From c6a8ae72ef94486d287477d6498e13e5443bbe82 Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Thu, 11 Jul 2019 16:46:51 +0200
Subject: [PATCH 01/13] Insert three function placeholders in pvsystem.py

---
 pvlib/pvsystem.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 35701bd3a2..7854da8645 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1064,6 +1064,20 @@ def physicaliam(aoi, n=1.526, K=4., L=0.002):
     return iam
 
 
+def iam_martin_ruiz(aoi, a_r):
+    '''
+    Determine the incidence angle modifier using the Martin & Ruiz model.
+    '''
+    raise NotImplementedError
+
+
+def iam_interp(aoi, measurements):
+    '''
+    Determine the incidence angle modifier by interpolating measured values.
+    '''
+    raise NotImplementedError
+
+
 def calcparams_desoto(effective_irradiance, temp_cell,
                       alpha_sc, a_ref, I_L_ref, I_o_ref, R_sh_ref, R_s,
                       EgRef=1.121, dEgdT=-0.0002677,
@@ -1994,6 +2008,14 @@ def pvsyst_celltemp(poa_global, temp_air, wind_speed=1.0, eta_m=0.1,
     return temp_cell
 
 
+def celltemp_faiman(poa_global, temp_air, wind_speed, u0, u1):
+    '''
+    Calculate cell temperature using an emperical heat loss factor model
+    in the form proposed by Faiman.
+    '''
+    raise NotImplementedError
+
+
 def sapm_spectral_loss(airmass_absolute, module):
     """
     Calculates the SAPM spectral loss coefficient, F1.

From 2177d78a111e287c86ee3d37f93ec0367dc30e7c Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Fri, 12 Jul 2019 17:39:56 +0200
Subject: [PATCH 02/13] transform docstring from matlab

---
 pvlib/pvsystem.py | 56 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 55 insertions(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 7854da8645..f10dc0ff2c 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1066,7 +1066,61 @@ def physicaliam(aoi, n=1.526, K=4., L=0.002):
 
 def iam_martin_ruiz(aoi, a_r):
     '''
-    Determine the incidence angle modifier using the Martin & Ruiz model.
+    Determine the incidence angle modifier using the Martin
+    and Ruiz incident angle model
+
+    iam_martin_ruiz calculates the incidence angle modifier (angular
+    factor) as described by Martin and Ruiz in [1]. The information
+    required is the incident angle (theta) and the angular losses
+    coefficient (ar). Please note that [1] has a corrigendum which makes the
+    document much simpler to understand.
+
+    The incident angle modifier is defined as
+    [1-exp(-cos(theta/ar))] / [1-exp(-1/ar)], which is
+    presented as AL(alpha) = 1 - IAM in equation 4 of [1]. Thus IAM is
+    equal to 1 at theta = 0, and equal to 0 at theta = 90. IAM is a
+    column vector with the same number of elements as the largest input
+    vector.
+
+    Parameters
+    ----------
+    aoi : numeric, degrees
+        The angle of incidence between the module normal vector and the
+        sun-beam vector in degrees. Theta must be a numeric scalar or vector.
+        For any values of theta where abs(theta)>90, IAM is set to 0. For any
+        values of theta where -90 < theta < 0, theta is set to abs(theta) and
+        evaluated. A warning will be generated if any(theta<0 or theta>90).
+
+    ar : numeric
+        The angular losses coefficient described in equation 3 of [1].
+        This is an empirical dimensionless parameter. Values of ar are
+        generally on the order of 0.08 to 0.25 for flat-plate PV modules. ar
+        must be a numeric scalar or vector with all values > 0. If ar
+        is a vector, it must be the same size as all other input vectors.
+
+    Returns
+    -------
+    iam : numeric
+        The incident angle modifier
+
+
+
+    References
+    ----------
+    [1] N. Martin and J. M. Ruiz, "Calculation of the PV modules angular
+    losses under field conditions by means of an analytical model", Solar
+    Energy Materials & Solar Cells, vol. 70, pp. 25-38, 2001.
+
+    [2] N. Martin and J. M. Ruiz, "Corrigendum to 'Calculation of the PV
+    modules angular losses under field conditions by means of an
+    analytical model'", Solar Energy Materials & Solar Cells, vol. 110,
+    pp. 154, 2013.
+
+    See Also
+    --------
+    getaoi
+    physicaliam
+    ashraeiam
     '''
     raise NotImplementedError
 

From f036041f4e66dd19eee62ef138a6c8c7a8f09256 Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Sat, 13 Jul 2019 18:43:46 +0200
Subject: [PATCH 03/13] Code for iam_martin and iam_interp using greek symbol
 for theta

---
 pvlib/pvsystem.py | 106 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 88 insertions(+), 18 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index f10dc0ff2c..b86aa0cc7b 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -17,7 +17,7 @@
 import pandas as pd
 
 from pvlib import atmosphere, irradiance, tools, singlediode as _singlediode
-from pvlib.tools import _build_kwargs
+from pvlib.tools import _build_kwargs, cosd
 from pvlib.location import Location
 
 
@@ -1064,7 +1064,7 @@ def physicaliam(aoi, n=1.526, K=4., L=0.002):
     return iam
 
