Physical Units and Nomenclature

Units

Unless an explicit statement to the contrary is given, all numerical quantities used in LightLike are expressed in MKS (meter-kilogram-second) units, and all angles are expressed in radians.  The documentation for individual library subsystems is usually explicit about the physical units for inputs, parameters and outputs.  (The individual-system documentation is the html page that appears when one clicks the "?" symbol below a subsystem icon in the System Editor window).  However, if a unit specification has been omitted, the user should feel confident in assuming MKS units.  This applies particularly to LightLike data that is saved in HDF 5 file. HDF 5 data does not have physical units information explicitly indicated in the HDF 5 file.  

One potential ambiguity that arises is the physical unit of wavefront optical path difference (OPD).  "MKS" in this case could refer either to absolute length (meters) or to radians of phase.  In LightLike, OPD is almost always expressed in meters.  The documentation of individual subsystems should be explicit in this regard.

Time in LightLike is always expressed in units of seconds.

Physical nomenclature

There are several important optics quantities for which no universally consistent nomenclature exists.  We mention several key items here, and compare commonly used names with the names that were adopted in LightLike.

Irradiance, intensity:  These terms are often used interchangeably for the quantity whose MKS units are W/m2.  "Irradiance" is the term recommended by specialists in radiometry and by the standards committees, but the use of "intensity" has a long history in physics texts.  In the LightLike documentation and system modules, these two terms will also appear interchangeably.

Exposure, integrated intensity:  The term "exposure" is often used for the product  (irradiance)*(exposure length), whose MKS units are (W/m2)*sec= J/m2.  In LightLike modules, this quantity is almost always referred to as "integrated intensity", where "integration" in this case refers to the time dimension.  "Integrated intensity" is the standard output of all LightLike intensity- or energy-type optical sensors.

Integrated complex field:  This is an unphysical quantity which appears in at least one important LightLike sensor module.  LightLike's SimpleFieldSensor module was provided to make the complex optical field directly accessible to the user.  For reasons of coding uniformity with respect to the other temporally-integrating sensors, it was originally decided to define the output of the field sensor as (complex field)*(exposure length), with units of (W/m2)1/2 * (sec).  Of course, physically this is not a meaningful quantity, and does not scale sensibly with exposure length.

The user must apply the appropriate scale factor to renormalize SimpleFieldSensor's output to a physically meaningful quantity.  Suppose icf denotes SimpleFieldSensor's integrated complex field output.  Then, in order to obtain the physically sensible integrated intensity in J/m2, one must compute in post-processing the quantity

    integrated_intensity = |icf|2 / exposure_length .

    (Note that |icf|2 / exposure_length = |c_field*exposure_length|2 / exposure_length

                                                             = |c_field|2 * exposure_length

                                                             = integrated_intensity ).