1.

INTRODUCTION

The importance of observations in the Mid Wave InfraRed (MWIR) spectral region, from agriculture to meteorological and climatic studies or for monitoring environmental risks such as fires and volcanic eruptions, has been highlighted by the scientific community. In particular, multispectral observations gain relevant importance when done at high spatial resolution [1]. For example, high temperature events like volcanic eruptions or wild fires are better characterized when observed at high spatial resolution [2 - 4]. The possibility to obtain multispectral, high spatial resolution observation in the MWIR range was investigated in the frame work of SISSI project funded by the Italian Space Agency. During the SISSI project, an innovative concept of optical payload, operating in the MWIR, based on two emerging technological approaches - super-resolution and Compressive Sensing (CS) - was studied to obtain particularly interesting performances in terms of spatial resolution on the ground and mitigation of some effects, such as saturation and blooming. Both approaches are based on the use of a Spatial Light Modulator (SLM), an optoelectronic device consisting of an array of electronically actuated micro-mirrors (MMA - MicroMirror Array).

The concept of super-resolution is based on the possibility of obtaining an image - with a larger number of pixels than the nominal ones provided by the chosen detector - thanks to the use of an SLM with a number of elements N times greater than the elements of the detector. The Compressive Sensing (CS) theory, on the other hand, exploits a priori knowledge on the sparsity characteristics of the image to acquire. Using CS approach fewer samples than those predicted by the Nyquist-Shannon theorem [5] are acquired and the image is (lossy) reconstructed. In general, it is always possible to find a domain in which the data to be acquired is sparse.

The operating principle of SISSI instrument is the one of the single pixel camera [6], in which a single element detector acquires, by multiple acquisitions, different modulations of the scene of interest, allowing the reconstruction of the original scene. Such modulation is usually performed via binary spatial masking. The use of CS architecture [7, 8] allows the scene reconstruction without loss of significant information by using a reduced number of acquisitions.

For SISSI, a bidimensional electronically actuated array of micro-mirrors (namely a Digital Micromirror Device (DMD)) was used to modulate the observed image using a binary on-off pattern. A sequence of such spatially-coded acquisitions is then integrated by a condenser lens and focused on the detector, each element of which acting, in parallel, as the single element detector of a single pixel camera. A contiguous ensemble of micromirrors subtends a so-called macropixel on the DMD plane, each macropixel corresponding to one detector element. Each micropixel subtends a portion of the scene corresponding, at ground, to the super-resolute ground sampling distance. Multispectral acquisition is obtained by applying a linear variable filter on the 2D detector, allowing the multiple coded acquisition of a generic group of across-track lines of macropixels through different spectral filters. The optical design of such imager required strict constraints on the optics involved. The collection optics, focusing on the DMD plane, must match with the condensing optics, coupling each macropixel on the DMD plane to the corresponding detector element. A major difficulty is added by the DMD working in reflection mode, making the optical design more complex due to the need of the impinging and reflected ray beams being separated and being collected by the condensing optics. A further requirement is given by the Airy disk size being comparable to the DMD micropixel. Such constraints lead to an upper limit for the optics f-number. A small f-number increases the collected number of photons but also the geometric aberrations of the optics.

In the following after a brief description of SISSI instrument operation mode, the optical design will be presented and the challenges faced during the design will be discussed.

2.

SISSI INSTRUMENT CONCEPT

The schematic of SISSI instrument operation mode is depicted in Figure 1. It is based on the CS architecture [7]: a single pixel detector collect the light modulated by a spatial light modulator (SLM) by mean of a lens acting as light condenser. Each acquisition is the product between the encoding binary pattern applied to the modulator and the corresponding element of the image focused on it and the condenser acts the integration on the single-pixel detector. The quality and the number of acquisitions needed for the reconstruction of the image depends on the sparsity of the observed image. CS merges acquisition and compression phases into a single step. It is worth to note that SISSI instrument performs a parallel acquisition of the images. Each detector pixel subtends an area of the scene on the ground corresponding to a macropixel on the DMD. Each DMD macropixel is made up of NxN micropixels, each of which corresponds to a micromirror of the DMD (Figure 1). Each DMD macropixel is made up of NxN micropixels – that corresponds to NxN micromirrors- and is encoded by a binary mask different for each frame. The final sampling of the reconstructed scene is increased by a NxN factor with respect to the native detector resolution, NxN being the super-resolution factor of the instrument.