 
-def iam_martin_ruiz(aoi, a_r):
+def iam_martin_ruiz(θ:'degrees', a_r=0.16):
     '''
     Determine the incidence angle modifier using the Martin
     and Ruiz incident angle model
@@ -1084,26 +1084,21 @@ def iam_martin_ruiz(aoi, a_r):
 
     Parameters
     ----------
-    aoi : numeric, degrees
+    θ : numeric, degrees
         The angle of incidence between the module normal vector and the
         sun-beam vector in degrees. Theta must be a numeric scalar or vector.
-        For any values of theta where abs(theta)>90, IAM is set to 0. For any
-        values of theta where -90 < theta < 0, theta is set to abs(theta) and
-        evaluated. A warning will be generated if any(theta<0 or theta>90).
+        iam is 0 where |θ| > 90
 
-    ar : numeric
+    a_r : numeric
         The angular losses coefficient described in equation 3 of [1].
         This is an empirical dimensionless parameter. Values of ar are
-        generally on the order of 0.08 to 0.25 for flat-plate PV modules. ar
-        must be a numeric scalar or vector with all values > 0. If ar
-        is a vector, it must be the same size as all other input vectors.
+        generally on the order of 0.08 to 0.25 for flat-plate PV modules. a_r
+        must be a positive numeric scalar.
 
     Returns
     -------
     iam : numeric
-        The incident angle modifier
-
-
+        The incident angle modifier(s)
 
     References
     ----------
@@ -1118,18 +1113,93 @@ def iam_martin_ruiz(aoi, a_r):
 
     See Also
     --------
-    getaoi
     physicaliam
     ashraeiam
+    iam_interp
     '''
-    raise NotImplementedError
+    # Contributed by Anton Driesse (@adriesse), PV Performance Labs. July, 2019
+
+    θ = np.asanyarray(θ)
+    a_r = np.asanyarray(a_r)
+
+    if not np.all(np.positive(a_r)):
+        raise RuntimeError("The parameter 'a_r' cannot be zero or negative.")
+
+    θ = np.clip(θ, -90, 90)
+    iam = (1 - np.exp(-cosd(θ)/ a_r)) / (1 - np.exp(-1 / a_r))
+
+    return iam
 
 
-def iam_interp(aoi, measurements):
+def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
     '''
-    Determine the incidence angle modifier by interpolating measured values.
+    Determine the incidence angle modifier by interpolating a set of
+    reference values, which are usually measured values.
+
+    Parameters
+    ----------
+    θ : numeric, degrees
+        The angle of incidence between the module normal vector and the
+        sun-beam vector in degrees.
+
+    θ_ref : numeric, degrees
+        Vector of angles at which the iam is known.
+
+    iam_ref :
+        iam values for each angle in θ_ref.
+
+    method :
+        Specifies the intrpolation method.
+        Useful options are: 'linear', 'quadratic','cubic'.
+        See scipy.interpolate.interp1d for more options.
+
+    normalize : boolean
+        When true, the interpolated values are divided by the interpolated
+        value at zero degrees.  This ensures that the iam and normal
+        incidence is equal to 1.0.
+
+    Notes:
+    ------
+    θ_ref must have two or more points and may span any range of angles, but
+    typically there will be a dozen or more points in the range 0-90 degrees.
+    iam beyond the range of θ_ref are extrapolated, but constrained to be
+    non-negative.
+
+    The sign of θ is ignore, only the magnitude is used.
+
+    Returns
+    -------
+    iam : numeric
+        The incident angle modifier(s)
+
+    See Also
+    --------
+    physicaliam
+    ashraeiam
+    iam_martin_ruiz
     '''
-    raise NotImplementedError
+    # Contributed by Anton Driesse (@adriesse), PV Performance Labs. July, 2019
+
+    from scipy.interpolate import interp1d
+
+    # Scipy doesn't give the clearest feedback, so check number of points here.
+    MIN_REF_VALS = {'linear':2, 'quadratic':3, 'cubic':4, 1:2, 2:3, 3:4}
+
+    if len(θ_ref) < MIN_REF_VALS.get(method, 2):
+        raise ValueError("Too few reference points defined "
+                         "for interpolation method '%s'." % method)
+
+    interpolator = interp1d(θ_ref, iam_ref, kind=method,
+                            fill_value='extrapolate')
+    θ = np.asanyarray(θ)
+    θ = np.abs(θ)
+    iam = interpolator(θ)
+    iam = np.clip(iam, 0, None)
+
+    if normalize:
+        iam /= interpolator(0)
+
+    return iam
 
 
 def calcparams_desoto(effective_irradiance, temp_cell,

From faecf0cd0719e4f482581b70845aeddb974f36ec Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Wed, 17 Jul 2019 15:28:16 +0200
Subject: [PATCH 04/13] Minor improvements.

---
 pvlib/pvsystem.py | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index b86aa0cc7b..01259d39c4 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1125,15 +1125,15 @@ def iam_martin_ruiz(θ:'degrees', a_r=0.16):
     if not np.all(np.positive(a_r)):
         raise RuntimeError("The parameter 'a_r' cannot be zero or negative.")
 
