first non compiling draft

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Lighting/HDLightEditor.Styles.cs

             public readonly GUIContent shapeWidthLine = new GUIContent("Length", "Length of the line light");
             public readonly GUIContent shapeWidthRect = new GUIContent("Size X", "SizeX of the rectangle light");
             public readonly GUIContent shapeHeightRect = new GUIContent("Size Y", "SizeY of the rectangle light");
+            public readonly GUIContent intensityPhysicalUnit = new GUIContent("intensity (in Lumens)", "Luminous power of the light in lumens, typical value is 600 for a light bulb - 1200 for a strong light bulb");
             public readonly GUIContent aspectRatioPyramid = new GUIContent("Aspect ratio", "");
             public readonly GUIContent shapeWidthBox = new GUIContent("Size X", "");
             public readonly GUIContent shapeHeightBox = new GUIContent("Size Y", "");

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Lighting/HDLightEditor.cs

         void DrawLightSettings()
         {
             settings.DrawColor();
-            settings.DrawIntensity();
+            // Customize the 
+            EditorGUI.BeginChangeCheck();
+            EditorGUILayout.PropertyField(settings.intensity, s_Styles.intensityPhysicalUnit);
+            if (EditorGUI.EndChangeCheck())
+            {
+                //settings.intensity.floatValue = 
+            }
             settings.DrawBounceIntensity();
             settings.DrawLightmapping();
 

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/LightUtils.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace UnityEngine.Experimental.Rendering.HDPipeline
+{
+    public class LightUtils
+    {
+        // Physical light unit helper
+        // All light unit are in lumens (Luminous power) and convert to cd/(m^2*steradian) (luminance)
+        // For an area light it is (Luminous Power / (Area * PI * steradian))
+        // In the case of a punctual light, there is no area, so theoretically it is Candela but math are the same
+
+        // Note:  area light calculation is done in Luminance, but punctual light calculation should be done in luminous intensity (Candela)
+        // Math are identical, so no need to make a distinction here, the conversion handle it.
+        // No change to shader code is required
+
+        public static float ConvertPointLightIntensity(float intensity)
+        {
+            return intensity / (4.0f * Mathf.PI);
+        }
+
+        // angle is the full angle, not the half angle in radiant
+        public static float ConvertSpotLightIntensity(float intensity, float angle, bool exact)
+        {
+            return exact ? intensity / (2.0f * (1.0f - Mathf.Cos(angle / 2.0f)) * Mathf.PI) : intensity / Mathf.PI;
+        }
+
+        // angleA and angleB are the full opening angle, not half angle
+        public static float ConvertFrustrumLightIntensity(float intensity, float angleA, float angleB)
+        {
+            return intensity / (4.0f * Mathf.Asin(Mathf.Sin(angleA / 2.0f) * Mathf.Sin(angleB / 2.0f)));
+        }
+
+        public static float ConvertSphereLightIntensity(float intensity, float sphereRadius)
+        {
+            return intensity / (4.0f * Mathf.PI * sphereRadius * sphereRadius * Mathf.PI * Mathf.PI);
+        }
+
+        public static float ConvertDiscLightIntensity(float intensity, float discRadius)
+        {
+            return intensity / (discRadius * discRadius * Mathf.PI * Mathf.PI);
+        }
+
+        public static float ConvertRectLightIntensity(float intensity, float width, float height)
+        {
+            return intensity / (width * height * Mathf.PI);
+        }
+
+        public static float calculateLineLightArea(float intensity, float lineRadius, float lineWidth)
+        {
+            return intensity / (2.0f * Mathf.PI * lineRadius * lineWidth * Mathf.PI);
+        }
+    }
+}