Simple but effective: strong efficiency boost for a linear variable filter-based spectrometer

Aliaksei Kobylinskiy,Hartmut Hillmer,Matthias Kraus,Xavier Uwurukundo,Robert Brunner

doi:10.1117/1.jom.1.4.044501

Abstract

A method to drastically enhance detection efficiency of a linear variable filter (LVF) sensor across an extended and continuous wavelength range is presented. The efficiency is increased by a wavelength preselection concept, where the incoming light is divided into partial spectra to reduce otherwise unavoidable reflection losses of filter-based spectrometers. The simple but effective setup uses selected and successively arranged dichroic beamsplitters, which ensures an optimized compromise between efficiency enhancement and minimum increasing complexity. When connected to a two-dimensional camera and combined with a tilted LVF, this compact optical system allows the continuous recording of the full wavelength range between 450 and 850 nm with a resolution of ∼19 nm at 508.6 nm. An efficiency enhancement factor of up to 5.7 is achieved in comparison to a conventional LVF setup. The working principle was verified by measuring the reflection spectra of different natural and artificial green leaves. The proposed approach for increasing the efficiency can be miniaturized and applied to a broad range of other filter-based sensors.

Highlights

Spectroscopic analysis is no longer confined to science and laboratories
Filter-based spectrometers can have an extremely compact and rigid design without any moving parts, consisting of only a filter array or longitudinally variable filter placed in front of a sensor array, for example, a chargecoupled device (CCD) or complementary metal–oxide–semiconductor, to capture a spectrum in one shot
The wavelength preselection principle was applied to create an efficiency-enhanced module employing an linear variable filter (LVF) as the spectral-resolving element

Summary

Introduction

Spectroscopic analysis is no longer confined to science and laboratories. There is a growing demand for spectroscopic instruments with very ambitious instrumental requirements to work in extended application fields: process control for manufacturing industries, such as food[1] and pharmaceutical[2] industries, and screening a broad range of factors for agriculture and the environment,[3] to name only a few. Among the several advantages of filter-based spectrometers, a fundamental disadvantage is its low detection efficiency This limited efficiency can be attributed to most of the broadband incident light passing through narrowband filters (or small regions of LVFs) being reflected or absorbed. One approach to improve the efficiency of such filter-based spectrometers utilizes a spectral preselection method, in which the incoming light to be analyzed is divided spectrally and spatially into partial ray bundles and delivered to appropriate filter areas, which only cover a limited spectral range. There are various methods to accomplish a spectral preselection, e.g., using Köster prisms or micro prisms Another simple approach uses multiple dichroic long-pass beamsplitters successively arranged and aligned to the filter arrays for the respective spectral subbands. The transmission and reflection data of the beamsplitters and mirrors were used to estimate the module losses or approximate the increase in efficiency achieved

Working Principle

Mechanical Layout

Experimental Verification

Achieved Efficiency Increase and Spectra Simulations

Findings

Conclusions