-    θ = np.clip(θ, -90, 90)
-    iam = (1 - np.exp(-cosd(θ)/ a_r)) / (1 - np.exp(-1 / a_r))
+    iam = (1 - np.exp(-cosd(θ) / a_r)) / (1 - np.exp(-1 / a_r))
+    iam = np.where(np.abs(θ) >= 90.0, 0.0, iam)
 
     return iam
 
 
 def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
     '''
-    Determine the incidence angle modifier by interpolating a set of
+    Determine the incidence angle modifier (iam) by interpolating a set of
     reference values, which are usually measured values.
 
     Parameters
@@ -1165,7 +1165,7 @@ def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
     iam beyond the range of θ_ref are extrapolated, but constrained to be
     non-negative.
 
-    The sign of θ is ignore, only the magnitude is used.
+    The sign of θ is ignored; only the magnitude is used.
 
     Returns
     -------
@@ -1189,6 +1189,10 @@ def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
         raise ValueError("Too few reference points defined "
                          "for interpolation method '%s'." % method)
 
+    if np.any(np.less(iam_ref, 0)):
+        raise ValueError("Negative value(s) found in 'iam_ref'. "
+                         "This is not physically possible.")
+
     interpolator = interp1d(θ_ref, iam_ref, kind=method,
                             fill_value='extrapolate')
     θ = np.asanyarray(θ)

From 054981e3b29dfcd6a0ebe8eca4668bf85c0460dc Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Wed, 17 Jul 2019 15:28:47 +0200
Subject: [PATCH 05/13] Tests for iam_martin_ruiz() and iam_interp()

---
 pvlib/test/test_pvsystem.py | 81 +++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index 17f70e9d12..03817bebe8 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -148,6 +148,87 @@ def test_PVSystem_physicaliam(mocker):
     assert iam < 1.
 
 
+def test_iam_martin_ruiz():
+
+    aoi = 45.
+    a_r = 0.16
+    expected = 0.98986965
+
+    # will fail of default values change
+    iam = pvsystem.iam_martin_ruiz(aoi)
+    assert_allclose(iam, expected)
+    # will fail of parameter names change
+    iam = pvsystem.iam_martin_ruiz(θ=aoi, a_r=a_r)
+    assert_allclose(iam, expected)
+
+    a_r = 0.18
+    aoi = [-100, -60, 0, 60, 100, np.nan, np.inf]
+    expected = [0.0, 0.9414631, 1.0, 0.9414631, 0.0, np.nan, 0.0]
+
+    with np.errstate(invalid='ignore'):
+        # check out of range of inputs as list
+        iam = pvsystem.iam_martin_ruiz(aoi, a_r)
+        assert_allclose(iam, expected, equal_nan=True)
+
+        # check out of range of inputs as array
+        iam = pvsystem.iam_martin_ruiz(np.array(aoi), a_r)
+        assert_allclose(iam, expected, equal_nan=True)
+
+        # check out of range of inputs as Series
+        iam = pvsystem.iam_martin_ruiz(pd.Series(aoi), a_r)
+        assert_allclose(iam, expected, equal_nan=True)
+
+    # check exception clause
+    with pytest.raises(RuntimeError):
+        pvsystem.iam_martin_ruiz(0.0, a_r=0.0)
+
+
+def test_iam_interp():
+
+    aoi_meas = [0.0, 45.0, 65.0, 75.0]
+    iam_meas = [1.0,  0.9,  0.8,  0.6]
+
+    # simple default linear method
+    aoi = 55.0
+    expected = 0.85
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
+    assert_allclose(iam, expected)
+
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
+    assert_allclose(iam, expected)
+
+    # simple non-default method
+    aoi = 55.0
+    expected = 0.8878062
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas, method='cubic')
+    assert_allclose(iam, expected)
+
+    # check with all reference values
+    aoi = aoi_meas
+    expected = iam_meas
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
+    assert_allclose(iam, expected)
+
+    # check normalization
+    iam_mult = np.multiply (0.9, [1.0,  0.9,  0.8,  0.6])
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_mult, normalize=True)
+    assert_allclose(iam, expected)
+
+    # check beyond reference values
+    aoi = [-45, 0, 45, 85, 90, 95, 100, 105, 110]
+    expected = [0.9, 1. , 0.9, 0.4, 0.3, 0.2, 0.1, 0. , 0. ]
+    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
+    assert_allclose(iam, expected)
+
+    # check exception clause
+    with pytest.raises(ValueError):
+        pvsystem.iam_interp(0.0, [0], [1])
+
+    # check exception clause
+    with pytest.raises(ValueError):
+        pvsystem.iam_interp(0.0, [0, 90], [1, -1])
+
+
 # if this completes successfully we'll be able to do more tests below.
 @pytest.fixture(scope="session")
 def sam_data():

From cd04ad1ebc3f2ceac8d57e1d24439f121ce1933e Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Tue, 23 Jul 2019 10:26:36 +0200
Subject: [PATCH 06/13] Removed undesirables; polished docstrings and code here
 and there.

