The Multi-Viewing, Multi-Channel, Multi-Polarisation Imager (3MI) is an imaging radiometer for the ESA/Eumetsat MetOp-SG programme. Based on the heritage of POLDER/PARASOL, 3MI will collect global observations of the top-of-atmosphere polarised bi-directional reflectance distribution function in 12 spectral bands, by observing the same target from multiple views using a push-broom scanning concept. In order to mitigate any technological risks associated with the 3MI instrument development, an Elegant Breadboard of representative form, function and performance to the 3MI VNIR lens was foreseen in the frame of the Optics Pre- Development (OPD) activity. The optical design and the performance results of the OPD VNIR lens are presented, from the top level requirements flow-down to the optical design solution and concept adopted. The large FOV and image irradiance uniformity, the extended VNIR spectral range, combined with the demanding polarisation and stray-light requirements are the main design drivers. The design concept is based on a Galilean telescope coupled to a focusing group. The aperture stop, placed in between, is located in such a way that the system is telecentric in image space. The system exhibits a fine control of the entrance pupil size as a function of the FOV, low distortion and correction of lateral chromatic aberration. Polarisation related performances are achieved by low polarisation sensitivity and low retardance anti-reflection coatings, as well as by a proper selection of glass material properties.
The Multi-Viewing, Multi-Channel, Multi-Polarisation Imager (3MI) is an imaging radiometer for the ESA/Eumetsat MeteOp-SG programme. Based on the heritage of the POLDER/PARASOL instrument, 3MI is designed to collect global observations of the top-of-atmosphere polarised bi-directional reflectance distribution function in 12 spectral bands, by observing the same target from multiple views using a pushbroom scanning concept. <p> </p>The demanding challenge of the 3MI optical design is represented by the polarisation and image irradiance fall-off (throughput uniformity) requirements. In a generic optical system, the image irradiance fall-off is a function of: target radiance distribution and polarisation, entrance pupil size and optical transmittance variations across the field of view (FOV), distortion and vignetting. In most applications these aspects can be considered as independent; however, when high image irradiance uniformity is required, they have to be considered as linked together. This is particularly true in case of a wide FOV polarimeter as 3MI is.<p> </p> In order to properly account for these aspects, an irradiance fall-off analytical model has been developed in the frame of 3MI Optics Pre-Development (OPD), whose aim is to mitigate any technological risks associated with the 3MI instrument development. It is shown how it is possible to control the image irradiance distribution acting on optical design parameters (e.g. distortion and entrance pupil size variation with FOV). Moreover, the impact of polarisation performances on irradiance fall-off is discussed.