Figure 1.

Super resolution for SISSI instrument: macro-pixel, and micro-pixel

Multispectral acquisition of the same scene is performed due to the application of spectral filters on several contiguous lines of the detector, arranged along the across-track dimension of the scene, and to the apparent movement of the scene in the direction along-track through the field of view (Figure 2). SISSI instrument will achieve a super-resolution factor equal to 4x4.

Figure 2.

Schematic of SISSI operation mode

The main characteristic of SISSI instruments are listed in Table 1.

Table 1.

SISSI payload main characteristics

SISSI payload was conceived as formed by an optical module (collection optics, DMD, condensing optics and detector) and the electronic section. For what concern CS algorithm [9], two different categories of reconstruction algorithms were taken into consideration: traditional and based [10] on deep learning [11]. In the following the activities related to the design of the optical module are described and the obtained results discussed.

3.

OPTICAL DESIGN

The design of the optical module took into account four key elements:

In Figure 3 the schematic of the main functional block of the optical section are shown.

Figure 3.

Functional blocks of SISSI instrument optical section

In the following the optical design of SISSI instrument is presented paying particular attention to the design of collection optics and condensing optics.

3.1

DMD selection

One of the key elements during the design of SISSI payload was the choice of DMD, since its characteristics such as micromirrors dimension, or tilt angle (δ) influence the optical layout and the dimensioning of the other optical elements. For example, tilt angle δ determines the direction where the focusing optic is placed. If the optical system is designed so that collection and condensing optics have the same f-number, in order to avoid the superimposition of impinging and outgoing beams the minimum, f-number is equal to:

Due to the dimension of micromirrors, the choice of DMD have to take into consideration the effects of light diffraction on the performance of the optical system. On the basis of simulations (not reported here), the optimal choice for the SISSI instrument was the DMD produced by Texas Instrument DLP 7000 was selected because it gives the better performance in terms of diffraction efficiency.

3.5

SISSI optical CAD

The optical cad of SISSI instrument is depicted in Figure 7. The use of a folding mirror avoids the collision of the reflected rays on the mechanical support of the primary mirror. A second folding mirrors is used for extracting the detector from the focusing cone of the aspheric mirror. It is worth to note the non-planarity due to the presence of DMD which reflects the light along a direction that does not lie in the plane of the incident optical axis. The use of folding mirrors avoids, respectively, the collision of the rays reflected by DMD with the primary mirror of the collection optics and (for the condensing-optics-focused rays) with the CaF2 corrector plate.

Figure 4.

SISSI instrument 3D optical CAD.

The use of mirrors minimizes the chromatic aberration and maximize the overall transmittance of the optical system. For the lenses CaF2 was preferred, when possible, for maximizing the transmittance.

4.

CONCLUSIONS

The optical design of SISSI instrument satisfies the nominal requirements requested by its compressive sensing architecture, representing an example of a multispectral imager MWIR implementing super-resolution thanks to the use of a DMD.

The system can be ideally divided into a collection optics that focuses the scene on the DMD plane, and a condensing optics performing the spatial coding and integration on the detector.

During the design of SISSI instrument, the use of free-form optics was necessary for reducing the number of optical surfaces involved, thus maximizing the instrument transmissivity.

The optical elements envisaged during the design are in large part easy to manufacture, mount and align, with the exception of the collection optics lenses, having large diameter with respect to their thickness. Aspheric surfaces could lead to further difficulties in both feasibility and alignment.