Hyperspectral Imaging

14 October 2024

Electromagnetic radiation is energy that travels as particles or waves, spreading out as it goes. The majority of the electromagnetic radiation that affects the Earth comes from the Sun. We can understand this radiation by looking at its range of wavelengths and frequencies, from the longer wavelength, low-frequency radio waves, to shorter wavelength, high-frequency gamma waves. Together, all of these different energy types, most of them invisible, are called the electromagnetic (EM) spectrum, or spectrum for short.

Spectral imaging is imaging that uses multiple bands across the electromagnetic spectrum. While an ordinary camera captures light across three wavelength bands in the visible spectrum, red, green, and blue (RGB), spectral imaging encompasses a wide variety of techniques that go beyond RGB.

Hyperspectral imaging is a powerful technology combining spectroscopy with imaging capability. It enables gathering detailed information about the composition and characteristics of objects and surfaces in a way that is impossible with conventional imaging systems.

Hyperspectral imaging collects and processes information from across the electromagnetic spectrum. The goal of hyperspectral imaging is to obtain the spectrum for each pixel in the image of a scene, with the purpose of finding objects, identifying materials, or detecting processes.

the human eye sees color of visible light in mostly three bands (long wavelengths, perceived as red; medium wavelengths, perceived as green; and short wavelengths, perceived as blue), spectral imaging divides the spectrum into many more bands. This technique of dividing images into bands can be extended beyond the visible. In hyperspectral imaging, the recorded spectra have fine wavelength resolution and cover a wide range of wavelengths. Hyperspectral imaging measures continuous spectral bands

Cameras developed for hyperspectral Imaging basically can be categorized into two as Spatial Scanning and Non Spatial Scanning. We call them as Push Broom Scanning and Snapshot scanning.

Push Broom Scanning:

Hyperspectral imaging (HSI) devices for spatial scanning obtain slit spectra by projecting a strip of the scene onto a slit and dispersing the slit image with a prism or a grating. With these line-scan cameras, the spatial dimension is collected through platform movement or scanning. This requires stabilized mounts or accurate pointing information to ‘reconstruct’ the image. Nonetheless, line-scan systems are particularly common in remote sensing, where it is sensible to use mobile platforms. Line-scan systems are also used to scan materials moving by on a conveyor belt. In spatial scanning, each two-dimensional (2D) sensor output represents a full slit spectrum (xλ).

SNAPSHOT Scanning:

In non-scanning, a single 2D sensor output contains all spatial (xy) and spectral (λ) data. HSI devices for non-scanning yield the full datacube at once, without any scanning. Figuratively speaking, a single snapshot represents a perspective projection of the datacube, from which its three-dimensional structure can be reconstructed. The most prominent benefits of these snapshot hyperspectral imaging systems are the snapshot advantage (higher light throughput) and shorter acquisition time.

Herewith we are providing a data shared in a research paper that shows simultaneous use of cameras with above referred technologies and how they capture spectral information.

Hyperspectral Camera Specifications

Two different HS cameras based on different acquisition techniques have been used to capture invivo brain images. The technical specifications of both cameras, sensors and lenses are presented on Table 1.

Table 1 Sensor and camera optics specifications of the different hyperspectral cameras used. Last five parameters are fixed during the acquisition of images at the operating room.