---
 pvlib/pvsystem.py           | 92 ++++++++++++++++++-------------------
 pvlib/test/test_pvsystem.py |  6 +--
 2 files changed, 49 insertions(+), 49 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 01259d39c4..6c9a9158de 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1064,42 +1064,42 @@ def physicaliam(aoi, n=1.526, K=4., L=0.002):
     return iam
 
 
-def iam_martin_ruiz(θ:'degrees', a_r=0.16):
+def iam_martin_ruiz(theta, a_r=0.16):
     '''
-    Determine the incidence angle modifier using the Martin
-    and Ruiz incident angle model
-
-    iam_martin_ruiz calculates the incidence angle modifier (angular
-    factor) as described by Martin and Ruiz in [1]. The information
-    required is the incident angle (theta) and the angular losses
-    coefficient (ar). Please note that [1] has a corrigendum which makes the
-    document much simpler to understand.
-
-    The incident angle modifier is defined as
-    [1-exp(-cos(theta/ar))] / [1-exp(-1/ar)], which is
-    presented as AL(alpha) = 1 - IAM in equation 4 of [1]. Thus IAM is
-    equal to 1 at theta = 0, and equal to 0 at theta = 90. IAM is a
-    column vector with the same number of elements as the largest input
-    vector.
+    Determine the incidence angle modifier (iam) using the Martin
+    and Ruiz incident angle model.
 
     Parameters
     ----------
-    θ : numeric, degrees
+    theta : numeric, degrees
         The angle of incidence between the module normal vector and the
         sun-beam vector in degrees. Theta must be a numeric scalar or vector.
-        iam is 0 where |θ| > 90
+        iam is 0 where |theta| > 90.
 
     a_r : numeric
         The angular losses coefficient described in equation 3 of [1].
-        This is an empirical dimensionless parameter. Values of ar are
-        generally on the order of 0.08 to 0.25 for flat-plate PV modules. a_r
-        must be a positive numeric scalar.
+        This is an empirical dimensionless parameter. Values of a_r are
+        generally on the order of 0.08 to 0.25 for flat-plate PV modules.
+        a_r must be a positive numeric scalar or vector (same length as theta).
 
     Returns
     -------
     iam : numeric
         The incident angle modifier(s)
 
+    Notes
+    -----
+    iam_martin_ruiz calculates the incidence angle modifier (iamangular
+    factor) as described by Martin and Ruiz in [1]. The information
+    required is the incident angle (theta) and the angular losses
+    coefficient (a_r). Please note that [1] has a corrigendum which makes
+    the document much simpler to understand.
+
+    The incident angle modifier is defined as
+    [1-exp(-cos(theta/ar))] / [1-exp(-1/ar)], which is
+    presented as AL(alpha) = 1 - IAM in equation 4 of [1]. Thus IAM is
+    equal to 1 at theta = 0, and equal to 0 at theta = 90.
+
     References
     ----------
     [1] N. Martin and J. M. Ruiz, "Calculation of the PV modules angular
@@ -1119,59 +1119,59 @@ def iam_martin_ruiz(θ:'degrees', a_r=0.16):
     '''
     # Contributed by Anton Driesse (@adriesse), PV Performance Labs. July, 2019
 
-    θ = np.asanyarray(θ)
+    theta = np.asanyarray(theta)
     a_r = np.asanyarray(a_r)
 
-    if not np.all(np.positive(a_r)):
+    if np.any(np.less_equal(a_r, 0)):
         raise RuntimeError("The parameter 'a_r' cannot be zero or negative.")
 
-    iam = (1 - np.exp(-cosd(θ) / a_r)) / (1 - np.exp(-1 / a_r))
-    iam = np.where(np.abs(θ) >= 90.0, 0.0, iam)
+    iam = (1 - np.exp(-cosd(theta) / a_r)) / (1 - np.exp(-1 / a_r))
+    iam = np.where(np.abs(theta) >= 90.0, 0.0, iam)
 
     return iam
 
 
-def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
+def iam_interp(theta, theta_ref, iam_ref, method='linear', normalize=True):
     '''
     Determine the incidence angle modifier (iam) by interpolating a set of
     reference values, which are usually measured values.
 
     Parameters
     ----------
-    θ : numeric, degrees
+    theta : numeric, degrees
         The angle of incidence between the module normal vector and the
         sun-beam vector in degrees.
 
-    θ_ref : numeric, degrees
+    theta_ref : numeric, degrees
         Vector of angles at which the iam is known.
 
     iam_ref :
-        iam values for each angle in θ_ref.
+        iam values for each angle in theta_ref.
 
     method :
-        Specifies the intrpolation method.
+        Specifies the interpolation method.
         Useful options are: 'linear', 'quadratic','cubic'.
         See scipy.interpolate.interp1d for more options.
 
     normalize : boolean
         When true, the interpolated values are divided by the interpolated
-        value at zero degrees.  This ensures that the iam and normal
+        value at zero degrees.  This ensures that the iam at normal
         incidence is equal to 1.0.
 
