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 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.

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/mm100um50um25um10um


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.

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.

TypeGrooves/mmBlaze WL [nm]Dispers. range [nm]Used range
060030HHologr.600300300 – 280UV-NIR
060060HHologr.600600300 – 280VIS-NIR
060100HHologr.6001000300 – 280VIS-NIR
090030HHologr.900300200 – 150UV-NIR
090060HHologr.900600200 – 150VIS-NIR
090080HHologr.900800200 – 150VIS-NIR
120020HHologr.1200200150 – 90UV-NIR
120040HHologr.1200400150 – 90UV-NIR
120080HHologr.1200800150 – 90VIS-NIR
160020HHologr.1600200100 – 30UV-NIR
160060HHologr.1600600100 – 30VIS-NIR
180040HHologr.180040090 – 25UV-NIR