from yotta (10^24) to yocto (10^-24). The Length unit also contains values from the Imperial system as well as internal values which are internal to OME: reference frame and pixel.

Unit fields

The following fields in the OMERO model have a unit type:

Class Field Type Default value
Channel EmissionWavelength Length nm
Channel ExcitationWavelength Length nm
Channel PinholeSize Length µm
Detector Voltage ElectricPotential V
DetectorSettings ReadOutRate Frequency MHz
DetectorSettings Voltage ElectricPotential V
ImagingEnvironment AirPressure Pressure mbar
ImagingEnvironment Temperature Temperature °C
Laser RepetitionRate Frequency Hz
Laser Wavelength Length nm
LightSource Power Power mW
LightSourceSettings Wavelength Length nm
Objective WorkingDistance Length µm
Pixels PhysicalSizeX Length µm
Pixels PhysicalSizeY Length µm
Pixels PhysicalSizeZ Length µm
Pixels TimeIncrement Time s
Plane DeltaT Time s
Plane ExposureTime Time s
Plane PositionX Length reference frame
Plane PositionY Length reference frame
Plane PositionZ Length reference frame
Plate WellOriginX Length reference frame
Plate WellOriginY Length reference frame
Shape FontSize Length pt
Shape StrokeWidth Length pixel
StageLabel X Length reference frame
StageLabel Y Length reference frame
StageLabel Z Length reference frame
TransmittanceRange CutIn Length nm
TransmittanceRange CutInTolerance Length nm
TransmittanceRange CutOut Length nm
TransmittanceRange CutOutTolerance Length nm
WellSample PositionX Length reference frame
WellSample PositionY Length reference frame

Units of type reference frame cannot be assumed comparable to units from other properties including those with type reference frame.

Unit objects

Each unit quantity consists of a double-precision scalar and an enumeration which chooses one of the pre-defined values from the model. In code, uppercase spellings of the enumerations are used, while in the schema, in OME-XML files, and in the database, the Unicode symbol for the unit is used.

Language Representation
Ice enum UnitsLength { MICROM, ... };
Java and Python omero.model.enums.UnitsLength.MICROM
C++ omero::model::enums::MICROM
PostgreSQL ‘µm’::unitslength

Defining a unit

Pixels p = ...; // Defined elsewhere
Length l = new LengthI(2.1, UnitsLength.MICROM); // µm

The above stores a Pixels object in the database with X and Y physical lengths of “µm”.

Converting a unit

Often a measurement will not be in the most convenient unit for display, e.g. 0.00001 mm. could better be expressed in microns. In order to convert between units, pass the measurement that you have available to a constructor of the same type, passing in the target unit that you would like to see:

Pixels p = ...; // As saved above
Length l1 = p.getPhysicalSizeX(); // 2.1 microns
Length l2 = new LengthI(x1, UnitsLength.NM); // As nanometers

Getting a symbol

The enumerations used in the “units” field of each measurement is of type omero.model.enums.UnitsNAME where NAME is Length, Temperature, etc. These members of that enumeration are all uppercased, code-safe versions of the unit name. To get the symbol as defined in the SI specification, for example, use the getSymbol method:

Length l1 = ...; // As above
l1.getSymbol(); // Returns "µm"

Querying units

In HQL queries, the scalar and the enumeration value can be separately retrieved.

select planeInfo.exposureTime.value from PlaneInfo planeInfo ...

will retrieve just the double scalar value while

select planeInfo.exposureTime.unit from PlaneInfo planeInfo ...

will retrieve a string representation of the enum which can be used in each language to create an enum object, e.g.:

UnitsTime.valueOf(unit); // Java
getattr(UnitsTime, unit) # Python

To load the symbolic representation of the enum which is used internally in the database and is more concise, use an HQL cast:

select cast(planeInfo.exposureTime.unit as text) from PlaneInfo planeInfo ...

Returning the entire unit quantity will result in a hash map with the various representations:

select planeInfo.exposureTime from PlaneInfo planeInfo ...
{'symbol': 's', 'unit': 'SECOND', 'value': 1.2000000476837158}