-    Notes:
-    ------
-    θ_ref must have two or more points and may span any range of angles, but
-    typically there will be a dozen or more points in the range 0-90 degrees.
-    iam beyond the range of θ_ref are extrapolated, but constrained to be
-    non-negative.
-
-    The sign of θ is ignored; only the magnitude is used.
-
     Returns
     -------
     iam : numeric
         The incident angle modifier(s)
 
+    Notes:
+    ------
+    theta_ref must have two or more points and may span any range of angles.
+    Typically there will be a dozen or more points in the range 0-90 degrees.
+    iam beyond the range of theta_ref are extrapolated, but constrained to be
+    non-negative.
+
+    The sign of theta is ignored; only the magnitude is used.
+
     See Also
     --------
     physicaliam
@@ -1183,9 +1183,9 @@ def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
     from scipy.interpolate import interp1d
 
     # Scipy doesn't give the clearest feedback, so check number of points here.
-    MIN_REF_VALS = {'linear':2, 'quadratic':3, 'cubic':4, 1:2, 2:3, 3:4}
+    MIN_REF_VALS = {'linear': 2, 'quadratic': 3, 'cubic': 4, 1: 2, 2: 3, 3: 4}
 
-    if len(θ_ref) < MIN_REF_VALS.get(method, 2):
+    if len(theta_ref) < MIN_REF_VALS.get(method, 2):
         raise ValueError("Too few reference points defined "
                          "for interpolation method '%s'." % method)
 
@@ -1193,11 +1193,11 @@ def iam_interp(θ:'degrees', θ_ref, iam_ref, method='linear', normalize=True):
         raise ValueError("Negative value(s) found in 'iam_ref'. "
                          "This is not physically possible.")
 
-    interpolator = interp1d(θ_ref, iam_ref, kind=method,
+    interpolator = interp1d(theta_ref, iam_ref, kind=method,
                             fill_value='extrapolate')
-    θ = np.asanyarray(θ)
-    θ = np.abs(θ)
-    iam = interpolator(θ)
+    theta = np.asanyarray(theta)
+    theta = np.abs(theta)
+    iam = interpolator(theta)
     iam = np.clip(iam, 0, None)
 
     if normalize:
diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index 03817bebe8..238f23d594 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -158,7 +158,7 @@ def test_iam_martin_ruiz():
     iam = pvsystem.iam_martin_ruiz(aoi)
     assert_allclose(iam, expected)
     # will fail of parameter names change
-    iam = pvsystem.iam_martin_ruiz(θ=aoi, a_r=a_r)
+    iam = pvsystem.iam_martin_ruiz(theta=aoi, a_r=a_r)
     assert_allclose(iam, expected)
 
     a_r = 0.18
@@ -210,13 +210,13 @@ def test_iam_interp():
     assert_allclose(iam, expected)
 
     # check normalization
-    iam_mult = np.multiply (0.9, [1.0,  0.9,  0.8,  0.6])
+    iam_mult = np.multiply (0.9, iam_meas)
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_mult, normalize=True)
     assert_allclose(iam, expected)
 
     # check beyond reference values
     aoi = [-45, 0, 45, 85, 90, 95, 100, 105, 110]
-    expected = [0.9, 1. , 0.9, 0.4, 0.3, 0.2, 0.1, 0. , 0. ]
+    expected = [0.9, 1.0, 0.9, 0.4, 0.3, 0.2, 0.1, 0.0, 0.0]
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
     assert_allclose(iam, expected)
 

From defc07311a497bef6fc5b9c7a01f3404ecc55bd4 Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Tue, 23 Jul 2019 10:32:35 +0200
Subject: [PATCH 07/13] Final stickler.

---
 pvlib/test/test_pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index 238f23d594..c36e7591c0 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -210,7 +210,7 @@ def test_iam_interp():
     assert_allclose(iam, expected)
 
     # check normalization
-    iam_mult = np.multiply (0.9, iam_meas)
+    iam_mult = np.multiply(0.9, iam_meas)
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_mult, normalize=True)
     assert_allclose(iam, expected)
 

From fcf267768e144b99aaca4b4aa2650f068fe7cf8b Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Tue, 23 Jul 2019 10:43:59 +0200
Subject: [PATCH 08/13] Update v0.7.0.rst

---
 docs/sphinx/source/whatsnew/v0.7.0.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/sphinx/source/whatsnew/v0.7.0.rst b/docs/sphinx/source/whatsnew/v0.7.0.rst
index a8b2f21ebd..723c1a1717 100644
--- a/docs/sphinx/source/whatsnew/v0.7.0.rst
+++ b/docs/sphinx/source/whatsnew/v0.7.0.rst
@@ -10,6 +10,11 @@ compatibility notes.
 **Python 2.7 support ended on June 1, 2019**. (:issue:`501`)
 **Minimum numpy version is now 1.10.4. Minimum pandas version is now 0.18.1.**
 
