# A new method to determine multi-angular reflectance factor from lightweight multispectral cameras with sky sensor in a target-less workflow applicable to UAV

## TL;DR

This paper introduces a physically based, target-less method for estimating hemispheric-directional reflectance factors from lightweight multispectral UAV cameras with sky sensors, combining radiometry and photogrammetry for accurate results.

## Contribution

It presents a novel approach that derives geometrically and radiometrically accurate reflectance data without targets, applicable under various sky conditions, using only imagery and a sky sensor.

## Key findings

- Achieved RMSE of 3% under clear sky conditions.
- Achieved RMSE of up to 5% under overcast conditions.
- Successfully retrieved both value and directionality of reflectance factor.

## Abstract

A new physically based method to estimate hemispheric-directional reflectance factor (HDRF) from lightweight multispectral cameras that have a downwelling irradiance sensor is presented. It combines radiometry with photogrammetric computer vision to derive geometrically and radiometrically accurate data purely from the images, without requiring reflectance targets or any other additional information apart from the imagery. The sky sensor orientation is initially computed using photogrammetric computer vision and revised with a non-linear regression comprising radiometric and photogrammetry-derived information. It works for both clear sky and overcast conditions. A ground-based test acquisition of a Spectralon target observed from different viewing directions and with different sun positions using a typical multispectral sensor configuration for clear sky and overcast showed that both the overall value and the directionality of the reflectance factor as reported in the literature were well retrieved. An RMSE of 3% for clear sky and up to 5% for overcast sky was observed.