## How do we write reciprocal and non-SI units?

The following table shows the physical dimensions typically used in reflectometry with the accepted units. The 4 last columns contain suggested shortcuts for the units to be used by ORSO: the very short notation, the short notation, the full word and a LaTeX type notation. The bold notations are those more or less recommended during the September workshop.

dimension unit   short long LaTeX-like
length meter `m`
milimeter `mm`
nanometer `nm`
Aangstroem `A` `Aa` `angstrom` `\AA`
reciprocal length 1/nanometer `1/nm`     `nm^{-1}`
1/Aangstroem `1/A`   `1/angstrom` `\AA^{-1}`
angle radian `1` `rad` `radian`
degree   `deg` `degree` `^o`
time second `s` `sec`

There is an obvious difference between strict SI units and related units. The Si units have a short, well defined and accepted notation. I.e. the discussion should concentrate on angstrom (or so).

## Where are units declared?

There are 2 principle approaches:

• Units are declared once at the beginning of the header and then apply to all quantities with the same dimension.
• Units are given for every quantity individually.

In the first case a conflict might arise between the slit size given in mm and the wavelength in nm.

The second approach has the advantage that there is no need to collect the constituents of a physical quantity from various places in the document.

## defaults

If no unit is given it is assumed to be `1`. Thus no `unit` must be defined for a ratio like `R` or an angle in radian (for `1 rad := 1`).

With respect to readibility, the best (and most compact) choice to put units is the conventional approach

``````#    quantity:  <magnitude> <unit>
``````

rather than

``````#    quantity:
#         magnitude: <magnitude>
#         unit: <unit>
``````

Is there a serious argument for the latter approach?