+Enhancements
+~~~~~~~~~~~~
+* Created two new incidence angle modifier functions: :py:func:`pvlib.pvsystem.iam_martin_ruiz`
+  and :py:func:`pvlib.pvsystem.iam_interp`. (:issue:`751`)
+
 Bug fixes
 ~~~~~~~~~
 * Fix handling of keyword arguments in `forecasts.get_processed_data`.
@@ -25,3 +30,4 @@ Contributors
 * Mark Campanellli (:ghuser:`markcampanelli`)
 * Will Holmgren (:ghuser:`wholmgren`)
 * Oscar Dowson (:ghuser:`odow`)
+* Anton Driesse (:ghuser:`adriesse`)

From 8a93f975d0510052d6916e0e5b794e9eb60e6174 Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Wed, 24 Jul 2019 00:01:10 +0200
Subject: [PATCH 09/13] Fix many oversights (also an old one).

---
 pvlib/pvsystem.py           | 60 +++++++++++++++++++------------------
 pvlib/test/test_pvsystem.py |  2 +-
 2 files changed, 32 insertions(+), 30 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 6c9a9158de..f23c6a64aa 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -946,7 +946,7 @@ def ashraeiam(aoi, b=0.05):
     iam = np.where(aoi_gte_90, 0, iam)
     iam = np.maximum(0, iam)
 
-    if isinstance(iam, pd.Series):
+    if isinstance(aoi, pd.Series):
         iam = pd.Series(iam, index=aoi.index)
 
     return iam
@@ -1064,23 +1064,23 @@ def physicaliam(aoi, n=1.526, K=4., L=0.002):
     return iam
 
 
-def iam_martin_ruiz(theta, a_r=0.16):
+def iam_martin_ruiz(aoi, a_r=0.16):
     '''
     Determine the incidence angle modifier (iam) using the Martin
     and Ruiz incident angle model.
 
     Parameters
     ----------
-    theta : numeric, degrees
+    aoi : numeric, degrees
         The angle of incidence between the module normal vector and the
         sun-beam vector in degrees. Theta must be a numeric scalar or vector.
-        iam is 0 where |theta| > 90.
+        iam is 0 where |aoi| > 90.
 
     a_r : numeric
         The angular losses coefficient described in equation 3 of [1].
         This is an empirical dimensionless parameter. Values of a_r are
         generally on the order of 0.08 to 0.25 for flat-plate PV modules.
-        a_r must be a positive numeric scalar or vector (same length as theta).
+        a_r must be a positive numeric scalar or vector (same length as aoi).
 
     Returns
     -------
@@ -1089,16 +1089,16 @@ def iam_martin_ruiz(theta, a_r=0.16):
 
     Notes
     -----
-    iam_martin_ruiz calculates the incidence angle modifier (iamangular
-    factor) as described by Martin and Ruiz in [1]. The information
-    required is the incident angle (theta) and the angular losses
+    iam_martin_ruiz calculates the incidence angle modifier (iam)
+    as described by Martin and Ruiz in [1]. The information
+    required is the incident angle (aoi) and the angular losses
     coefficient (a_r). Please note that [1] has a corrigendum which makes
     the document much simpler to understand.
 
     The incident angle modifier is defined as
-    [1-exp(-cos(theta/ar))] / [1-exp(-1/ar)], which is
+    [1-exp(-cos(aoi/ar))] / [1-exp(-1/ar)], which is
     presented as AL(alpha) = 1 - IAM in equation 4 of [1]. Thus IAM is
-    equal to 1 at theta = 0, and equal to 0 at theta = 90.
+    equal to 1 at aoi = 0, and equal to 0 at aoi = 90.
 
     References
     ----------
@@ -1119,36 +1119,41 @@ def iam_martin_ruiz(theta, a_r=0.16):
     '''
     # Contributed by Anton Driesse (@adriesse), PV Performance Labs. July, 2019
 
-    theta = np.asanyarray(theta)
+    aoi_input = aoi
+
+    aoi = np.asanyarray(aoi)
     a_r = np.asanyarray(a_r)
 
     if np.any(np.less_equal(a_r, 0)):
         raise RuntimeError("The parameter 'a_r' cannot be zero or negative.")
 
-    iam = (1 - np.exp(-cosd(theta) / a_r)) / (1 - np.exp(-1 / a_r))
-    iam = np.where(np.abs(theta) >= 90.0, 0.0, iam)
