diff --git a/tests/unit_tests/test_illumination.py b/tests/unit_tests/test_illumination.py
index 1c675c0a..a7e02ee1 100644
--- a/tests/unit_tests/test_illumination.py
+++ b/tests/unit_tests/test_illumination.py
@@ -1,5 +1,7 @@
 """Tests for the Illumination thing."""
 
+from unittest.mock import call, patch
+
 import pytest
 
 from labthings_fastapi.testing import create_thing_without_server
@@ -10,12 +12,71 @@ from openflexure_microscope_server.things.illumination import (
     SimulatorIllumination,
 )
 
+# Test the BaseClass before testing specific implementation
 
-def test_base_illumination_flash_not_implemented():
-    """Check that calling flash on the base Illumination raises NotImplementedError."""
+
+def test_base_illumination_set_led_not_implemented():
+    """Check that calling set_led on the base Illumination raises NotImplementedError."""
     ill = create_thing_without_server(Illumination)
-    with pytest.raises(NotImplementedError, match="Flashing the LED can only be done"):
-        ill.flash(number_of_flashes=1, dt=0.01)
+    with pytest.raises(
+        NotImplementedError,
+        match="Turning the LED on and off should be implemented by child classes.",
+    ):
+        ill.set_led(led_on=True)
+
+
+def test_flash_calls_set_led_and_raises():
+    """Check that calling flash from Baseclass calls flash with right arguments."""
+    ill = create_thing_without_server(Illumination)
+
+    number_of_flashes = 3  # smaller number for test speed
+    dt = 0  # avoid sleeping
+
+    with pytest.raises(NotImplementedError):
+        ill.flash(number_of_flashes=number_of_flashes, dt=dt)
+
+    # Patch set_led to just record calls without raising
+    with patch.object(ill, "set_led") as mock_set_led:
+        # Call flash normally
+        ill.flash(number_of_flashes=number_of_flashes, dt=dt)
+
+        # Each flash calls set_led twice: False, True
+        expected_sequence = [call(False), call(True)] * number_of_flashes
+        assert mock_set_led.call_args_list == expected_sequence
+
+        # Also check total number of calls
+        assert mock_set_led.call_count == number_of_flashes * 2
+
+
+def test_negative_flash_calls():
+    """Check that calling flash with negative number doesn't call set_led."""
+    ill = create_thing_without_server(Illumination)
+
+    number_of_flashes = -33  # smaller number for test speed
+    dt = 0  # avoid sleeping
+
+    # Patch set_led to just record calls without raising
+    with patch.object(ill, "set_led") as mock_set_led:
+        # Call flash normally
+        ill.flash(number_of_flashes=number_of_flashes, dt=dt)
+
+        # Also check total number of calls
+        assert mock_set_led.call_count == 0
+
+
+def test_negative_flash_dt():
+    """Check that calling flash with negative dt raises ValueError."""
+    ill = create_thing_without_server(Illumination)
+
+    number_of_flashes = 3  # smaller number for test speed
+    dt = -5  # set a negative delay
+
+    # Patch set_led to just record calls without raising
+    with (
+        patch.object(ill, "set_led"),
+        pytest.raises(ValueError, match="sleep length must be non-negative"),
+    ):
+        ill.flash(number_of_flashes=number_of_flashes, dt=dt)
 
 
 @pytest.fixture
@@ -43,8 +104,8 @@ def test_sanga_flash_calls_led(sanga_mock_stage):
     # 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
+    for led_call, expected in zip(calls, expected_sequence, strict=True):
+        args, kwargs = led_call
         assert args[0] == expected
 
 
@@ -61,6 +122,6 @@ def test_sim_flash_calls_led(sim_mock_cam):
     # 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
+    for led_call, expected in zip(calls, expected_sequence, strict=True):
+        args, kwargs = led_call
         assert args[0] == expected