diff --git a/qpretrieve/fourier/__init__.py b/qpretrieve/fourier/__init__.py
index e1d4383..c135f66 100644
--- a/qpretrieve/fourier/__init__.py
+++ b/qpretrieve/fourier/__init__.py
@@ -1,5 +1,6 @@
 # flake8: noqa: F401
 import warnings
+from typing import Type
 
 from .base import FFTFilter
 from .ff_numpy import FFTFilterNumpy
@@ -12,7 +13,7 @@
 PREFERRED_INTERFACE = None
 
 
-def get_available_interfaces() -> list:
+def get_available_interfaces() -> list[Type[FFTFilter]]:
     """Return a list of available FFT algorithms"""
     interfaces = [
         FFTFilterPyFFTW,
@@ -25,7 +26,7 @@ def get_available_interfaces() -> list:
     return interfaces_available
 
 
-def get_best_interface():
+def get_best_interface() -> Type[FFTFilter]:
     """Return the fastest refocusing interface available
 
     If `pyfftw` is installed, :class:`.FFTFilterPyFFTW`
diff --git a/qpretrieve/interfere/base.py b/qpretrieve/interfere/base.py
index 05a1c44..7bd4323 100644
--- a/qpretrieve/interfere/base.py
+++ b/qpretrieve/interfere/base.py
@@ -1,5 +1,6 @@
 import warnings
 from abc import ABC, abstractmethod
+from typing import Type
 
 import numpy as np
 
@@ -25,7 +26,7 @@ class BaseInterferogram(ABC):
     }
 
     def __init__(self, data: np.ndarray,
-                 fft_interface: str | FFTFilter = "auto",
+                 fft_interface: str | Type[FFTFilter] = "auto",
                  subtract_mean=True, padding=2, copy=True,
                  **pipeline_kws) -> None:
         """
diff --git a/qpretrieve/interfere/if_qlsi.py b/qpretrieve/interfere/if_qlsi.py
index a8af42f..ed1ceaf 100644
--- a/qpretrieve/interfere/if_qlsi.py
+++ b/qpretrieve/interfere/if_qlsi.py
@@ -24,12 +24,11 @@ class QLSInterferogram(BaseInterferogram):
 
     def __init__(self, data, reference=None, *args, **kwargs):
         super(QLSInterferogram, self).__init__(data, *args, **kwargs)
-        ff_iface = get_best_interface()
 
         if reference is not None:
-            self.fft_ref = ff_iface(data=reference,
-                                    subtract_mean=self.fft.subtract_mean,
-                                    padding=self.fft.padding)
+            self.fft_ref = self.ff_iface(data=reference,
+                                         subtract_mean=self.fft.subtract_mean,
+                                         padding=self.fft.padding)
         else:
             self.fft_ref = None