+    iam = (1 - np.exp(-cosd(aoi) / a_r)) / (1 - np.exp(-1 / a_r))
+    iam = np.where(np.abs(aoi) >= 90.0, 0.0, iam)
+
+    if isinstance(aoi_input, pd.Series):
+        iam = pd.Series(iam, index=aoi_input.index)
 
     return iam
 
 
-def iam_interp(theta, theta_ref, iam_ref, method='linear', normalize=True):
+def iam_interp(aoi, theta_ref, iam_ref, method='linear', normalize=True):
     '''
     Determine the incidence angle modifier (iam) by interpolating a set of
     reference values, which are usually measured values.
 
     Parameters
     ----------
-    theta : numeric, degrees
+    aoi : numeric, degrees
         The angle of incidence between the module normal vector and the
         sun-beam vector in degrees.
 
     theta_ref : numeric, degrees
         Vector of angles at which the iam is known.
 
-    iam_ref :
+    iam_ref : numeric, unitless
         iam values for each angle in theta_ref.
 
-    method :
+    method : str, default 'linear'
         Specifies the interpolation method.
         Useful options are: 'linear', 'quadratic','cubic'.
         See scipy.interpolate.interp1d for more options.
@@ -1170,7 +1175,7 @@ def iam_interp(theta, theta_ref, iam_ref, method='linear', normalize=True):
     iam beyond the range of theta_ref are extrapolated, but constrained to be
     non-negative.
 
-    The sign of theta is ignored; only the magnitude is used.
+    The sign of aoi is ignored; only the magnitude is used.
 
     See Also
     --------
@@ -1195,14 +1200,19 @@ def iam_interp(theta, theta_ref, iam_ref, method='linear', normalize=True):
 
     interpolator = interp1d(theta_ref, iam_ref, kind=method,
                             fill_value='extrapolate')
-    theta = np.asanyarray(theta)
-    theta = np.abs(theta)
-    iam = interpolator(theta)
+    aoi_input = aoi
+
+    aoi = np.asanyarray(aoi)
+    aoi = np.abs(aoi)
+    iam = interpolator(aoi)
     iam = np.clip(iam, 0, None)
 
     if normalize:
         iam /= interpolator(0)
 
+    if isinstance(aoi_input, pd.Series):
+        iam = pd.Series(iam, index=aoi_input.index)
+
     return iam
 
 
@@ -2136,14 +2146,6 @@ def pvsyst_celltemp(poa_global, temp_air, wind_speed=1.0, eta_m=0.1,
     return temp_cell
 
 
-def celltemp_faiman(poa_global, temp_air, wind_speed, u0, u1):
-    '''
-    Calculate cell temperature using an emperical heat loss factor model
-    in the form proposed by Faiman.
-    '''
-    raise NotImplementedError
-
-
 def sapm_spectral_loss(airmass_absolute, module):
     """
     Calculates the SAPM spectral loss coefficient, F1.
diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index c36e7591c0..27aaf4ed5a 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -158,7 +158,7 @@ def test_iam_martin_ruiz():
     iam = pvsystem.iam_martin_ruiz(aoi)
     assert_allclose(iam, expected)
     # will fail of parameter names change
-    iam = pvsystem.iam_martin_ruiz(theta=aoi, a_r=a_r)
+    iam = pvsystem.iam_martin_ruiz(aoi=aoi, a_r=a_r)
     assert_allclose(iam, expected)
 
     a_r = 0.18

From 396ab4cfe26c1b4585a254f935b09b21ba9c5dcc Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Wed, 24 Jul 2019 10:13:32 +0200
Subject: [PATCH 10/13] Final(?) documentation touch-ups.

---
 docs/sphinx/source/whatsnew/v0.7.0.rst | 1 +
 pvlib/pvsystem.py                      | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/docs/sphinx/source/whatsnew/v0.7.0.rst b/docs/sphinx/source/whatsnew/v0.7.0.rst
index 723c1a1717..5531712dad 100644
--- a/docs/sphinx/source/whatsnew/v0.7.0.rst
+++ b/docs/sphinx/source/whatsnew/v0.7.0.rst
@@ -19,6 +19,7 @@ Bug fixes
 ~~~~~~~~~
 * Fix handling of keyword arguments in `forecasts.get_processed_data`.
   (:issue:`745`)
+* Fix output as Series feature in :py:func:`pvlib.pvsystemashraeiam`.
 
