diff --git a/src/aarecommon/models/models.py b/src/aarecommon/models/models.py
index 493f922..47e3a38 100644
--- a/src/aarecommon/models/models.py
+++ b/src/aarecommon/models/models.py
@@ -61,7 +61,11 @@ class DataCollectionParameters(BaseModel):
     totalangle: int | None = Field(  # was totalrange
         default=None, validation_alias=AliasChoices("totalangle", "totalrange")
     )  # Only accept positive integers between 0 and 360
-    transmission: int | None = None  # Only accept positive integers between 0 and 100
+    # Fraction of full beam (1.0 = full transmission), NOT a percentage.
+    # Values in (1.0, 100.0] fail validation by design: they are percent-scale
+    # leftovers and must be divided by 100 at whatever boundary produced them
+    # (user spreadsheets stay percent for humans; converters live in AareDB).
+    transmission: Annotated[float, Field(ge=0.0, le=1.0)] | None = None
     targetresolution: float | None = None  # Only accept positive float
     beamsize: str | None = None
     aperture: int | None = None  # Optional string field
diff --git a/src/aarecommon/models/raster_grid.py b/src/aarecommon/models/raster_grid.py
index db37e4a..fd0c02d 100644
--- a/src/aarecommon/models/raster_grid.py
+++ b/src/aarecommon/models/raster_grid.py
@@ -11,7 +11,8 @@ from aarecommon.models.gridscan_decision import GridScanDecision
 
 class RasterGridRequest(BaseModel):
     dtz: float | None = None
-    transmission: float | None = None
+    # Fraction of full beam (1.0 = full transmission), NOT a percentage.
+    transmission: Annotated[float, Field(ge=0.0, le=1.0)] | None = None
     file_prefix: str | None = None
     exp_time_s: float
 
diff --git a/src/aarecommon/models/rotation_scan.py b/src/aarecommon/models/rotation_scan.py
index 008ced3..8936ecf 100644
--- a/src/aarecommon/models/rotation_scan.py
+++ b/src/aarecommon/models/rotation_scan.py
@@ -1,12 +1,15 @@
+from typing import Annotated
+
 from jfjoch_client.models.scan_result import ScanResult
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from aarecommon.math.coordinate import SmargonCoordinate
 
 
 class RotationScanRequest(BaseModel):
     dtz: float | None = None
-    transmission: float | None = None
+    # Fraction of full beam (1.0 = full transmission), NOT a percentage.
+    transmission: Annotated[float, Field(ge=0.0, le=1.0)] | None = None
     file_prefix: str | None = None
     exp_time_s: float
 
diff --git a/tests/test_data_collection_parameters.py b/tests/test_data_collection_parameters.py
index 20a0b90..56c274a 100644
--- a/tests/test_data_collection_parameters.py
+++ b/tests/test_data_collection_parameters.py
@@ -71,3 +71,26 @@ def test_datacollectionparameters_accepts_new_fields():
     assert params.processingresolution == 1.2
     assert params.pdbmodel == "model.pdb"
     assert params.cloud is False
+
+
+def test_transmission_is_a_fraction_of_full_beam():
+    params = DataCollectionParameters(transmission=0.35)
+    assert params.transmission == 0.35
+
+
+def test_transmission_boundaries_are_valid():
+    assert DataCollectionParameters(transmission=0.0).transmission == 0.0
+    assert DataCollectionParameters(transmission=1.0).transmission == 1.0
+
+
+def test_transmission_rejects_percent_scale_values():
+    # Percent-scale leftovers (1.0, 100.0] must fail loudly, never be
+    # silently reinterpreted — converters live at the producing boundary.
+    for percent in (1.5, 35, 100):
+        with pytest.raises(ValidationError):
+            DataCollectionParameters(transmission=percent)
+
+
+def test_transmission_rejects_negative_values():
+    with pytest.raises(ValidationError):
+        DataCollectionParameters(transmission=-0.1)