diff --git a/examples/example_material.xomp b/examples/example_material.xomp
index 50788383..c75d3270 100644
--- a/examples/example_material.xomp
+++ b/examples/example_material.xomp
@@ -8,12 +8,13 @@
     "openMaterialVersion": "1.0.0",
     "copyright": "(C) 2023-2024, Example Company",
     "license": "MPL-2.0",
-    "author": "john.doe@asam.net",
+    "author": "john.doe@asam.net"
   },
   "materialProperties": {
     "surfaceRoughness": {
         "surfaceHeightRms": 0.8,
-        "surfaceCorrelationLength": 1.0
+        "surfaceCorrelationLength": 1.0,
+        "source": "estimate"
     },
     "emissivityData": {
         "emissivityCoefficient": 0.07,
diff --git a/examples/example_material_emp.xompt b/examples/example_material_emp.xompt
index cdbd4158..6eab3dfb 100644
--- a/examples/example_material_emp.xompt
+++ b/examples/example_material_emp.xompt
@@ -9,6 +9,7 @@
     "copyright": "(C) 2023-2024, Example Company",
     "license": "MPL-2.0",
     "author": "john.doe@asam.net",
+    "source": "estimate"
   },
   "electromagneticProperties": [
     [500e-9, 293.0, 50.0, 0.000001256665, 0.0, -56.0, 24.0],
diff --git a/schemas/material_emp_schema.json b/schemas/material_emp_schema.json
index 9e433a66..9c8c18d0 100644
--- a/schemas/material_emp_schema.json
+++ b/schemas/material_emp_schema.json
@@ -45,6 +45,10 @@
         "author": {
           "type": "string",
           "description": "Name or email address of the author of this material. In case of multiple authors, use comma-separation. The author can also be a company name."
+        },
+        "source": {
+          "type": "string",
+          "description": "Source of the electromagnetic property data."
         }
       },
       "required": [
@@ -56,7 +60,8 @@
         "openMaterialVersion",
         "copyright",
         "license",
-        "author"
+        "author",
+        "source"
       ]
     },
     "electromagneticProperties": {
diff --git a/schemas/material_schema.json b/schemas/material_schema.json
index af38f096..da112db4 100644
--- a/schemas/material_schema.json
+++ b/schemas/material_schema.json
@@ -74,11 +74,16 @@
             "surfaceCorrelationLength": {
               "type": "number",
               "description": "Correlation length of the surface height deviations."
+            },
+            "source": {
+              "type": "string",
+              "description": "Source of the surface roughness data."
             }
           },
           "required": [
             "surfaceHeightRms",
-            "surfaceCorrelationLength"
+            "surfaceCorrelationLength",
+            "source"
           ]
         },
         "emissivityData": {