 Testing
 ~~~~~~~
diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index f23c6a64aa..fec0066499 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1098,7 +1098,9 @@ def iam_martin_ruiz(aoi, a_r=0.16):
     The incident angle modifier is defined as
     [1-exp(-cos(aoi/ar))] / [1-exp(-1/ar)], which is
     presented as AL(alpha) = 1 - IAM in equation 4 of [1]. Thus IAM is
-    equal to 1 at aoi = 0, and equal to 0 at aoi = 90.
+    equal to 1 at aoi = 0, and equal to 0 at aoi = 90.  This equation is only
+    valid for -90 <= aoi <= 90, therefore iam must be constrained to 0.0
+    beyond this range.
 
     References
     ----------

From f5d653f5c21e589ebff74baa11c92c31ab7040ad Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Thu, 25 Jul 2019 16:56:14 +0200
Subject: [PATCH 11/13] Fix tests and nan handling.

---
 pvlib/pvsystem.py           |  5 +++--
 pvlib/test/test_pvsystem.py | 27 ++++++++++++---------------
 2 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index fec0066499..9000b9f611 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1129,8 +1129,9 @@ def iam_martin_ruiz(aoi, a_r=0.16):
     if np.any(np.less_equal(a_r, 0)):
         raise RuntimeError("The parameter 'a_r' cannot be zero or negative.")
 
-    iam = (1 - np.exp(-cosd(aoi) / a_r)) / (1 - np.exp(-1 / a_r))
-    iam = np.where(np.abs(aoi) >= 90.0, 0.0, iam)
+    with np.errstate(invalid='ignore'):
+        iam = (1 - np.exp(-cosd(aoi) / a_r)) / (1 - np.exp(-1 / a_r))
+        iam = np.where(np.abs(aoi) >= 90.0, 0.0, iam)
 
     if isinstance(aoi_input, pd.Series):
         iam = pd.Series(iam, index=aoi_input.index)
diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index 27aaf4ed5a..8373ca4e85 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -165,24 +165,24 @@ def test_iam_martin_ruiz():
     aoi = [-100, -60, 0, 60, 100, np.nan, np.inf]
     expected = [0.0, 0.9414631, 1.0, 0.9414631, 0.0, np.nan, 0.0]
 
-    with np.errstate(invalid='ignore'):
-        # check out of range of inputs as list
-        iam = pvsystem.iam_martin_ruiz(aoi, a_r)
-        assert_allclose(iam, expected, equal_nan=True)
+    # check out of range of inputs as list
+    iam = pvsystem.iam_martin_ruiz(aoi, a_r)
+    assert_allclose(iam, expected, equal_nan=True)
 
-        # check out of range of inputs as array
-        iam = pvsystem.iam_martin_ruiz(np.array(aoi), a_r)
-        assert_allclose(iam, expected, equal_nan=True)
+    # check out of range of inputs as array
+    iam = pvsystem.iam_martin_ruiz(np.array(aoi), a_r)
+    assert_allclose(iam, expected, equal_nan=True)
 
-        # check out of range of inputs as Series
-        iam = pvsystem.iam_martin_ruiz(pd.Series(aoi), a_r)
-        assert_allclose(iam, expected, equal_nan=True)
+    # check out of range of inputs as Series
+    iam = pvsystem.iam_martin_ruiz(pd.Series(aoi), a_r)
+    assert_allclose(iam, expected, equal_nan=True)
 
     # check exception clause
     with pytest.raises(RuntimeError):
         pvsystem.iam_martin_ruiz(0.0, a_r=0.0)
 
 
+@requires_scipy
 def test_iam_interp():
 
     aoi_meas = [0.0, 45.0, 65.0, 75.0]
@@ -194,9 +194,6 @@ def test_iam_interp():
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
     assert_allclose(iam, expected)
 
-    iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
-    assert_allclose(iam, expected)
-
     # simple non-default method
     aoi = 55.0
     expected = 0.8878062
@@ -209,8 +206,8 @@ def test_iam_interp():
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_meas)
     assert_allclose(iam, expected)
 
-    # check normalization
-    iam_mult = np.multiply(0.9, iam_meas)
+    # check normalization and Series
+    iam_mult = pd.Series(np.multiply(0.9, iam_meas))
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_mult, normalize=True)
     assert_allclose(iam, expected)
 

From 1de4228d1513944141c06128056ad65520249107 Mon Sep 17 00:00:00 2001
From: Anton Driesse <anton.driesse@pvperformancelabs.com>
Date: Fri, 26 Jul 2019 09:59:42 +0200
Subject: [PATCH 12/13] Final(?) test fixes.

---
 pvlib/test/test_pvsystem.py | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/pvlib/test/test_pvsystem.py b/pvlib/test/test_pvsystem.py
index 8373ca4e85..7de8930f73 100644
--- a/pvlib/test/test_pvsystem.py
+++ b/pvlib/test/test_pvsystem.py
@@ -174,8 +174,10 @@ def test_iam_martin_ruiz():
     assert_allclose(iam, expected, equal_nan=True)
 
     # check out of range of inputs as Series
-    iam = pvsystem.iam_martin_ruiz(pd.Series(aoi), a_r)
-    assert_allclose(iam, expected, equal_nan=True)
+    aoi = pd.Series(aoi)
+    expected = pd.Series(expected)
+    iam = pvsystem.iam_martin_ruiz(aoi, a_r)
+    assert_series_equal(iam, expected)
 
     # check exception clause
     with pytest.raises(RuntimeError):
@@ -207,9 +209,11 @@ def test_iam_interp():
     assert_allclose(iam, expected)
 
     # check normalization and Series
-    iam_mult = pd.Series(np.multiply(0.9, iam_meas))
+    aoi = pd.Series(aoi)
+    expected = pd.Series(expected)
+    iam_mult = np.multiply(0.9, iam_meas)
     iam = pvsystem.iam_interp(aoi, aoi_meas, iam_mult, normalize=True)
-    assert_allclose(iam, expected)
+    assert_series_equal(iam, expected)
 
     # check beyond reference values
     aoi = [-45, 0, 45, 85, 90, 95, 100, 105, 110]

From b893e03b7a7215a595028ba12b0dfba00579601c Mon Sep 17 00:00:00 2001
From: Cliff Hansen <cwhanse@sandia.gov>
Date: Fri, 26 Jul 2019 11:05:30 -0600
Subject: [PATCH 13/13] Update v0.7.0.rst

---
 docs/sphinx/source/whatsnew/v0.7.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/sphinx/source/whatsnew/v0.7.0.rst b/docs/sphinx/source/whatsnew/v0.7.0.rst
index fe2adf68ef..505bd00825 100644
--- a/docs/sphinx/source/whatsnew/v0.7.0.rst
+++ b/docs/sphinx/source/whatsnew/v0.7.0.rst
@@ -19,7 +19,7 @@ Bug fixes
 ~~~~~~~~~
 * Fix handling of keyword arguments in `forecasts.get_processed_data`.
   (:issue:`745`)
-* Fix output as Series feature in :py:func:`pvlib.pvsystemashraeiam`.
+* Fix output as Series feature in :py:func:`pvlib.pvsystem.ashraeiam`.
 
 Testing
 ~~~~~~~