diff --git a/camtools/io.py b/camtools/io.py
index 46a52263..722f0154 100644
--- a/camtools/io.py
+++ b/camtools/io.py
@@ -5,7 +5,7 @@
 import cv2
 import numpy as np
 from pathlib import Path
-from typing import Union, Optional
+from typing import Union, Optional, Literal
 from jaxtyping import UInt8, Float
 
 from . import sanity
@@ -185,7 +185,7 @@ def imwrite_depth(
 
 def imread(
     im_path: Union[str, Path],
-    alpha_mode: Optional[str] = None,
+    alpha_mode: Optional[Literal["keep", "ignore", "white", "black"]] = None,
 ) -> Union[
     Float[np.ndarray, "h w"],
     Float[np.ndarray, "h w 3"],
diff --git a/camtools/metric.py b/camtools/metric.py
index 86febfea..0bbf1784 100644
--- a/camtools/metric.py
+++ b/camtools/metric.py
@@ -6,7 +6,7 @@
 from skimage.metrics import peak_signal_noise_ratio
 from skimage.metrics import structural_similarity
 from pathlib import Path
-from typing import Tuple, Optional, Union
+from typing import Tuple, Optional, Union, Literal
 from jaxtyping import Float
 
 from . import image
@@ -200,7 +200,7 @@ def load_im_pd_im_gt_im_mask_for_eval(
     im_pd_path: Union[str, Path],
     im_gt_path: Union[str, Path],
     im_mask_path: Optional[Union[str, Path]] = None,
-    alpha_mode: str = "white",
+    alpha_mode: Literal["white", "keep"] = "white",
 ) -> Tuple[
     Float[np.ndarray, "h w 3"], Float[np.ndarray, "h w 3"], Float[np.ndarray, "h w"]
 ]: