.. index:: 
   single: Parameters; 1D Penumbra 8020 Left

1D Penumbra 8020 Left
=====================

The left 80%-20% penumbra is defined as the distance from the first point on the profile at 80% of the CAX value to the first point that reaches 20% of the CAX value on the left side of the profile:

.. math:: Penumbra(80-20) = |d(P_{20\%}) - d(P_{80\%})|

.. math:: P_{20\%} = CAX*0.2

.. math:: P_{80\%} = CAX*0.8
   
Where d(P\ :sub:`20%`) and d(P\ :sub:`80%`) are the linear distances to the 20% and 80% profile values respectively as defined above.

Since version 0.5 BeamScheme no longer does automatic grounding of the profile, i.e. reducing the minimum to zero. This means that if you want the grounded field size you must *explicitly* normalise or window the image.

**Protocol invocation name**: 1D Penumbra 8020 Left

|Note| Profiles are no longer grounded automatically. This can lead to differences in results from previous version.

|Note| Noisy data can lead to an inaccurate penumbra value being reported.

.. |Note| image:: _static/Note.png