Measurement principle and arrangement for the determination of spectral channel-specific angle dependencies for multispectral resolving filter-on-chip CMOS cameras

Abstract

Filter-On-Chip CMOS sensor equipped cameras are a convenient, reliable and affordable approach for the parallel acquisition of spatial and spectral information. The combination of pixel-arranged spectral filter matrices on CMOS sensors increases their integration density and system complexity by several times compared to standard RGB cameras. Due to their system design, these cameras have an increased spectral crosstalk and specific dependencies from the angle of radiation. The paper will show how to develop and set up a measurement arrangement for the characterization of the channel specific spectral sensitivities under different angles of radiation. These characterizations are necessary to develop a more robust model for the camera pixel-value correction to ensure the comparability and reproducibility of the measured values. Therefore, a measurement setup to investigate the influence of the angle of incident radiation on filter-on-chip CMOS sensors was developed. After initial investigations with a setup in which the camera was simply rotated and investigations with a lens and changed f-number confirmed that the angle influence results in a measurable difference in the sensor response, a new measurement arrangement was developed to investigate this behavior more precisely. The developed measurement arrangement allows multispectral resolving image sensors to be radiated with collimated light at reproducible angles of incidence and with adjustable wavelengths. By comparing the measured values with illuminances measured using a calibrated photodiode in the same setup and with the same parameters, it is possible to evaluate the angle dependence based on quantum efficiency curves according to the EMVA 1288 standard. The investigations carried out, the developed principles and the realized semi-automatic measurement arrangement will be shown and explained to characterize the capabilities of multispectral resolving filter-on-chip CMOS sensor equipped cameras for applications in industry and biomedicine.

Multispectral Imaging Flow Cytometry with Spatially and Spectrally Resolving Snapshot-Mosaic Cameras for the Characterization and Classification of Bioparticles

“…Furthermore, a calibrated detector head RW-3705 for the measurement of irradiance in W/m 2 also from Gigahertz Optik GmbH have been used [27]. The methods to measure and interpret the characterization results are described in [28][29][30].…”

“…Prior to the experiments, an extended camera characterization was performed in accordance with [28][29][30]. This is necessary because the behavior of the used snapshotmosaic camera concerning the spectral sensitivity differs from the generalized provided manufacturers data.…”

Detailed characterization of a mosaic based hyperspectral snapshot imager

“…Since the filters applied on the sensor are interference filters whose characteristics strongly depend on the angle of incidence, the behavior under oblique light incidence was investigated. With the following formula, the wavelength at which constructive interference appears inside a Fabry-Pérot filter, can be calculated with the cavity length L, the angle of incidence θ, and the diffraction order m. 5,13 It is obvious that the wavelength λ is proportional to the cosine of θ. This relation is confirmed by the recorded hyperspectral data: Fig.…”

“…The angle of incidence of the illumination is adjustable. Dittrich et al 5 presented a system that allows collimated illumination at different angles of incidence with the help of a pinhole array. The illumination is tunable by the use of a monochromator.…”

Resonant waveguide vs Fabry-Perot cavity: A comparative study for CMOS spectral sensor technology

“…Among these, ambient light sensors (ALS), especially for operation in the far red + near-infrared ranges, are of interest in smartphone cameras, color correction, screen-brightness adjustment and intended surveillance and monitoring systems [6,7,8,9,10,11,12,13]. The embedded filters that come atop each large pixel of an ALS could be made by variable thickness Fabry-Perot, but this choice incurs too costly multiple deposition+masking processes for more than 3-5 channels in the CMOS context that is unavoidable to get low-cost mass production [14,15,16]. A more rational choice within this demand rests on the interaction of light with subwavelength structures (dielectric or metallic) [17,18,19,20].…”