A reflectance panel has known spectral properties and can be used to convert imagery to reflectance values. The process of converting imagery into reflectance values allows for time series comparisons of imagery because it normalizes the data for different lighting conditions.

Before Starting

Instructions

1. Power on the drone and remote controller.

2. Place the reflectance panel in an open and unobstructed area.

3. Connect your iPad to the remote controller and open the DJI GS Pro app on the iPad.

4. In the DJI GS Pro app Press the "My Missions" button.

1. Press the drone icon

1. Press the camera view.

1. Press the three line icon to open the image settings panel. Ensure that the camera is set to Auto.

1. Pick up aircraft and hold at waist height over calibration panel.

   • Ensure aircraft is not shadowing the reflectance panel.

2. Press the image capture button on the remote controller or in the DJI GS Pro app to take the calibration photo.

3. Set down the aircraft and pick up the panel.

Before Starting

Instructions

1. Power on the drone and remote controller.

2. Place the reflectance panel in an open and unobstructed area.

3. Open the DJI Pilot 2 App on the remote controller.

4. Press the Camera View Button.

5. Verify Settings and Change as needed.

   1. Set the focus to AFS (Auto Focus Single). The Drone may be set to MF or AFC mode. Tap the letters to switch between focus modes.

   2. Set the zoom to 1.0X
6. Pick up aircraft and hold at waist height over calibration panel.

   • Ensure aircraft is not shadowing the reflectance panel.

   • Ensure the light sensor is not shadowed by your body.
7. Press the image capture button on the remote controller to take the calibration photo.

8. Set down the aircraft and pick up the panel.

Calibrated Reflectance Panel

SKU: 21226-00

• Calibrated Reflectance Panel

• Protective Paper

Labels

The calibrated reflectance panel may have one of the following labels on it.

Care

Maintenance

Cleaning can be done with a photo brush, oil free air, or nitrogen.

Panel Types & Coefficients

Sentera offers two reflectance panels with different coefficients for each band. Check the cover of your panel to verify which coefficients you should use. You can also consult the image on the inside of your panel to determine which version you have, and use the following values.

See the 6X Series sensors user guide for instructions on how to use the calibrated reflectance panel with 6X.

Introduction

When using a reflectance panel, raw images should first be run through a Radiometric Corrections Workflow.

Click below to view the GitHub page for this tool.

Download Options

There are two options to setup and perform these corrections.

1. Running the stand alone ImageryCorrector.exe in the command prompt

2. Installing the scripts and libraries in python, then running commands in the environment you have created.

Stand Alone Executable Setup Instructions

Python Installation Instructions

git clone https://github.com/SenteraLLC/py-radiometric-corrections.git

cd py-radiometric-corrections/

conda env create -f environment.yml

conda activate imgcorrect-venv

pip install .

How To Use

Stand Alone Executable Usage

cd C:\Users\Mbohman\Downloads\ImageryCorrector

 ImageryCorrector.exe -h

 ImageryCorrector.exe "input_path" --output_path "output_path"

Python Usage

cd py-radiometric-corrections/ 

 conda activate imgcorrect-venv 

 python scripts\correct_images.py -h  

 python scripts\correct_images.py "input_path" --output_path "output_path" 

Arguments

input_path

Path to image files taken from supported sensors. Choose the session folder of your images. This will cause the script to correct the images in each subfolders for all 5 multispectral band and ignore the rgb folder.

--calibration_id "CALIBRATION_ID", -c CALIBRATION_ID

Identifier in the name of the image that denotes it is from the calibration set. If not specified, defaults to "CAL".

--output_path OUTPUT_PATH, -o OUTPUT_PATH

Path to output folder at which the corrected images will be stored. If not supplied, corrected images will be placed into the input directory.

--no_ils_correct, -i

If selected, Incident Light Sensor correction will not be applied to the images.

--no_reflectance_correct, -r

If selected, reflectance correction will not be applied to the images.

--delete_original, -d

Overwrite original 12-bit images with the corrected versions. If selected, corrected images are renamed to their original names. If not, an extension is added.

--exiftool_path EXIFTOOL_PATH, -e EXIFTOOL_PATH

Path to ExifTool executable. ExifTool is required for the conversion; if not passed, the script will use a bundled ExifTool executable.

--uint16_output, -u

If selected, scale of output values will be adjusted to 0-65535 and dtype will be changed to uint16.

Arugment Usage

[-h] [--calibration_id CALIBRATION_ID] [--output_path OUTPUT_PATH] [--no_ils_correct] [-no_reflectance_correct] [--delete_original] [--exiftool_path EXIFTOOL_PATH] [--uint16_output] [--version] input_path 

The correction is done in 3 steps:

1. Autoexposure correction.

2. Incident Light Sensor correction.

3. Reflectance correction.

Autoexposure Correction

Sensors simulate longer exposures and wider apertures for subjects with lower upwelling radiance. These scripts correct by dividing pixel values by EXIF ISOSpeedRatings * EXIF ExposureTime

Incident Light Sensor (ILS) Correction

Downwelling radiance may change over the course of a flight as clouds pass overhead. These scripts correct by dividing pixel values by a rolling average of Incident Light Sensor readings (Camera:SunSensor) on images taken within 3 seconds of the corrected image. Correcting for ILS will standardize the DN (digital number) of your images based on the differences in incident light.

Reflectance Correction

Sensors measure upwelling radiance, not reflectance. Radiance is dependent on unpredictable environmental conditions, and without a reference point with known reflectance, it is impossible to calculate. By beginning a flight with a photo of a panel with known reflectance, that panel's upwelling radiance can be used to standardize measurements.

These scripts correct by multiplying pixel values by slope coefficient (calculated based on known calibration panel reflectance values).