The OME system of units

Implementation

The ome.xsd schema file defines new type enumerations: UnitsLength, UnitsTime, UnitsPressure, UnitsAngle, UnitsTemperature, UnitsElectricPotential, UnitsPower and UnitsFrequency. These types are used throughout the model. Each new unit attribute is in a pair with an existing attribute, e.g. PinholeSize and PinholeSizeUnit. The xsd-fu code generator then processes these attribute pairs into a single unit aware attribute in the output java classes. The java classes make use of the types from the new ome.units.* classes.

The classes in ome.units are inspired by but not a direct implementation of the specification for a Units of Measurement API.

These classes have been constructed to allow for future wrapping of the Units of Measurement API implementation developed at https://www.eclipse.org/uomo/ if future work on that project provides a more complete implementation.

See also

OMERO developer documentation on units for further details of how this aspect of the Data Model is implemented in OMERO.

Default and Alternate OME-XML samples demonstrating the use of instrument units.

OME system of measurements

A system of measurements is defined for:

Angle

Is based on the following unit:

• rad - radian (the SI unit of angle)

And from this are derived the following units:

• deg - degree
• gon - gradian

All angle units are freely convertible.

Electric potential (commonly called voltage)

Is based on the following unit:

• V - volt (the SI unit of electric potential)

And from this are derived the following units:

• All the SI range for volt from 10²⁴ yotta (YV) to 10^-24 yocto (yV)

All electric potential units are freely convertible.

Frequency

Is based on the following unit:

• Hz - hertz (the SI unit of frequency)

And from this are derived the following units:

• All the SI range for hertz from 10²⁴ yotta (YHz) to 10^-24 yocto (yHz)

All frequency units are freely convertible.

Length

Is based on the following units:

• m - meter (the si unit of length)
• pixel - a length measured in terms of the image pixel size
• reference frame - a length measured in terms of an arbitrary unit based on the equipment used

And from these are derived the following units:

• All the SI range for meter from 10²⁴ yotta (Ym) to 10^-24 yocto (ym)
• Å - ångströms
• in - inch (the Imperial/US unit of length)
• thou - thou (or mil, 1/1000 of an inch)
• li - line (1/12 of an inch)
• in - inch
• ft - foot
• yd - yard
• mi - terrestrial mile
• ua - astronomical unit - The official term is ua as the SI standard assigned AU to absorbance unit.
• ly - light year
• pc - parsec
• pt - typography point - This unit should be limited to font sizes.

All units are freely convertible except for pixel and reference frame, both of which are of unspecified length.

Power

Is based on the following unit:

• W - watt (the SI unit of power)

And from this are derived the following units:

• All the SI range for watt from 10²⁴ yotta (YW) to 10^-24 yocto (yW)

All power units are freely convertible.

Pressure

Is based on the following unit:

• Pa - pascal (the si unit of pressure)

And from this are derived the following units:

• All the SI range for pascal from 10²⁴ yotta (YPa) to 10^-24 yocto (yPa)
• Mbar - mega bar
• kbar - kilo bar
• dbar - deci bar
• cbar - centi bar
• mbar - milli bar
• atm - standard atmosphere
• psi - pounds per square inch
• Torr - torr
• mTorr - milli torr
• mm Hg - millimetre of mercury

All pressure units are freely convertible.

Temperature

Is based on the following unit:

• K - kelvin (the SI unit of temperature)

And from this are derived the following units:

• °C - degree Celsius
• °F - degree Fahrenheit
• °R - degree Rankine

The degree sign and word was dropped from kelvin in 1968. https://en.wikipedia.org/wiki/Kelvin

All temperature units are freely convertible.

Time

Is based on the following unit:

• s - second (the SI unit of time)

And from this are derived the following units:

• All the SI range for second from 10²⁴ yotta (Ys) to 10^-24 yocto (ys)
• min - minute
• h - hour
• d - day

All time units are freely convertible.

General points

Unit abbreviations

The string used for each unit is the standard abbreviation for that unit. In a few cases these do not seem obvious but the set of abbreviations has been chosen by the scientific community to avoid abbreviation clashes.

Unit names

The name and spelling used for each unit in long form is not defined by the scientific community, rather the abbreviation is standardised. For example, the unit of length with the symbol m is equally valid written as meter, metre, metr, or metro.

The SI range of prefixes

The following unit abbreviation are defined:

• Y - 10²⁴ - yotta
• Z - 10²¹ - zetta
• E - 10¹⁸ - exa
• P - 10¹⁵ - peta
• T - 10¹² - tera
• G - 10⁹ - giga
• M - 10⁶ - mega
• k - 10³ - kilo
• h - 10² - hecto
• da - 10¹ - deca
• d - 10^-1 - deci
• c - 10^-2 - centi
• m - 10^-3 - milli
• µ - 10^-6 - micro
• n - 10^-9 - nano
• p - 10^-12 - pico
• f - 10^-15 - femto
• a - 10^-18 - atto
• z - 10^-21 - zepto
• y - 10^-24 - yocto