diff --git a/src/backend/api/routes.py b/src/backend/api/routes.py
index 40a819c..2b44601 100644
--- a/src/backend/api/routes.py
+++ b/src/backend/api/routes.py
@@ -269,7 +269,13 @@ def _frame_to_dict(frame: CameraFrame, include_images: bool = True) -> dict:
 
 
 def _calibration_payload() -> dict | None:
-    """Full calibration (intrinsics + distortion + extrinsics) or None."""
+    """Full calibration (intrinsics + distortion + extrinsics) or None.
+
+    Enough to align depth to color outside the SDK: per stream width, height
+    and distortion model (rs.distortion member name), depth_scale_m (meters per
+    raw unit). extrinsics_ir_to_rgb: p_rgb = R p_ir + t, R row-major (the SDK's
+    is column-major), t in millimeters.
+    """
     if not _camera or not _camera.intrinsics:
         return None
     i = _camera.intrinsics
@@ -280,19 +286,26 @@ def _calibration_payload() -> dict | None:
     def _d(d):
         return None if d is None else {"k1": d.k1, "k2": d.k2, "k3": d.k3, "p1": d.p1, "p2": d.p2}
 
+    def _stream(name):
+        width, height = _camera.stream_sizes.get(name, (None, None))
+        return {"width": width, "height": height, "model": _camera.distortion_models.get(name)}
+
     return {
         "ir": {
             "fx": i.fx_ir, "fy": i.fy_ir, "cx": i.cx_ir, "cy": i.cy_ir,
             "distortion": _d(dir_),
+            **_stream("ir"),
         },
         "rgb": {
             "fx": i.fx_rgb, "fy": i.fy_rgb, "cx": i.cx_rgb, "cy": i.cy_rgb,
             "distortion": _d(drgb),
+            **_stream("rgb"),
         },
         "extrinsics_ir_to_rgb": None if e is None else {
             "rotation": list(e.rotation),
             "translation": list(e.translation),
         },
+        "depth_scale_m": _camera.depth_scale(),
     }
 
 
diff --git a/src/backend/services/camera.py b/src/backend/services/camera.py
index 744fd91..316f63c 100644
--- a/src/backend/services/camera.py
+++ b/src/backend/services/camera.py
@@ -79,6 +79,11 @@ class RawFrame:
     depth: np.ndarray             # (H, W) uint16 millimeters, depth sensor grid, own memory
 
 
+def _model_name(model) -> str:
+    """'distortion.inverse_brown_conrady' -> 'inverse_brown_conrady' (a member of rs.distortion)."""
+    return str(model).split(".")[-1]
+
+
 def _domain_name(domain) -> str:
     """'timestamp_domain.hardware_clock' -> 'hardware_clock'."""
     return str(domain).split(".")[-1]
@@ -173,6 +178,10 @@ class RealSenseD435:
         self.extrinsics: Optional[CameraExtrinsics] = None
         self.distortion_ir: Optional[Distortion] = None
         self.distortion_rgb: Optional[Distortion] = None
+        # Per stream ("ir" = depth, "rgb" = color): image size and distortion model name,
+        # what an align outside the SDK needs besides the intrinsics above.
+        self.stream_sizes: dict[str, tuple[int, int]] = {}
+        self.distortion_models: dict[str, str] = {}
 
     # ------------------------------------------------------------------
     # Lifecycle
@@ -446,6 +455,12 @@ class RealSenseD435:
 
         self.distortion_ir = _to_distortion(list(di.coeffs))
         self.distortion_rgb = _to_distortion(list(ci.coeffs))
+        self.stream_sizes = {"ir": (di.width, di.height), "rgb": (ci.width, ci.height)}
+        self.distortion_models = {"ir": _model_name(di.model), "rgb": _model_name(ci.model)}
+
+    def depth_scale(self) -> float:
+        """Meters per raw z16 depth unit."""
+        return self._depth_scale
 
     # ------------------------------------------------------------------
     # Frame capture