diff --git a/HISTORY.rst b/HISTORY.rst
index 1f3a68e7..a9ddc7ab 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -2,6 +2,21 @@
 History
 =======
 
+1.1.0 (2023-02-20)
+------------------
+
+* Introduction of simple model language that can be used to describe
+  sample structures. The module *orsopy.fileio.model_language* is used to implement
+  and parse the model language.
+  See https://www.reflectometry.org/projects/simple_model for specifications.
+  Sample model examples can be found in the examples folder together
+  with scripts using the orsopy module to parse and plot the data.
+* Add polarization channels for x-ray experiments
+* Implement ErrorValue class for optional description of errors
+  on values within the file header.
+* Update of .ort standard according to discussions with community.
+  (E.g. rename of column attribute "dimension" to "physical_quantity")
+
 1.0.1 (2022-06-28)
 ------------------
 
@@ -9,6 +24,8 @@ History
 * Update the schema files for released .ort standard.
 * Sample.sample_parameters keys to be strings and values restricted to
   Value, ValueRange, ValueVector or ComplexValue.
+* Add *as_unit* method to value classes that uses the *pint* library to convert
+  values to supplied unit automatically.
 
 1.0.0 (2022-06-10)
 ------------------
diff --git a/examples/simple_model_example_3.yml b/examples/simple_model_example_3.yml
index 76e28642..caa04301 100644
--- a/examples/simple_model_example_3.yml
+++ b/examples/simple_model_example_3.yml
@@ -6,10 +6,10 @@ sample:
           substrate:
             sequence:
             - material: Si
-              sigma: 2
+              roughness: 2
             - material: SiO2
               thickness: 5
-              sigma: 3
+              roughness: 3
           film:
             repetitions: 5
             stack: head_group 4 | tail | tail | head_group 4
@@ -35,7 +35,7 @@ sample:
               H2O: 0.3
               D2O: 0.7
         globals:
-          sigma: {magnitude: 5, unit: angstrom}
+          roughness: {magnitude: 5, unit: angstrom}
           length_unit: angstrom
           mass_density_unit: g/cm^3
           sld_unit: 1/angstrom^2
diff --git a/orsopy/__init__.py b/orsopy/__init__.py
index a5923eec..02f80e2a 100644
--- a/orsopy/__init__.py
+++ b/orsopy/__init__.py
@@ -1,3 +1,3 @@
 """Top-level package for orsopy."""
 
-__version__ = "1.0.1"
+__version__ = "1.1.0"
diff --git a/orsopy/dataclasses.py b/orsopy/dataclasses.py
index b4174745..a66bc5ea 100644
--- a/orsopy/dataclasses.py
+++ b/orsopy/dataclasses.py
@@ -9,3 +9,14 @@
 
     def _field_init(f, frozen, locals, self_name):
         return _field_init_real(f, frozen, locals, self_name, False)
+
+
+elif sys.version_info < (3, 7, 0):
+    # fix bug in python 3.6 when using default_factory for dataclass objects
+    _orig_field = field
+
+    def field(*args, **opts):
+        if "default_factory" in opts and not opts["default_factory"] in [list, tuple]:
+            return opts["default_factory"]()
+        else:
+            return _orig_field(*args, **opts)
diff --git a/orsopy/fileio/model_language.py b/orsopy/fileio/model_language.py
index cb89feea..dca1d827 100644
--- a/orsopy/fileio/model_language.py
+++ b/orsopy/fileio/model_language.py
@@ -8,6 +8,7 @@
 
 from typing import Any, Dict, List, Optional, Union
 
+from ..dataclasses import field
 from ..utils.chemical_formula import Formula
 from ..utils.density_resolver import DensityResolver
 from .base import ComplexValue, Header, Value, orsodataclass
@@ -26,12 +27,12 @@ def find_idx(string, start, value):
 
 @orsodataclass
 class ModelParameters(Header):
-    roughness: Optional[Value] = Value(0.3, "nm")
-    length_unit: Optional[str] = "nm"
-    mass_density_unit: Optional[str] = "g/cm^3"
-    number_density_unit: Optional[str] = "1/nm^3"
-    sld_unit: Optional[str] = "1/angstrom^2"
-    magnetic_moment_unit: Optional[str] = "muB"
+    roughness: Value = field(default_factory=lambda: Value(0.3, "nm"))
+    length_unit: str = "nm"
+    mass_density_unit: str = "g/cm^3"
+    number_density_unit: str = "1/nm^3"
+    sld_unit: str = "1/angstrom^2"
+    magnetic_moment_unit: str = "muB"
 
 
 @orsodataclass
@@ -152,6 +153,8 @@ def get_sld(self, xray_energy=None) -> complex:
 class Composit(Header):
     composition: Dict[str, float]
 
+    original_name = None
+
     def resolve_names(self, resolvable_items):
         self._composition_materials = {}
         for key, value in self.composition.items():
@@ -288,6 +291,8 @@ class SubStack(Header):
     arguments: Optional[List[Any]] = None
     keywords: Optional[Dict[str, Any]] = None
 
