Design and Test of Portable Hyperspectral Imaging Spectrometer

We design and implement a portable hyperspectral imaging spectrometer, which has high spectral resolution, high spatial resolution, small volume, and lowweight.The flight test has been conducted, and the hyperspectral images are acquired successfully. To achieve high performance, small volume, and regular appearance, an improved Dyson structure is designed and used in the hyperspectral imaging spectrometer. The hyperspectral imaging spectrometer is suitable for the small platform such as CubeSat and UAV (unmanned aerial vehicle), and it is also convenient to use for hyperspectral imaging acquiring in the laboratory and the field.


Introduction
Hyperspectral imaging spectrometer can acquire hundreds of inhomogeneous spectral images.Compared to the other sensors, much more information could be excavated from the massive data.Owing to the characters above, the demand for the hyperspectral imaging spectrometer is put forward in many different tasks such as accurate mapping of wide areas, target detection, process monitoring and control, object identification and recognition, clinical diagnosis imaging, and environment assessment and management.After decades of development, the application areas of the hyperspectral imaging spectrometer have extended to ecology, geology, agriculture, medicine, military, security, oceanography, manufacturing, urban studies, and others [1][2][3][4].
With the development of machinery and electronics technology [5], the unmanned aerial vehicle (UAV) and the Cube-Sat have made great progress.Due to the miniaturization of platform, the small, compact, portable hyperspectral imaging spectrometer becomes a development direction [6][7][8][9].
In this paper, we design and implement a portable hyperspectral imaging spectrometer.Using the hyperspectral imaging spectrometer, we conduct the flight test experiment and acquire the hyperspectral image of the bared soil, roofs, green wheat, and so on.
The final implemented hyperspectral imaging spectrometer can provide an instantaneous FOV of 0.22 mrad in 6.57 degrees and a spectral sampling of 1.6 nm and covers the range of 450 to 850 nm.

Considerations and Design Specifications
The hyperspectral imaging spectrometer designed and implemented in the paper is used for acquiring experimental hyperspectral imaging data in the field and laboratory.Another application is used for remote sensing installed in the small platform such as UAV and CubeSat.
To be suited for the small platform and the convenience of the field experiment, the hyperspectral imaging spectrometer must have a small volume, a light weight, and a regular appearance.
Two forms of the instrument are considered, the whiskbroom sensor and the pushbroom sensor.The whiskbroom sensor can achieve the highest spectral and spatial uniformity.However, the whiskbroom sensor records the spectrum of every point on a single linear detector array.The pushbroom sensor disperses the image of a slit onto a two-dimensional array detector.It is clear that the efficiency of the pushbroom sensor is much higher than the whiskbroom sensor.Thus the pushbroom imaging spectrometers are becoming a preferred form for many remote and laboratory sensing applications.The typical pushbroom imaging spectrometer consists of a telescope, a slit, a dispersing spectrometer, and an array detector.
There are many methods to achieve the dispersing spectrometer of the pushbroom hyperspectral imaging spectrometer.For high performance and compact structure, the Dyson structure is utilized, as is shown in Figure 1.In 1959, Dyson first proposed that a simple concentric arrangement of a planoconvex lens and concave mirror would be free of all Seidel aberrations at the design wavelength and center of a field imaged at 1 : 1 magnification [10].Since then, many papers have advanced the concept to fully working systems [11].In recent years, lots of research institutions develop many compact spectrometer based on the Dyson structure [12][13][14][15][16][17].The advantageous features of this particular concentric configuration are as follows: (1) sharp imagery due to the inherent absence of Seidel aberrations; (2) high numerical aperture; (3) flat field; (4) wide unvignetted field having linear dispersion as a function of wavelength; (5) nonanamorphic field; (6) telecentric; (7) no central obscuration of the pupil; (8) no aspherical optical surfaces being required [18,19].
Based on the above considerations, we design a high resolution hyperspectral imaging spectrometer in both spectral dimension and spatial dimension.The specifications are presented in Table 1.

Optical Design
To prove the advantages of the system above, we design and implement the hyperspectral imaging spectrometer.

Telescope Design.
The telescope is designed using a refraction system.The specifications are shown in Table 2.
We use three kinds of glasses from the CDGM material catalogs of ZEMAX: CAF2, H-LAK2A, and TF3.The telescope is designed as Figure 2, the total length of the telescope is 95.9 mm, and the stop is at the first glass of the system whose diameter is 13.2 mm.The exit pupil position is −20000 mm from the image surface.
It can be seen that the MTF is higher than 0.75 at the Nyquist frequency (67.6 lp/mm) of the sensor from Figure 3.
The energy is included mostly in the pixel range from the spot diagram shown in Figure 4, and the RMS of the spot  diagram is 1.754 m.The distortion is less than 0.27% shown in Figure 5.

