The radiometric calibration target allows the user to calibrate and correct the images' reflectance, taking into consideration the illumination and some of the characteristics of the sensor. It is recommended to use such a target when generating index maps. To see how to use the information of the reflectance target within Pix4Dmapper: 203891879.
- What is a radiometric calibration target?
- Should the radiometric calibration image be taken from the ground or while flying?
- Should the radiometric calibration image be taken before or after flying?
- How to convert the spectroradiometer information to albedo values?
- Should the drawing of the radiometric target's region be inside the gray target, or should it cover the entire box with the QR code?
- Using the calibration target with Sequoia
- How to calibrate your images using the calibration target?
Practically, the radiometric calibration target is a white balance card.
The problem with white balance cards used in photography is that, normally, the manufacturer provides the albedo values for Red, Green, Blue. An 18% Gray Card, for example, is known to have albedo values of 0.18 for the visible range, i.e. Red = 0.18, Green = 0.18, Blue =0.18. However, an 18% Gray Card does not necessarily have albedo values of 0.18 for other bands, including Near Infrared or Red Edge.
When radiometrically calibrating a camera that captures different bands, it is required to know the albedo of the material at each band that is captured by the camera.
The radiometric calibration target should cover enough pixels to get good statistics. If a target is big enough to be properly visible from the air, then the image can be taken from the air. If this is not the case, it is recommended to take the image from the ground.
The radiometric calibration image can be taken before, during or after flying:
- Before or after flying: It does not matter if it is taken before or after flying. It is important that the weather and lighting conditions are as similar as possible to the ones during the flight.
- During the flight: The target should be big enough to be properly visible and cover many pixels.
The spectroradiometer gives the spectral power distribution of a source. Having this information, the spectral reflectance can be calculated: https://en.wikipedia.org/wiki/Reflectance
The albedo values are calculated based on the spectral reflectance. The albedo values take into account the spectral reflectance, the sensitivity of the camera, and the incoming light.
Should the drawing of the region of the target be inside the gray target, or the entire box with QR code?
When drawing the region of the target after importing the image containing the radiometric calibration target into Pix4Dmapper, the region should be well inside the gray target and exclude all borders.
If a calibration target is used and is not visible from the sky (handheld target), a picture of it should be taken shortly before or after the flight under conditions similar to those during the flight.
- The target should be fully visible and should not have any shade projected on it.
- The target should be level with the ground, and not at an angle.
- The calibration region should not be over/under exposed in any of the bands. It sometimes helps to have the target surrounded by reflective surfaces while taking the calibration target to minimize the likelihood of overexposure.
In the case of the handheld Airinov target, the picture of the target is automatically recognized by the software: 211984703.
In order to calibrate your images and thus take into account the information from a picture with radiometric calibration target:
1. On the menu bar click Process > Processing Options..., the pop-up Processing Options appears.
2. Click 3. DSM, Orthomosaic and select the tab Index > Index Calculator
3. In the section Radiometric Processing and Calibration, the different available discrete bands are displayed. Under each band, click Calibrate...
3.1 Click Browse... to import the corresponding image of the calibration target for the specific band you have selected.
3.2 Draw a region on the image that will define the radiometric calibration area, the region should be well inside the gray target and exclude all borders.
3.3 Insert the Reflectance Factor value for the specific band that you have selected. The Reflectance Factor value should be the one furnished by the target manufacturer.
3.4 Click OK and continue for each band with the same procedure.
4. Once you have repeated the same procedure for all the discrete bands, done click OK and run again Step 3. DSM, Orthomosaic and Index