diff --git a/qpretrieve/fourier/__init__.py b/qpretrieve/fourier/__init__.py
index bfb4143..3ef04d6 100644
--- a/qpretrieve/fourier/__init__.py
+++ b/qpretrieve/fourier/__init__.py
@@ -1,6 +1,7 @@
 # flake8: noqa: F401
 import warnings
 
+from .base import FFTFilter
 from .ff_numpy import FFTFilterNumpy
 
 try:
@@ -11,7 +12,7 @@
 PREFERRED_INTERFACE = None
 
 
-def get_available_interfaces():
+def get_available_interfaces() -> list:
     """Return a list of available FFT algorithms"""
     interfaces = [
         FFTFilterPyFFTW,
@@ -24,7 +25,7 @@ def get_available_interfaces():
     return interfaces_available
 
 
-def get_best_interface():
+def get_best_interface() -> FFTFilter:
     """Return the fastest refocusing interface available
 
     If `pyfftw` is installed, :class:`.FFTFilterPyFFTW`
diff --git a/qpretrieve/fourier/base.py b/qpretrieve/fourier/base.py
index c8f6e91..0df9b77 100644
--- a/qpretrieve/fourier/base.py
+++ b/qpretrieve/fourier/base.py
@@ -41,7 +41,7 @@ def __init__(self,
                  data: np.ndarray,
                  subtract_mean: bool = True,
                  padding: int = 2,
-                 copy: bool = True):
+                 copy: bool = True) -> None:
         r"""
         Parameters
         ----------
@@ -135,13 +135,13 @@ def __init__(self,
         self.fft_used = None
 
     @property
-    def shape(self):
+    def shape(self) -> tuple:
         """Shape of the Fourier transform data"""
         return self.fft_origin.shape
 
     @property
     @abstractmethod
-    def is_available(self):
+    def is_available(self) -> bool:
         """Whether this method is available given current hardware/software"""
         return True
 
@@ -169,7 +169,7 @@ def _init_fft(self, data):
 
     def filter(self, filter_name: str, filter_size: float,
                freq_pos: (float, float),
-               scale_to_filter: bool | float = False):
+               scale_to_filter: bool | float = False) -> np.ndarray:
         """
         Parameters
         ----------
diff --git a/qpretrieve/fourier/ff_numpy.py b/qpretrieve/fourier/ff_numpy.py
index af3d960..0ac86c8 100644
--- a/qpretrieve/fourier/ff_numpy.py
+++ b/qpretrieve/fourier/ff_numpy.py
@@ -10,7 +10,7 @@ class FFTFilterNumpy(FFTFilter):
     # always available, because numpy is a dependency
     is_available = True
 
-    def _init_fft(self, data):
+    def _init_fft(self, data: np.ndarray) -> np.ndarray:
         """Perform initial Fourier transform of the input data
 
         Parameters
@@ -25,6 +25,6 @@ def _init_fft(self, data):
         """
         return np.fft.fft2(data, axes=(-2, -1))
 
-    def _ifft(self, data):
+    def _ifft(self, data: np.ndarray) -> np.ndarray:
         """Perform inverse Fourier transform"""
         return np.fft.ifft2(data, axes=(-2, -1))
diff --git a/qpretrieve/fourier/ff_pyfftw.py b/qpretrieve/fourier/ff_pyfftw.py
index 56eda93..7ecd056 100644
--- a/qpretrieve/fourier/ff_pyfftw.py
+++ b/qpretrieve/fourier/ff_pyfftw.py
@@ -1,4 +1,5 @@
 import multiprocessing as mp
+import numpy as np
 
 import pyfftw
 
@@ -11,7 +12,7 @@ class FFTFilterPyFFTW(FFTFilter):
     # always available, because numpy is a dependency
     is_available = True
 
-    def _init_fft(self, data):
+    def _init_fft(self, data: np.ndarray) -> np.ndarray:
         """Perform initial Fourier transform of the input data
 
         Parameters
@@ -33,13 +34,13 @@ def _init_fft(self, data):
         fft_obj()
         return out_arr
 
-    def _ifft(self, data):
+    def _ifft(self, data: np.ndarray) -> np.ndarray:
         """Perform inverse Fourier transform"""
         in_arr = pyfftw.empty_aligned(data.shape, dtype='complex128')
-        ou_arr = pyfftw.empty_aligned(data.shape, dtype='complex128')
-        fft_obj = pyfftw.FFTW(in_arr, ou_arr, axes=(-2, -1),
+        out_arr = pyfftw.empty_aligned(data.shape, dtype='complex128')
+        fft_obj = pyfftw.FFTW(in_arr, out_arr, axes=(-2, -1),
                               direction="FFTW_BACKWARD",
                               )
         in_arr[:] = data
         fft_obj()
-        return ou_arr
+        return out_arr