On one hand, the snapshot HS camera has a complementary metal oxide semiconductor (CMOS) sensor holding a 5×5-mosaic pattern with a pixel size of 5.5 μm (MQ022HG-IM-SM5X5-NIR, Ximea GmbH, Germany). The analogue to digital converter (ADC) of the sensor provides images with a resolution of 8 bits. Additionally, a long pass filter (FELH0650, Thorlabs, Inc., USA) with a 650 nm cut-on wavelength is placed in front of the lens to remove non-negligible secondary harmonics, which were determined by the manufacturer in the spectral response curves during sensor production. Although the sensor resolution is 2048×1088 pixels, the active area of the sensor with the built-in spectral filters has 2045×1085 pixels. This reduced area is called the active filter zone, which omits the last 3 rows and last 3 columns from the total sensor resolution. Each capture with this snapshot camera produces an image containing the spatial and spectral resolution due to the 5×5-mosaic pattern. Each mosaic contains approximately the same spatial pixel at different 25 wavelength bands within the NIR spectrum, specifically in the 660 to 950 nm spectral range. Additionally, these bands are spaced among each other with a mean and standard deviation value of 12.11+2.64 nm. Hence, the mosaic pattern reduces the two-dimensional output spatial resolution by a factor of 5 to obtain a three-dimensional HS cube. In particular, the image with 2045×1085 pixels generated by the sensor is arranged into a 409×217×25 HS cube to perform the spectral analysis. The main advantage of the snapshot camera is its capability for real-time solutions, understood as processing a sequence of HS images to provide a live video of the scene. On the other hand, the other camera is based on the pushbroom line scan technology (Micro-Hyperspec® E-Series, HeadWall Photonics Inc., USA), which holds a scientific CMOS sensor with an ADC of 16 bits and a pixel size of 6.5 μm. The sensor acquires a single spatial line with 1600 pixels and 394 wavelengths of information. Thus, the camera needs to be moved with an actuator to scan an image with as many lines as desired. All in-vivo brain captures used in this study were scanned with 500 lines, producing images with a spatial resolution of 1600×500 pixels and 394 spectral

bands. Besides, the exposure time and frame period were set to 150 ms and 160 ms, respectively.

It is worth noting that captures from both cameras were cropped spatially to help neurosurgeons during the labelling processing. Therefore, the spatial resolution of the HS line scan or snapshot captures are smaller than 1600×500 pixels or 409×217 pixels, respectively. Although the pusbroom line scan camera provides more spatial resolution and spectral information, it is not suitable for real-time solutions due to the scanning procedure. The time spent to scan a brain image requires, approximately, 1 minute and 40 seconds, while an image captured with the HS snapshot camera takes 100 ms.

Ref: Spectral Analysis Comparison of Pushbroom and Snapshot Hyperspectral Cameras for In-Vivo Brain Tissues and Chromophores Identification.

The above is just used to show an example to differentiate the usage of Push Broom and Snapshot technology of hyperspectral Cameras in the way they form images and spectral information and the places where they can be used.

To summarize more…

According to the scanning principle of hyperspectral camera, it can be divided into four kinds, which are also the most common hyperspectral cameras on the market: point scanning hyperspectral camera, line scanning hyperspectral camera, spectral scanning hyperspectral camera and snapshot hyperspectral camera. In the process of image acquisition and color detection, these four kinds of hyperspectral cameras can obtain images with different wavelengths.

Point scanning is to obtain the spectral data of one point at a time. The imaging equipment is a spectrometer. It can be used on satellites and requires degrees of freedom in two directions.

Line scanning is to obtain spectral data on one line at a time. The imaging equipment is a spectrometer and gray camera. Due to its high spectral resolution and fast imaging, it is most widely used at present.

Spectral scanning is to obtain images of one band at a time. The imaging equipment is an adjustable filter and gray camera.

Snapshot is to obtain a three-dimensional hyperspectral image at a time. At present, the application is mostly realized by multi-channel filter. Fast imaging, but low spectral resolution.

This advanced technique offers valuable insights across various industries, including food processing, agriculture, environmental monitoring, healthcare, and mining. By utilizing hyperspectral imaging, professionals can enhance productivity, ensure safety, and support sustainable practices in their respective fields.

 

Image Courtesy: SPECIM Web site*

 

We Online Solutions (Imaging) Pvt. Ltd., Chennai India represent Teledyne DALSA and CHN Spec China (for their Push broom Hyperspectral Imaging Cameras.

We can be reached at [email protected] with application details and we can provide suitable camera (Hyperspectral) solutions with all needed accessories including software.

*Disclaimer: All the details used in this blog are collection of data from different web sites and blogs all round. The main aim of this blog is to provide a comprehensive idea about hyper spectral Imaging with some examples to explore and not for any commercial purposes. Contents can be removed or changed if any one has any objection to any contents shared here. Any claims and apprehensions can be sent to [email protected]

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