diff --git a/superxas_bec/scans/timepix_scan.py b/superxas_bec/scans/timepix_scan.py
index 802442f..5e51e27 100644
--- a/superxas_bec/scans/timepix_scan.py
+++ b/superxas_bec/scans/timepix_scan.py
@@ -47,20 +47,23 @@ class TimepixScan(LineScan):
         # fmt: off
         time_roi_step: Annotated[int, ScanArgument(display_name="Time ROI step", description="Bin size", gt=0)],
         time_roi_n: Annotated[int, ScanArgument(display_name="Time ROI n", description="Number of time points", gt=0)],
-        idx_off_start: Annotated[int, ScanArgument(display_name="Index Off Start", description="Start Index for Off Level", ge=0)],
-        idx_off_stop: Annotated[int, ScanArgument(display_name="Index Off Stop", description="Stop Index for Off Level", ge=0)],
-        idx_on_start: Annotated[int, ScanArgument(display_name="Index On Start", description="Start Index for On Level", ge=0)],
-        idx_on_stop: Annotated[int, ScanArgument(display_name="Index On Stop", description="Stop Index for On Level", ge=0)],
+        idx_off_start: Annotated[int, ScanArgument(display_name="Index Off Start", description="Start Index for Off Level (first time bin)", ge=0)],
+        idx_off_stop: Annotated[int, ScanArgument(display_name="Index Off Stop", description="Stop Index for Off Level (exclusive)", ge=0)],
+        idx_on_start: Annotated[int, ScanArgument(display_name="Index On Start", description="Start Index for On Level (first time bin)", ge=0)],
+        idx_on_stop: Annotated[int, ScanArgument(display_name="Index On Stop", description="Stop Index for On Level (exclusive)", ge=0)],
         # fmt: on
         **kwargs,
     ):
-        if idx_off_start > idx_off_stop:
+        # The on/off windows are half-open index ranges [start, stop) over the time_roi_n time
+        # bins; the Timepix device averages each window into xes_level_*. An empty window
+        # (start == stop) would only produce NaN, so it is rejected here as well.
+        if idx_off_start >= idx_off_stop:
             raise ValueError(
-                f"idx_off_start ({idx_off_start}) cannot be bigger than idx_off_stop ({idx_off_stop})!"
+                f"idx_off_start ({idx_off_start}) must be smaller than idx_off_stop ({idx_off_stop})!"
             )
-        if idx_on_start > idx_on_stop:
+        if idx_on_start >= idx_on_stop:
             raise ValueError(
-                f"idx_on_start ({idx_on_start}) cannot be bigger than idx_on_stop ({idx_on_stop})!"
+                f"idx_on_start ({idx_on_start}) must be smaller than idx_on_stop ({idx_on_stop})!"
             )
         for name, idx in (
             ("idx_off_start", idx_off_start),
diff --git a/tests/tests_scans/test_timepix_scan.py b/tests/tests_scans/test_timepix_scan.py
new file mode 100644
index 0000000..87c4dce
--- /dev/null
+++ b/tests/tests_scans/test_timepix_scan.py
@@ -0,0 +1,75 @@
+# pylint: skip-file
+import pytest
+from bec_lib.bec_errors import ScanInputValidationError
+from bec_server.scan_server.tests.scan_fixtures import *
+
+from superxas_bec.scans.timepix_scan import TimepixScan
+
+TIMEPIX_PARAMETERS = {
+    "time_roi_step": 1,
+    "time_roi_n": 10,
+    "idx_off_start": 0,
+    "idx_off_stop": 4,
+    "idx_on_start": 5,
+    "idx_on_stop": 10,
+}
+
+
+def _assemble_timepix_scan(scan_assembler, **overrides):
+    params = {**TIMEPIX_PARAMETERS, **overrides}
+    return scan_assembler(
+        "timepix_scan", "samx", -1.0, 1.0, steps=3, relative=False, exp_time=0.1, **params
+    )
+
+
+def test_timepix_scan_forwards_parameters_to_scan_info(scan_assembler):
+    """The Timepix device reads the parameters from additional_scan_parameters."""
+    scan = _assemble_timepix_scan(scan_assembler)
+
+    assert isinstance(scan, TimepixScan)
+    assert scan.scan_info.scan_name == "timepix_scan"
+    for name, value in TIMEPIX_PARAMETERS.items():
+        assert scan.scan_info.additional_scan_parameters[name] == value
+
+
+@pytest.mark.parametrize(
+    "overrides, match",
+    [
+        ({"idx_off_start": 5, "idx_off_stop": 4}, "idx_off_start"),
+        ({"idx_on_start": 9, "idx_on_stop": 6}, "idx_on_start"),
+        # start == stop would be an empty window
+        ({"idx_off_start": 3, "idx_off_stop": 3}, "idx_off_start"),
+        ({"idx_on_start": 7, "idx_on_stop": 7}, "idx_on_start"),
+    ],
+)
+def test_timepix_scan_rejects_start_not_smaller_than_stop(scan_assembler, overrides, match):
+    with pytest.raises(ValueError, match=match):
+        _assemble_timepix_scan(scan_assembler, **overrides)
+
+
+@pytest.mark.parametrize(
+    "overrides, name",
+    [
+        ({"idx_off_stop": 11}, "idx_off_stop"),
+        ({"idx_on_stop": 11}, "idx_on_stop"),
+        ({"idx_on_start": 11, "idx_on_stop": 12}, "idx_on_start"),
+    ],
+)
+def test_timepix_scan_rejects_index_above_time_roi_n(scan_assembler, overrides, name):
+    with pytest.raises(ValueError, match=f"{name} with value"):
+        _assemble_timepix_scan(scan_assembler, **overrides)
+
+
+def test_timepix_scan_accepts_stop_equal_to_time_roi_n(scan_assembler):
+    """stop is exclusive, so a window may end at time_roi_n."""
+    scan = _assemble_timepix_scan(scan_assembler, idx_on_start=9, idx_on_stop=10)
+
+    assert scan.scan_info.additional_scan_parameters["idx_on_stop"] == 10
+
+
+@pytest.mark.parametrize(
+    "overrides", [{"idx_off_start": -1}, {"time_roi_n": 0}, {"time_roi_step": 0}]
+)
+def test_timepix_scan_argument_bounds_are_validated(scan_assembler, overrides):
+    with pytest.raises(ScanInputValidationError):
+        _assemble_timepix_scan(scan_assembler, **overrides)