diff --git a/tests/test_scheludko.py b/tests/test_scheludko.py
index 58e1b6e..57128d5 100644
--- a/tests/test_scheludko.py
+++ b/tests/test_scheludko.py
@@ -9,7 +9,9 @@ from optifik.analysis import smooth_intensities
 from optifik.io import load_spectrum
 
 
-def test_scheludko_4peaks():
+
+@pytest.fixture
+def dataset1():
     FOLDER = os.path.join('tests', 'basic')
     FILE_NAME = '000005253.xy'
     expected = 777.07
@@ -17,8 +19,57 @@ def test_scheludko_4peaks():
     spectrum_path = os.path.join(FOLDER, FILE_NAME)
     lambdas, raw_intensities = load_spectrum(spectrum_path, lambda_min=450)
     smoothed_intensities = smooth_intensities(raw_intensities)
-    r_index =  1.324188 + 3102.060378 / (lambdas**2)
+    r_index = 1.324188 + 3102.060378 / (lambdas**2)
 
+    return {
+        "expected": expected,
+        "lambdas": lambdas,
+        "smoothed_intensities": smoothed_intensities,
+        "r_index": r_index,
+    }
+
+
+
+def test_interference_order_positive(dataset1):
+    expected = dataset1['expected']
+    lambdas = dataset1['lambdas']
+    smoothed_intensities = dataset1['smoothed_intensities']
+    r_index = dataset1['r_index']
+    prominence = 0.02
+
+    w_start, w_stop = 300, 500
+    with pytest.raises(ValueError):
+        result = thickness_from_scheludko(lambdas,
+                                          smoothed_intensities,
+                                          refractive_index=r_index,
+                                          wavelength_start=w_start,
+                                          wavelength_stop=w_stop,
+                                          interference_order=-1,
+                                          plot=False)
+
+def test_start_stop_swapped(dataset1):
+    expected = dataset1['expected']
+    lambdas = dataset1['lambdas']
+    smoothed_intensities = dataset1['smoothed_intensities']
+    r_index = dataset1['r_index']
+    prominence = 0.02
+
+    w_start, w_stop = 500, 300
+    with pytest.raises(ValueError):
+        result = thickness_from_scheludko(lambdas,
+                                          smoothed_intensities,
+                                          refractive_index=r_index,
+                                          wavelength_start=w_start,
+                                          wavelength_stop=w_stop,
+                                          interference_order=None,
+                                          plot=False)
+
+
+def test_scheludko_4peaks(dataset1):
+    expected = dataset1['expected']
+    lambdas = dataset1['lambdas']
+    smoothed_intensities = dataset1['smoothed_intensities']
+    r_index = dataset1['r_index']
     prominence = 0.02