Polarization/asymmetry function #97
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esmith1729
commented
Jan 30, 2025
esmith1729
commented
- Polarization/asymmetry #58
esmith1729
changed the title
| @@ -120,6 +120,50 @@ async def sum_spectra_with_wavelength( | |||
| return sum_spectra_with_wavelength | |||
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| def polarization(a: sc.Variable, b: sc.Variable) -> sc.Variable: | |||
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Sorry this is my fault as I specified precisely this, but thinking more I think the signature is better expressed as sc.typing.VariableLike (actual types unchanged, it just means pyright will be happy for us to use sc.DataArray and others too in future). Again actual types don't change, just the type hints in the signature.
| On SANS instruments e.g. LARMOR, A and B correspond to intensity in different DAE | ||
| periods (before/after switching a flipper) |
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| On SANS instruments e.g. LARMOR, A and B correspond to intensity in different DAE | |
| periods (before/after switching a flipper) | |
| On SANS instruments e.g. LARMOR, A and B correspond to intensity in different DAE | |
| periods (before/after switching a flipper) and the output is interpreted as a neutron polarization ratio |
| On SANS instruments e.g. LARMOR, A and B correspond to intensity in different DAE | ||
| periods (before/after switching a flipper) | ||
| Or reflectometry instruments e.g. POLREF, the situation is the same as on LARMOR | ||
| On muon instruments, A and B correspond to measuring from different detector banks |
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| On muon instruments, A and B correspond to measuring from different detector banks | |
| On muon instruments, A and B correspond to measuring from forward/backward detector banks, and the output is interpreted as a muon asymmetry |
| ("a", "b", "variance_a", "variance_b", "expected_value", "expected_uncertainty"), | ||
| [ | ||
| # Case 1: Symmetric case with equal uncertainties | ||
| (5.0, 3.0, 0.1, 0.1, 0.25, 0.05762215285808055), # comment where tf this number came from |
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comment where tf this number came from
good idea
| @pytest.mark.parametrize( | ||
| ("a", "b", "variance_a", "variance_b", "expected_value", "expected_uncertainty"), | ||
| [ | ||
| # Case 1: Symmetric case with equal uncertainties |
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What does "symmetric" mean here?
| # Case 2: Asymmetric case with different uncertainties | ||
| (10.0, 6.0, 0.2, 0.3, 0.25, 0.04764984588117782), | ||
| # Case 3: Case with larger values and different uncertainty magnitudes | ||
| (100.0, 60.0, 1.0, 2.0, 0.25, 0.0120017902310447), |
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In our chat we also had some "simple" example cases - the sort which could be worked out easily "in your head". Those would make good test cases to add here.
| var_a = sc.Variable(dims=[], values=a, variances=variance_a, unit="", dtype="float64") | ||
| var_b = sc.Variable(dims=[], values=b, variances=variance_b, unit="", dtype="float64") |
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From https://scipp.github.io/generated/classes/scipp.Variable.html#scipp.Variable:
This constructor is meant primarily for internal use. Use one of the Specialized creation functions instead. See in particular scipp.array() and scipp.scalar().
(scalar is what you want here) - it still gives you a sc.Variable object in the end though.
| polarization(var_a, var_b) | ||
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| # test cases uncert = 0 |
| @pytest.mark.parametrize( | ||
| ("a", "b", "variances_a", "variances_b", "expected_values", "expected_uncertainties"), | ||
| [ | ||
| # Case 1: Symmetric case with equal uncertainties |
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Not sure what "symmetric" means here - could you clarify?
Also the equal uncertainties seems not true.
Copy/paste?
| Polarization refers to the property of transverse waves which specifies the geometrical orientation of the | ||
| oscillations. | ||
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| The polarization funtion provided will calculate the polarization between two values, A and B, which | ||
| have different definitions based on the instrument context. | ||
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| Instrument-Specific Interpretations | ||
| SANS Instruments (e.g., LARMOR) | ||
| A: Intensity in DAE period before switching a flipper. | ||
| B: Intensity in DAE period after switching a flipper. | ||
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| Reflectometry Instruments (e.g., POLREF) | ||
| Similar to LARMOR, A and B represent intensities before and after flipper switching. | ||
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| Muon Instruments | ||
| A and B refer to Measurements from different detector banks. |
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I think we need to re-word this section a bit. Not your fault, this is kind of specific science knowledge here - which I'm no expert on myself but have picked up bits & pieces from talking to scientists.
How about something like:
Polarization/asymmetry
ibex_bluesky_coreprovides a helper method,
{py:obj}ibex_bluesky_core.devices.simpledae.reducers.polarization, for calculating the quantity(a-b)/(a+b). This quantity is used, for example, in neutron polarization measurements, and in calculating asymmetry for muon measurements.For this expression,
scipp's default uncertainty propagation rules cannot be used as the uncertainties on(a-b)are correlated with those of(a+b)in the division step - butscippassumes uncorrelated data. This helper method calculates the uncertainties following linear error propagation theory, using the partial derivatives of the above expression.The partial derivatives are:
[INSERT MATHS HERE - and see the fitting models docs pages for how to render nice maths equations in docs]
Which then means the variances computed by this helper function are:
[INSERT MATHS HERE]
