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Need of a system capable of measuring up-welling radiance (from the target Earth surface) and down-welling (reference standard measuring solar irradiance on the target surface) simultaneously and across different spectral regions by using two different spectrometers: one for the spectral region where sun induced chlorophyll fluorescence estimation (SIF) is possible and one across the wider visible near-infrared region (VNIR).


Double bifurcated fibre optic assembly routing light alternately from two different sources to two different spectrometers simultaneously and these configured for different wavelength ranges (SIF approximately 640 nm to 800 nm) and VNIR (approximately 400 nm to 950 nm).

Alker contribution: double bifurcated assembly

Alker contribution: double bifurcated assembly
This assembly has two fibres side by side in each of its two input legs. Both input legs hold fibres of different types: f1 and f2 (f1f2, f1f2). In the junction/central block they cross over into two legs each containing side by side fibres of the same type, f1f1 and f2f2. The legs are armoured and water-resistant. A 4 core ribbon electrical cable in one of the output legs splits in 2 core ribbon cable in each one of the input legs. The output legs are keyed for alignment with spectrometer entrance slit(by special construction/design of the connector ferrule). Each input leg has a weatherproof fore optic housing an electronically controlled optical shutter.

The whole assembly is armoured and water-resistant with the actual connectors protected by an IP68 glands on all fibre leg ends.


  • 2×2 assembly;
  • Both aligned: connectors with keyway polarisation, and unaligned fibre legs;
  • SMA905 custom connectors specially design to sit both fibres, keyed for alignment;
  • 400um and 600um core fibre;
  • Armoured;
  • Water-resistant;
  • IP68 gland;
  • 4 Ribbon cable in one input leg that splits in 2ribbon cable in each output leg;
  • Lightweight design for use on in unmanned aircraft (UAV), less than 80g per assembly.

Custom Versions Available

  • Variations on input and output legs: 2×1, 3×2, etc;
  • Custom fibre: The fibre used in each leg can be the same or different, based on the optimisation of fibre core or wavelength range or both;
  • Custom connectors with ferrules able to sit a different number of fibres;

Problem explained in detail

Measurements of sun-induced chlorophyll fluorescence (SIF) have the potential for the assessment of actual plant photosynthesis from space.
However, corresponding near-ground measurements are required to validate these SIF estimates since the correction for atmospheric effects plays a significant role in SIF estimation from air and spaceborne measurements.
Very high spectral resolution spectrometers are currently available that measure across the spectral region containing the O2-A and O2-B bands, where SIF estimation is possible, and other spectrometers are available for measuring across the wider visible-near infrared region, used to determine indices such as the Photochemical Reflectance Index, an alternative indicator of photosynthetic activity.
However, the very high spectral resolution systems used to date often have weak signal to noise and low sensitivity, requiring measurements to be made at different integration times for each O2 band.
Besides, current methods either rely on sequential measurements of a reference standard (to estimate solar irradiance) and the target Earth surface, both using a restricted field-of-view (FOV) foreoptic, or sequential measurements of upwelling radiance and downwelling irradiance using a cosine corrected foreoptic with a hemispherical FOV.
With both these approaches, there are time delays (at best multiple seconds) between measuring down-welling and upwelling radiances. These delays add to measurement uncertainties due to changes in sky conditions between the individual measurements, particularly in northern Europe.
Furthermore, the current systems, while possibly suitable for fixed location deployment and high-frequency temporal measurements, are not designed, and would be impractical to use, to capture the spatial variability necessary to spectrally characterise the footprint of carbon flux observation systems.

How does the product work?

This instrument is a dual-field-of-view system able to incorporate multiple spectrometers covering different spectral ranges, with a cosine-corrected foreoptic to capture down-welling irradiance and the up-welling channel configured with a view angle-limited foreoptic. [1]
A double bifurcated fibre optic is used to transfer light from the foreoptic to the spectrometers, therefore, each spectrometer receives light from the same Earth surface area (each measurement has the same support).[1]
This assembly has two fibre optic cables, in each of its input legs (one leg for up-welling light and the other for down-welling light), and these cross over at the central block so that one up-welling and one down-welling fibre go to each spectrometer.[2]
As there is an electro-mechanical shutter in each fore optic, one can be closed to exclude light entering the system while the other is open. Hence, by switching between the two shutters, down-welling and up-welling light can be recorded sequentially.[2]

[1]A. Mac Arthur, A.; Robinson, I.; Rossini, M.; Davis, N.; MacDonald, K. A dual-field-of-view spectrometer system for reflectance and fluorescence measurements (Piccolo Doppio) and correction of etaloning. In Proceedings of the 5th International Workshop on Remote Sensing of Vegetation Fluorescence, Paris, France, 22–24 April 2014.
[2] Mihai, L., Mac Arthur, A., Hueni, A., Robinson, I. and Sporea, D. (2018). Optimized Spectrometers Characterization Procedure for Near Ground Support of ESA FLEX Observations: Part 1 Spectral Calibration and Characterisation. Remote Sensing, 10(2), p.289.

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