# Analysis of a Fiber-Coupled RGB Color Sensor for Luminous Flux Measurement of LEDs

## TL;DR

This paper explores using a compact RGB sensor system to accurately measure LED light output, offering a portable and cost-effective alternative to traditional methods.

## Contribution

The work introduces a fiber-coupled RGB photodiode system for traceable luminous flux estimation through detailed optical modeling and calibration.

## Key findings

- A fiber-coupled RGB photodiode system can estimate luminous flux with scientifically meaningful accuracy.
- Optical effects like Fresnel losses and fiber alignment are quantified and incorporated into uncertainty budgets.
- Calibration using SPD and responsivity functions enables practical flux estimation for white LEDs.

## Abstract

Accurate measurement of luminous flux from solid-state light sources typically requires spectroradiometric equipment or integrating spheres. This work investigates a compact alternative based on a fiber-coupled RGB photodiode system and develops the optical, spectral, and geometric foundations required to obtain traceable flux estimates from reduced-channel measurements. The system under study comprises an LED with known spectral power distribution (SPD), optical head, optical fiber, a protective sensor window, and a photodiode matrix type sensor. A complete end-to-end analysis of the optical path is presented, including geometric coupling efficiency, fiber transmission and angular redistribution, Fresnel losses in the sensor window, and the mosaic structure of the sensor. Additional effects such as fiber–sensor alignment, fiber-facet tilt, air gaps, and LED placement tolerances are quantified and incorporated into a formal uncertainty budget. Using the manufacturer-supplied SPD of the reference LED together with the measured R, G, and B channel responsivity functions of the sensor, a calibration-based mapping is established to reconstruct photopic luminous flux from the three-channel outputs. These results demonstrate that, with appropriate modeling and calibration of all optical stages, a fiber-coupled RGB photodiode mosaic can provide practical and scientifically meaningful luminous-flux estimation for white LEDs, offering a portable and cost-effective alternative to conventional photometric instrumentation in mid-accuracy applications. Further optimization of computation speed can enable fully integrated measurement systems in resource-constrained environments.