# Advanced Exposure Calibration
### Why complete an advanced exposure calibration?
This process helps with maintaining colour accuracy and an even exposure.
### **Prerequisites**
Before starting, ensure you have:
* A **colour checker,** check compatible colour checkers [here](https://docs.m-xr.com/marso-measure/resources/capturing-a-calibration-image/grey-cards-or-known-values)
* Your **chosen lighting preset** configured
* Final **camera settings** locked in (shutter speed, ISO, aperture)
* A capture space with **textured surfaces and markers** for a clean photogrammetry solve
***
### **Notes & Best Practices**
* Always use **consistent lighting** between calibration and real captures
* Recreate the exposure solve whenever lighting presets or camera settings change
* Ensure the grey tile mesh is smooth and evenly reconstructed - clean the tile before capturing if there are any scuffs or marks on the grey tile you want to capture
* Use plenty of markers to guarantee scale accuracy
***
### **1. Capture your grey card**
The goal is to create a high-quality reconstruction of the grey tile under your chosen lighting conditions.
{% hint style="success" %}
Use a flat, detailed surface **on the ground** and surround the chart with **markers and measurements** for alignment accuracy.
{% endhint %}
### **1. Capture**
1. **Set the lighting and camera exposure**\
Use the exact lighting strength, flash mode, and exposure settings you intend to use in production captures.
2. **Top-down orthographic pass**\
Capture the colour checker in a grid pattern from above.
* Purpose: the grey value should be lit evenly from all frontal angles.
3. **Ring formation pass**\
Capture the colour checker in a circular sweep at multiple heights.
* Purpose: to provide data from different incidence angles for a more robust 3D solve.
#### Example capture
{% hint style="info" %}
You will see on the image below, we have placed the colour card on a pair of jeans with detail that helps align the mesh. You can use any object that has enough detail, avoid glossy or featureless floors.
{% endhint %}
.png?alt=media\&token=e74a0adc-f065-4157-afef-6ff8f1bab565)
***
### **2. Process the Images Through Marso Measure Develop**
Once all photos are captured:
1. Run the images through **Marso Measure Develop step**\
→ this converts the raw files into a format suitable for your photogrammetry software.
***
### **3. Create a Mesh of the Colour Checker**
Import your processed images into your photogrammetry software (Metashape or Reality Capture).
#### **Steps**
1. Solve alignment
2. Build the dense cloud and mesh
3. Use markers and scale bars for maximum accuracy
4. Ensure the colour checker surface is reconstructed cleanly
***
### **4. Isolate the Grey Tile**
The Advanced Exposure System requires a **mesh containing only the grey tile**.
#### **How to Isolate**
**Metashape:**
* Select the grey region using the bounding box
* Right-click the mesh → **Duplicate**
* **Clip by Region** to isolate the patch
**Reality Capture:**
* Use:\
`Scene 3D > Tools > Mesh Model > Cut by Box`
***
### **5. Export Required Files**
Export the following from your isolated grey tile:
| File Type | Purpose |
| ------------------ | ----------------------------------------- |
| **Alembic (.abc)** | 3D mesh of the grey square |
| **ST Map (EXR)** | Lens distortion map from solve |
| **PNG UV Mask** | Isolates the grey patch during processing |
***
### **6. Create the Advanced Exposure System in Marso Measure**
If you haven’t already, [create a **Light System** for your light](https://docs.m-xr.com/marso-measure/resources/creating-a-light-system)
Then, in Marso Measure, navigate to:
**Exposure Systems → New Exposure System → Toggle from “Basic” to “Advanced”**
***
### **7. Configure the Advanced Exposure System**
Fill in the following fields:
#### **Exposure System Name**
Use a clear naming scheme including camera model + exposure + light preset.\
e.g.,\
`A7R4_ISO100_F11_1/125_PtLight70%`
#### **Light System**
Select the light system previously created for your rig.
#### **Raw Calibration Image(s)**
Raw images used for the colour checker photoscan.
#### **Raw Image Extension**
#### **Colour Card Mesh File**
The Alembic mesh of the isolated grey tile.
#### **Reconstruction Tool**
Photogrammetry tool used:
* Metashape
* Reality Capture
#### **ST Map File**
EXR export from your grey square mesh.
#### **Colour Card UV Mask**
PNG mask defining the grey region.
#### **Grey Value**
Decimal place of the percentage of the grey value of your selected grey tile.\
Reference values can be found [here](https://docs.m-xr.com/marso-measure/resources/capturing-a-calibration-image/grey-cards-or-known-values).
***
### **8. Finalise the Exposure System**
Click **Create Exposure System**\
→ Marso will now use the 3D geometry and lighting information to accurately calculate grey values across your capture workflow.
