Confusion about the reflectance calibration of DJI P4 multispectral images

[Leonardo]

Hi, friends,

Is there anyone who is familar with the reflectance calibration of DJI P4M images? I attempted to calibrate the original DN value (16-bit, unitless) into spectral radiance (unit: W/m2/sr) and reflectance using either spectralon panels or the stocked sun sensor. However, a series of problems obstacles my step. After the review of DJI P4M image processing guide and metashape processing tutorial, I wonder:

(1) Did the metashape follow the equation 4 showed in P4M guide to calculate reflectance in case of sun sensor? https://dl.djicdn.com/downloads/p4-multispectral/20200717/P4_Multispectral_Image_Processing_Guide_EN.pdf If yes, how to know the value of ρ_NIR? I didn't find the corresponding explanation in Exif XMP metadata.
(2) Is there any reported evidence supporting the radiometric accuracy of DJI P4M? wihch calibration is more reliable? sun sensor or spectralon panel.
(3) Did metashape correct the vignetting effect as the DJI Exif-XMP has provided the polynomial correction coefficients? I guess this function must be involved in metashape but I didn't find it.
(4) Could I export the radiometrically calibrated images rather than orthomosaic from metashape after reflectance calibration? How to do that?

Thanks in advance.
Linyuan

[nprokofyev]

Hi Leonardo,
I haven't got P4M but I am very interested in its calibration procedure because our customers keep asking us about P4M.

(1) I've read the guide that you'd attached and it seems like ρNIR = 𝑝𝐶𝑎𝑚NIR/𝑝𝐿𝑆NIR but... they don't provide 𝑝𝐿𝑆NIR, they only provide it multiplied to NIRLS in [Irradiance]
I inspected metadata from P4M and didn't find anything that looks like  either  ρNIR or  𝑝𝐿𝑆NIR. I've used quite old data (dec 2019), maybe it is added in later firmware versions. If you have P4M images with recent firmware, would you please share it so I could look at their metadata?

BTW, I beleive there is (or was) no way to convert P4M images to spectral irradiance in SI units because DJI do not provide corresponding convertion constants. There is a tag XMP.Camera.RadiometricCalibration which contain such constants in case of Micasense RedEdge camera. Unfortunately, DJI simply stores additional copy of XMP.dji-drone.SensorGainAdjustment which couldn't be used in "Micasense-style" radiometric calibration  with SI units in output.

(2)  In case of Micasense and clear sky or equally overcast sky panels perform great. Moreover, if you use reflectance panel, you don't need to worry about intermediate step with spectral irradiance, because panel calibration is something like inverted opertaion to sensor response calibration plus additional correction for incident light.  Do you use Spectralon with ~99% reflectance? Most cameras will overexposure such targets, something less reflective (~50%) would be fine.

(3) Yes, Metashape corrects vignetting. You'll see corresponding information in console right after adding photos. And you may also open Tools - Camera Calibration, then right click on the camera and open Distortion plots. Vignetting plots are also there.

(4) Yes, File - Export - Convert Images  and select required checkboxes.

[uarsteam]

Is anyone else having trouble understanding the reflectance observed for NIR band images. There appears to be a significant brightness artifact located somewhat centrally to the image with a considerable drop-off as you approach the periphery of the image. It sort of follows the shape of a vignette process, however the brightness doesn't appear to be systematic from image to image. I've received no assistance from the manufacturer on the topic. Any thoughts whatsoever are greatly appreciated. Compressed JPG (not orig TIF) attached for your convenience.

UARS