Add reflectance calculations.

CHANGELOG.rst

@@ -2,6 +2,12 @@
 Changelog
 =========
 
+0.3.5 (2020-10-01)
+------------------
+
+* Add functions for calculating reflectance and transmission - useful for correcting for lack of AR coatings in
+  GRASP PO calculations of lens and dielectrics.
+
 0.3.4 (2020-10-01)
 ------------------
 

src/graspfile/analysis/reflectance.py

@@ -0,0 +1,41 @@
+import numpy as np
+
+
+def reflectance(n1, n2):
+    """Calculate the amplitude reflection coefficient due to moving from media with index n1 to media with index n2.
+
+    Args:
+        n1 (float:): Refractive index of media 1.
+        n2 (float:): Refractive index of media 2.
+
+    Returns:
+        float: Reflection coefficient"""
+    return (n1 - n2) / (n1 + n2)
+
+
+def transmittance(n1, n2):
+    """Calculate the amplitude transmission coefficient due to moving from media with index n1 to media with index n2.
+
+    Args:
+        n1 (float:): Refractive index of media 1.
+        n2 (float:): Refractive index of media 2.
+
+    Returns:
+        float: Transmission coefficient"""
+    return np.sqrt(1.0 - ((n1 - n2) / (n1 + n2)) ** 2)
+
+
+def total_transmittance(ns):
+    """Calculate the total transmission coeffficient due to moving through media with listed indices.  No interference
+    is included, making this calculation suitable for cascaded PO calculations in GRASP.
+
+    Args:
+        ns (list: of float:): List of refractive indices of media
+
+    Returns:
+        float: total transmittance."""
+    t = 1.0
+    for i in range(len(ns) - 1):
+        t *= transmittance(ns[i], ns[i + 1])
+
+    return t

tests/test_analysis_reflectance.py

@@ -0,0 +1,35 @@
+import numpy as np
+import pytest
+
+from graspfile.analysis import reflectance
+
+
+@pytest.fixture
+def indices():
+    """Return a list of refractive indices for testing."""
+    return [1.0, 3.0, 1.0, 3.0, 1.0]
+
+
+def test_reflectance():
+    """Test single reflectance value"""
+    r = reflectance.reflectance(1.0, 2.0)
+
+    assert pytest.approx(r) == (1.0 - 2.0)/(1.0 + 2.0)
+
+
+def test_transmittance():
+    """Test single transmittance value"""
+    t = reflectance.transmittance(1.0, 2.0)
+
+    assert pytest.approx(t) == np.sqrt(1.0 - ((1.0 - 2.0)/(1.0 + 2.0))**2)
+
+
+def test_total_transmittance(indices):
+    """Test cascaded transmittance value"""
+    t = reflectance.total_transmittance(indices)
+
+    t_test = 1.0
+    for i in range(len(indices)-1):
+        t_test *= reflectance.transmittance(indices[i], indices[i+1])
+
+    assert pytest.approx(t) == t_test