+    original_name = None
+
     def resolve_names(self, resolvable_items):
         if self.stack is None and self.sequence is None:
             raise ValueError("SubStack has to either define stack or sequence")
@@ -319,6 +324,7 @@ def resolve_names(self, resolvable_items):
                             obj.thickness = thickness
                     else:
                         obj = Layer(material=item, thickness=thickness)
+                        obj.original_name = item
                 if hasattr(obj, "resolve_names"):
                     obj.resolve_names(resolvable_items)
                 output.append(obj)
@@ -375,6 +381,8 @@ def __post_init__(self):
     def resolvable_items(self):
         output = {}
         if self.sub_stacks:
+            for key, ssi in self.sub_stacks.items():
+                ssi.original_name = key
             output.update(self.sub_stacks)
         if self.layers:
             for key, li in self.layers.items():
@@ -385,6 +393,8 @@ def resolvable_items(self):
                 mi.original_name = key
             output.update(self.materials)
         if self.composits:
+            for key, ci in self.composits.items():
+                ci.original_name = key
             output.update(self.composits)
         return output
 
@@ -421,6 +431,7 @@ def resolve_stack(self):
                         obj.thickness = thickness
                 else:
                     obj = Layer(material=item, thickness=thickness)
+                    obj.original_name = item
             if hasattr(obj, "resolve_names"):
                 obj.resolve_names(ri)
             if hasattr(obj, "resolve_defaults"):
diff --git a/orsopy/fileio/schema/refl_header.schema.json b/orsopy/fileio/schema/refl_header.schema.json
index a187bfe0..e3700221 100644
--- a/orsopy/fileio/schema/refl_header.schema.json
+++ b/orsopy/fileio/schema/refl_header.schema.json
@@ -384,6 +384,361 @@
         "real"
       ]
     },
+    "Material": {
+      "title": "Material",
+      "type": "object",
+      "properties": {
+        "formula": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "mass_density": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "number_density": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "sld": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/ComplexValue"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "magnetic_moment": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "relative_density": {
+          "type": [
+            "number",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    },
+    "Composit": {
+      "title": "Composit",
+      "type": "object",
+      "properties": {
+        "composition": {
+          "additionalProperties": {
+            "type": "number"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "composition"
+      ]
+    },
+    "Layer": {
+      "title": "Layer",
+      "type": "object",
+      "properties": {
+        "thickness": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "roughness": {
+          "anyOf": [
+            {
+              "type": "number"
+            },
+            {
+              "$ref": "#/definitions/Value"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "material": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/Material"
+            },
+            {
+              "$ref": "#/definitions/Composit"
+            },
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "composition": {
+          "additionalProperties": {
+            "type": "number"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    },
+    "SubStack": {
+      "title": "SubStack",
+      "type": "object",
+      "properties": {
+        "repetitions": {
+          "default": 1,
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "stack": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "sequence": {
+          "items": {
+            "$ref": "#/definitions/Layer"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        },
+        "represents": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "arguments": {
+          "items": {},
+          "type": [
+            "array",
+            "null"
+          ]
+        },
+        "keywords": {
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    },
+    "ModelParameters": {
+      "title": "ModelParameters",
+      "type": "object",
+      "properties": {
+        "roughness": {
+          "default": {
+            "magnitude": 0.3,
+            "unit": "nm",
+            "error": null,
+            "comment": null
+          },
+          "allOf": [
+            {
+              "$ref": "#/definitions/Value"
+            }
+          ]
+        },
+        "length_unit": {
+          "default": "nm",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "mass_density_unit": {
+          "default": "g/cm^3",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "number_density_unit": {
+          "default": "1/nm^3",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "sld_unit": {
+          "default": "1/angstrom^2",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "magnetic_moment_unit": {
+          "default": "muB",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    },
+    "SampleModel": {
+      "title": "SampleModel",
+      "type": "object",
+      "properties": {
+        "stack": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "origin": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "sub_stacks": {
+          "additionalProperties": {
+            "$ref": "#/definitions/SubStack"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "layers": {
+          "additionalProperties": {
+            "$ref": "#/definitions/Layer"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "materials": {
+          "additionalProperties": {
+            "$ref": "#/definitions/Material"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "composits": {
+          "additionalProperties": {
+            "$ref": "#/definitions/Composit"
+          },
+          "type": [
+            "object",
+            "null"
+          ]
+        },
+        "globals": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ModelParameters"
+            }
+          ]
+        },
