diff --git a/tests/unit_tests/test_illumination.py b/tests/unit_tests/test_illumination.py
new file mode 100644
index 00000000..f8edbd8a
--- /dev/null
+++ b/tests/unit_tests/test_illumination.py
@@ -0,0 +1,58 @@
+"""Tests for the Illumination thing."""
+
+import pytest
+
+from labthings_fastapi.testing import create_thing_without_server
+
+from openflexure_microscope_server.things.illumination import (
+    SangaIllumination,
+    SimulatorIllumination,
+)
+
+
+@pytest.fixture
+def sanga_mock_stage():
+    """Return a SangaIllumination with a mocked stage."""
+    return create_thing_without_server(SangaIllumination, mock_all_slots=True)
+
+
+@pytest.fixture
+def sim_mock_cam():
+    """Return a SimulatorIllumination with a mocked camera."""
+    return create_thing_without_server(SimulatorIllumination, mock_all_slots=True)
+
+
+def test_sanga_flash_calls_led(sanga_mock_stage):
+    """Test that flashing calls set_led with correct parameters."""
+    number_of_flashes = 3
+    dt = 0.01  # tiny sleep for test speed
+
+    sanga_mock_stage.flash(number_of_flashes=number_of_flashes, dt=dt)
+
+    # Each flash calls set_led False then True
+    assert sanga_mock_stage._stage.set_led.call_count == 2 * number_of_flashes
+
+    # Check that False and True are alternating when sent to _stage.set_led
+    calls = sanga_mock_stage._stage.set_led.call_args_list
+    expected_sequence = [False, True] * number_of_flashes
+    for call, expected in zip(calls, expected_sequence, strict=True):
+        args, kwargs = call
+        assert args[0] == expected
+
+
+def test_sim_flash_calls_led(sim_mock_cam):
+    """Test that simulator flash calls set_led correctly."""
+    number_of_flashes = 2
+    dt = 0.01
+
+    sim_mock_cam.flash(number_of_flashes=number_of_flashes, dt=dt)
+
+    # Each flash calls set_led False then True
+    assert sim_mock_cam._cam.set_led.call_count == 2 * number_of_flashes
+
+    # Check that False and True are alternating when sent to _cam.set_led
+    calls = sim_mock_cam._cam.set_led.call_args_list
+    expected_sequence = [False, True] * number_of_flashes
+    for call, expected in zip(calls, expected_sequence, strict=True):
+        args, kwargs = call
+        assert args[0] == expected