Rhea configurable spectroradiometers

The Rhea series is a modular spectroradiometer and its diversity is almost unlimited. Admesy can configure the Rhea with almost any grating or slit and deliver it with calibration as spectroradiometer or without as spectrometer. A large amount of standard gratings and slits are available. Next to that additional detectors will be available in the near future. Please consult our distributor network or sales engineers for your specific configuration. For OEM customers we additionally offer customized electrical, mechanical and firmware designs as well as system integration support.

Rhea optical bench

Rhea at a glance

The Rhea spectroradiometer has a Czerny-Turner configuration. Its optical bench consists of a slit, a mirror, a grating, a second mirror, a collector lens, a linear variable filter [OSF] and a cooled detector. In front of the optical bench a filter wheel with ND filters is located. Additional components [fibre, lens] are used to improve total system performance.

System response & detector

The system response is mainly determined by the grating and the detector response. In the Rhea specification you can see the system response for each grating. In the system response we have included the detector, grating and the mirrors. This data is based on simulation data and is only meant for realistic indication purposes. The Rhea uses cooled CCD detectors from Hamamatsu. Currently the Rhea supports detectors for the range of 200-1100nm. Quantum efficiency of the detector is illustrated in the graph below.

Sensor response

Slit size

The slit size defines the amount of light entering the optical bench and influence the FWHM. Admesy’s OEM spectrometer engines can be configured with various slit sizes. The table below shows the approximated FWHM values per slit size and grating type.

FWHM [nm] S7031-1006 detector [1024 pixels]
Grating Grooves/mm 100um 50um 25um 10um
200 3.7 2.7 2.4 2.3
300 2.6 1.8 1.6 1.5
500 1.5 1.1 0.95 0.85
600 1.3 0.9 0.8 0.7
900 0.90-0.65 0.60-0.45 0.55-0.40 0.45-0.36
1200 0.68-0.40 0.45-0.27 0.40-0.24 0.32-0.19
1600 0.45-0.14 0.30-0.10 0.26-0.08 0.21-0.06
1800 0.41-0.12 0.27-0.08 0.24-0.07 0.19-0.05


The grating disperses the light into individual wavelengths. The amount of dispersion is determined by the number of grooves. The blaze wavelength determines the optimal efficiency at a certain wavelength. Admesy keeps on extending its stock of gratings. If a grating is not on the list or you require something different please contact our distributor or sales engineers to see what the possibilities are.

Dispersion range

The higher the number of grooves the wider the dispersion. However, this also limits the range of wavelengths which can be resolved because the detector has a fixed width. For a broad wavelength range low groove gratings can be used and for a detailed analysis of a small wavelength range high grooves gratings can be used. This range is defined as the dispersion range. The number of grooves also has influence on the FWHM.

Filter wheel

The Rhea contains a filter wheel with 4 ND filters (OD1, OD2, OD3 and OD4) to enable a huge dynamic range. The fifth and sixth position on the filter wheel are for fully open and close. Each ND filter is fully calibrated so no spectral difference is visible between ND filter wheel positions.

Filter wheel

Start & stop wavelength

Start and stop wavelength are depending on the grating choice and alignment. For example, a grating with a dispersion range of 200nm can be used from 300nm to 500nm as well as from 400nm to 600nm. In order to keep every system the same we have also implemented a start and stop wavelength. These start and stop wavelengths are defined by an additional configuration order code which, combined with the Rhea order code, ensures that the Rhea is configured with the exact required wavelength range.

System configuration

The table below describes each grating with its corresponding range. The Code is specified as the grooves per mm and Blaze wavelength. Both are divided by 10 in the code. Additionally the type of grating can be chosen. H stands for holographic and R for ruled. Currently Admesy supports only holographic gratings. Note 1: Combine this data with the system response graphs to check whether your required range is feasible with respect to the total spectral system efficiency. Note 2: The dispersion range includes a safety margin. More detailed information can be found in the Rhea specification.

Type Grooves/mm Blaze WL [nm] Dispers. range [nm] Used range
020030H Hologr. 200 300 900 UV-NIR
030040H Hologr. 300 400 600 UV-NIR
030060H Hologr. 300 600 600 VIS-NIR
030100H Hologr. 300 1000 600 VIS-NIR
050030H Hologr. 500 300 350 UV-NIR
060030H Hologr. 600 300 300 – 280 UV-NIR
060060H Hologr. 600 600 300 – 280 VIS-NIR
060100H Hologr. 600 1000 300 – 280 VIS-NIR
090030H Hologr. 900 300 200 – 150 UV-NIR
090060H Hologr. 900 600 200 – 150 VIS-NIR
090080H Hologr. 900 800 200 – 150 VIS-NIR
120020H Hologr. 1200 200 150 – 90 UV-NIR
120040H Hologr. 1200 400 150 – 90 UV-NIR
120080H Hologr. 1200 800 150 – 90 VIS-NIR
160020H Hologr. 1600 200 100 – 30 UV-NIR
160060H Hologr. 1600 600 100 – 30 VIS-NIR
180040H Hologr. 1800 400 90 – 25 UV-NIR