We design and implement a portable hyperspectral imaging spectrometer, which has high spectral resolution, high spatial resolution, small volume, and low weight. 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.
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 [
With the development of machinery and electronics technology [
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.
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
The Dyson spectrometer.
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
Design specifications.
Parameter | Value |
---|---|
Principle | Pushbroom |
Spectral range (nm) | 450–850 |
Spectral sampling (nm) | 1.6 |
Field of view (degrees) | 6.57 |
Instantaneous FOV (mrad) | 0.22 |
|
2.5 |
Pixel size ( |
7.4 |
Spatial swath (pixels) | 1024 |
Spectral pixels | 512 |
Slit width ( |
13.2 |
To prove the advantages of the system above, we design and implement the hyperspectral imaging spectrometer.
The telescope is designed using a refraction system. The specifications are shown in Table
Telescope specifications.
Parameter | Value |
---|---|
Working range (nm) | 450–850 |
Focal length (mm) | 66 |
Field of view (degrees) | >6.6° |
|
2.5 |
Other demand | Telecentric |
We use three kinds of glasses from the CDGM material catalogs of ZEMAX: CAF2, H-LAK2A, and TF3. The telescope is designed as Figure
Ray trace of the telescope.
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
MTF of the telescope.
The energy is included mostly in the pixel range from the spot diagram shown in Figure
Ensquared energy of the telescope.
Distortion of the telescope.
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
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
Ray trace of the first step of the design of the spectrometer.
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
Ray trace of the second step of the design of the spectrometer.
The Dyson block.
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 337.3 mm 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.
The ray trace of the hyperspectral imaging spectrometer is shown in Figure
Simulation of the hyperspectral imaging spectrometer.
The matrix spot diagram shows that the spots diagram of all waves are less than the width of the slit (13.2
Matrix spot diagram of the hyperspectral imaging spectrometer.
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
Distortion (keystone) of the spectrometer.
The maximal smile of the hyperspectral imaging spectrometer occurs in the wavelength of 850 nm, which could be got from Figure
The smile of the hyperspectral imaging spectrometer at 850 nm.
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 [
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
The slit accomplished by two blades.
The hyperspectral imaging spectrometer utilizes a CMOS camera of the DALSA Corporation. The specifications of the array are shown in Table
Focal plane array specifications.
Parameter | Value |
---|---|
Active resolution | 1024 × 1024 pixels |
Pixel | 7.4 um |
Frame Rate (frames/s) | 120 |
Responsivity | 30 DN/(nJ/cm2) |
Data format | 8 bit |
Dynamic range | 50 dB |
Mass | <175 g |
Size | 44 × 44 × 44 mm |
Power supply | 12 V to 15 V DC |
Power dissipation | <3 W |
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
The quantum efficiency of the DALSA camera.
The final hyperspectral imaging spectrometer is designed as Figure
Diagram of the final hyperspectral imaging spectrometer.
The compact hyperspectral imaging spectrometer.
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
Test of the hyperspectral imaging spectrometer in lab.
The flight experiment was accomplished at meridiem in a sunshiny day. The image shown in Figure
The image composed by the red, blue, and green spectrums without processing.
The hyperspectral datacube acquired is shown in Figure
The hyperspectral image and the spectrums of typical objects acquired by the flight test.
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.
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.
The authors declare that they have no conflicts of interest.
This work is supported by the National Natural Science Foundation of China under Grant 61501456.