+        "reference": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "comment": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "stack"
+      ]
+    },
     "Sample": {
       "title": "Sample",
       "type": "object",
@@ -451,6 +806,13 @@
             "null"
           ]
         },
+        "model": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/SampleModel"
+            }
+          ]
+        },
         "comment": {
           "type": [
             "string",
diff --git a/orsopy/fileio/schema/refl_header.schema.yaml b/orsopy/fileio/schema/refl_header.schema.yaml
index d436e34e..afb82162 100644
--- a/orsopy/fileio/schema/refl_header.schema.yaml
+++ b/orsopy/fileio/schema/refl_header.schema.yaml
@@ -51,6 +51,22 @@ definitions:
     - real
     title: ComplexValue
     type: object
+  Composit:
+    properties:
+      comment:
+        type:
+        - string
+        - 'null'
+      composition:
+        additionalProperties:
+          type: number
+        type:
+        - object
+        - 'null'
+    required:
+    - composition
+    title: Composit
+    type: object
   DataSource:
     properties:
       comment:
@@ -256,6 +272,73 @@ definitions:
     - wavelength
     title: InstrumentSettings
     type: object
+  Layer:
+    properties:
+      comment:
+        type:
+        - string
+        - 'null'
+      composition:
+        additionalProperties:
+          type: number
+        type:
+        - object
+        - 'null'
+      material:
+        anyOf:
+        - $ref: '#/definitions/Material'
+        - $ref: '#/definitions/Composit'
+        - type: string
+        - type: 'null'
+      roughness:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+      thickness:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+    title: Layer
+    type: object
+  Material:
+    properties:
+      comment:
+        type:
+        - string
+        - 'null'
+      formula:
+        type:
+        - string
+        - 'null'
+      magnetic_moment:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+      mass_density:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+      number_density:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+      relative_density:
+        type:
+        - number
+        - 'null'
+      sld:
+        anyOf:
+        - type: number
+        - $ref: '#/definitions/ComplexValue'
+        - $ref: '#/definitions/Value'
+        - type: 'null'
+    title: Material
+    type: object
   Measurement:
     properties:
       additional_files:
@@ -295,6 +378,47 @@ definitions:
     - data_files
     title: Measurement
     type: object
+  ModelParameters:
+    properties:
+      comment:
+        type:
+        - string
+        - 'null'
+      length_unit:
+        default: nm
+        type:
+        - string
+        - 'null'
+      magnetic_moment_unit:
+        default: muB
+        type:
+        - string
+        - 'null'
+      mass_density_unit:
+        default: g/cm^3
+        type:
+        - string
+        - 'null'
+      number_density_unit:
+        default: 1/nm^3
+        type:
+        - string
+        - 'null'
+      roughness:
+        allOf:
+        - $ref: '#/definitions/Value'
+        default:
+          comment: null
+          error: null
+          magnitude: 0.3
+          unit: nm
+      sld_unit:
+        default: 1/angstrom^2
+        type:
+        - string
+        - 'null'
+    title: ModelParameters
+    type: object
   Person:
     properties:
       affiliation:
@@ -483,6 +607,9 @@ definitions:
         type:
         - array
         - 'null'
+      model:
+        anyOf:
+        - $ref: '#/definitions/SampleModel'
       name:
         type:
         - string
@@ -505,6 +632,55 @@ definitions:
     - name
     title: Sample
     type: object
+  SampleModel:
+    properties:
+      comment:
+        type:
+        - string
+        - 'null'
+      composits:
+        additionalProperties:
+          $ref: '#/definitions/Composit'
+        type:
+        - object
+        - 'null'
+      globals:
+        anyOf:
+        - $ref: '#/definitions/ModelParameters'
+      layers:
+        additionalProperties:
+          $ref: '#/definitions/Layer'
+        type:
+        - object
+        - 'null'
+      materials:
+        additionalProperties:
+          $ref: '#/definitions/Material'
+        type:
+        - object
+        - 'null'
+      origin:
+        type:
+        - string
+        - 'null'
+      reference:
+        type:
+        - string
+        - 'null'
+      stack:
+        type:
+        - string
+        - 'null'
+      sub_stacks:
+        additionalProperties:
+          $ref: '#/definitions/SubStack'
+        type:
+        - object
+        - 'null'
+    required:
+    - stack
+    title: SampleModel
+    type: object
   Software:
     properties:
       comment:
@@ -527,6 +703,42 @@ definitions:
     - name
     title: Software
     type: object
+  SubStack:
+    properties:
+      arguments:
+        items: {}
+        type:
+        - array
+        - 'null'
+      comment:
+        type:
+        - string
+        - 'null'
+      keywords:
+        type:
+        - object
+        - 'null'
+      repetitions:
+        default: 1
+        type:
+        - integer
+        - 'null'
+      represents:
+        type:
+        - string
+        - 'null'
+      sequence:
+        items:
+          $ref: '#/definitions/Layer'
+        type:
+        - array
+        - 'null'
+      stack:
+        type:
+        - string
+        - 'null'
+    title: SubStack
+    type: object
   Value:
     properties:
       comment:
diff --git a/tests/test_fileio/test_model_language.py b/tests/test_fileio/test_model_language.py
index 186fcf6f..57d657f4 100644
--- a/tests/test_fileio/test_model_language.py
+++ b/tests/test_fileio/test_model_language.py
@@ -358,7 +358,7 @@ def test_resolve_stack(self):
         stack = sm.resolve_stack()
         assert len(stack) == 1
         assert stack[0] == ml.Layer(
-            ml.Value(0.0, "nm"), roughness=ml.ModelParameters.roughness, material=ml.Material(formula="Si")
+            ml.Value(0.0, "nm"), roughness=defaults.roughness, material=ml.Material(formula="Si")
         )
 
     def test_resolve_to_layers(self):