Spectrometer Design.
The Dyson spectrometer is compact and has high performances.But if we use the prototype, there is no space to install the array detector and the mechanical slit shown in Section 4. For the regular appearance of the system and to avoid the overlapping of the slit assembly and the detector, we design a new form of the Dyson spectrometer based on the prototype.First, we separate the object surface and the image surface of the spectrometer along the optical axis.A meniscus lens is added before the concave grating to correct the aberration brought by the separation at the same time, as is shown in Figure 6.
Second, we add a reflective surface to the Dyson block; the dispersing light is reflected to the bottom of the system and then received by the detector, as is shown in Figure 7.The slit and the detector are successfully separated to the two perpendicular surfaces.The Dyson block is shown in Figure 8.The material of the Dyson block and the meniscus lens is H-K9L in the CDGM glass catalog, which has the same parameters as the N-BK7 of SCHOTT.In the Dyson block, the thickness is 50.23 mm, radius is 53.46 mm, the distance between the left endpoint of the reflect surface and the axis is 1.4 mm, and the angle between the reflective surface and the axis is 45 ∘ .The radii of the meniscus lens are 337.3mm and 398.1 mm and the thickness of the meniscus lens is 6 mm.The concave grating is a holographic Rowland grating, whose groove density is 83 lines/mm and radius is 173.9 mm.The distance between the Dyson block and the meniscus lens is 105.45 mm, and the distance between the meniscus lens and the grating is 6 mm.

Simulation of the Hyperspectral Imaging Spectrometer.
The ray trace of the hyperspectral imaging spectrometer is shown in Figure 9.The total length of the system is 246.7 mm, and the largest diameter is 47.4 mm.
The matrix spot diagram shows that the spots diagram of all waves are less than the width of the slit (13.2 m) (Figure 10).
At the maximal field of the hyperspectral imaging spectrometer, the maximal distortion is 0.32% occurring in the 450 nm wavelength, and the minimal distortion is 0.22% occurring in the 850 nm wavelength.Thus the keystone of the system could be calculated as follows: 0.32% − 0.22% = 0.1% (Figure 11).The maximal smile of the hyperspectral imaging spectrometer occurs in the wavelength of 850 nm, which could be got from Figure 12: 6.473 mm − 6.468 mm = 0.005 mm.

Slit Assembly
The slit assembly is a critical element of the overall design.It could be accomplished with a lithographic technique that creates the slit on a silicon nitride membrane supported on a Si wafer [20], and it could also be made by two mechanical blades installed on the base and adjusted accurately under the microscope.
The slit accomplished by the lithographic technique is straight and uniform within 100 nm or better, but the Si wafer has a high reflection and would amplify the stray light if measures are not taken, because light reflected from the detector is returned toward the slit at high efficiency and can then be redirected toward the spectrometer.
The slit made by two mechanical blades can absorb the light reflected by the detector because the mechanical blade is dyed black.And it is used to implement the instrument in the paper, as is shown in Figure 13.

Focal Plane Array
The hyperspectral imaging spectrometer utilizes a CMOS camera of the DALSA Corporation.The specifications of the array are shown in Table 3.
The spatial swath contains 1024 pixels and the spectral dispersing direction utilizes 512 pixels effectively.
The quantum efficiency of the camera product is shown in Figure 14.

Implementation
The final hyperspectral imaging spectrometer is designed as Figure 15.And the instrument is shown in Figure 16.The total volume of the instrument is 90 mm × 120 mm × 260 mm, and the weight is 1.7 kg.We tested the hyperspectral imaging spectrometer by acquiring the spectrum and the image of a black textile painting two butterflies before the flight test.Since the focal plane of the telescope is corresponding to the infinity and the position of the telescope is fixed by glue to adapt to the vibration of the airplane, the image of the butterflies is a little obscured, as is shown in Figure 17.The textile is illuminated by a halogen lamp.

Flight Test
The flight experiment was accomplished at meridiem in a sunshiny day.The image shown in Figure 18 is composed by the red, blue, and green spectrums without other processing.It should be declared that there is a small dust falling on the slit of the spectrometer without knowing.Thus a line of pixels is blocked to receive the light of the ground, and a dark line is generated in the color image and the hyperspectral image.

Conclusions
In this paper, a portable hyperspectral imaging spectrometer is designed and implemented.The spectral resolution of the instrument is of 1.6 nm in the spectral range of 450 nm to 850 nm; the spatial resolution is of 0.22 mrad instantaneous FOV in the range of 6.57 degrees.The total volume of the    instrument is 90 mm × 120 mm × 260 mm, and the weight is 1.7 kg.The hyperspectral imaging spectrometer could be used for the laboratory experiment data acquiring and remote sensing on the UAV and the CubeSat.

Figure 2 :
Figure 2: Ray trace of the telescope.

Figure 15 :
Figure 15: Diagram of the final hyperspectral imaging spectrometer.

Figure 17 :
Figure 17: Test of the hyperspectral imaging spectrometer in lab.

Figure 18 :
Figure 18: The image composed by the red, blue, and green spectrums without processing.

Figure 19 :
Figure 19: The hyperspectral image and the spectrums of typical objects acquired by the flight test.
Figure 12: The smile of the hyperspectral imaging spectrometer at 850 nm.