What is Hyperspectral Imaging?
Hyperspectral Imaging (HSI), is an advanced technology that combines machine vision and spectroscopy. It allows for capturing detailed information about the spectrum of an object, offering a wide range of application possibilities across various industries. Let's start from the beginning...
The Development of Imaging Technology
The development of imaging technology began with monochrome cameras capable of capturing only intensity information without colour information. Over time, RGB cameras were developed, which can distinguish three different spectral ranges in the visible region of the electromagnetic spectrum. These cameras mimic the spectral response of the human eye, providing better visual representation of the image for human observation.
In comparison to monochrome cameras, RGB cameras provide significantly more information and are the natural choice for applications where human visual perception is relevant. Although they can be considered multispectral cameras, they are not usually referred to as such.
Multispectral cameras offer a larger number of spectral bands compared to RGB cameras, typically ranging from four to ten bands. These bands can be customized depending on the application and offer a cost-effective option for simple sorting applications. However, the bands in multispectral cameras are usually not directly adjacent or evenly distributed, making them unsuitable for spectroscopy.
Hyperspectral cameras take the final step from being solely imaging cameras to actual spectrometers. They provide dozens to hundreds of spectral bands, usually directly adjacent and evenly distributed. This allows hyperspectral systems to apply established spectroscopic techniques such as derivative spectroscopy, chemometrics, and others. As a result, they become highly versatile tools for various applications in chemical detection, analytics, and sensor-based sorting.
Originally, hyperspectral imaging was developed for remote sensing applications in satellites and airborne measurement systems. As a pioneer of hyperspectral imaging for industrial applications, EVK introduced the first generation of HELIOS Hyperspectral cameras for the recycling industry in 2006. EVK HELIOS Hyperspectral cameras measure in the near-infrared to short-wavelength infrared range from 930 nm to 1700 nm, which is particularly useful for material identification.
With spectrally calibrated hyperspectral cameras, which have become the norm in the machine vision industry since their introduction by EVK, the distinction between a hyperspectral camera and an imaging spectrometer is no longer fundamental but rather emphasizes specific performance metrics. Hyperspectral imaging is a powerful method for machine vision applications such as in-line measurement and monitoring, sensor-based sorting, in-line quality control, foreign body detection, and many more.
Hyperspectral Camera Features
The main optical features of a hyperspectral camera for machine vision applications include:
- Spectral range
- Frame rate
- Spatial resolution (optical, Along-Track, Across-Track)
- Spectral resolution (optical)
- Spatial sampling (number of pixels)
- Spectral sampling (number of spectral bins)
- Temporal sampling (framerate)
- Light efficiency
- Light responsivity
- Signal-to-noise ratio
- Dynamic range
While often a lot of emphasis is devoted to the typical features of spatial sampling (number of pixels) and frame rate, responsivity and signal-to-noise ratio are often more important for proper data quality, especially in high-speed applications with short exposure time, where high illumination powers are needed.
Additional important non-imaging features include ruggedness for industrial environments, such as dust, vibrations, temperature, and moisture, calibration stability, temperature stability, and ease of device replacement.
EVK Hyperspectral cameras are industry-proven, rugged devices without moving mechanical parts, optimized for industrial environments and 24/7 operation. These cameras deliver high-quality, calibrated spectral data in high speeds with high spectral resolution, very high light responsivity, excellent signal-to-noise ratio, and dynamic range.
The real time computing engines deliver state of the art spectral preprocessing (e.g. filtering, derivatives, normalization) and integrated chemometrics, to give a real-time chemical imaging stream output in RGB false-colour encoding.
Types of Hyperspectral Cameras
Hyperspectral cameras come in different types, each with distinct features, advantages, and disadvantages. They are generally classified based on their functioning into the following types:
Snapshot Hyperspectral Imagers
Snapshot hyperspectral imagers capture a multispectral two-dimensional image with a single shot. These cameras are typically structured similarly to an RGB sensor. Their advantage is their lightweight and compact design. They usually capture 16 to 64 different spectral bands. However, the actual spatial resolution suffers in this type of hyperspectral camera and is not optimally suited for chemometric applications due to the low number of measured spectral bands.
Whisk-Broom Hyperspectral Imagers
Whisk-Broom hyperspectral imagers are single point spectrometers, which generate imaging information by scanning over a sample. As these systems rely on moving parts, they are susceptible to vibrations and are not maintenance-free.
Spectral Scanning Hyperspectral Imagers
Spectral scanning hyperspectral cameras are image sensors that sequentially measure different wavelengths, e.g. by filter wheels or variable filters. Their frame-rate will be reduced by the number of spectral bins recorded. Artifacts may occur when objects move and different wavelengths are captured at different times.
Push-Broom Hyperspectral Imagers
Push-Broom hyperspectral cameras are line-scan cameras that project a straight line of image information through a dispersing system, typically an imaging spectrograph. One dimension of the sensor corresponds to the position on the scanned line, while the other represents the wavelength. This way, a spectrum is recorded directly for each point on the scanned line. Push-Broom cameras are free from measurement errors caused by fast moving objects, making them ideal for continuous processes and continuous image streams. Push-broom imagers are the predominantly used hyperspectral system in sensor-based sorting, inline monitoring and analysis.
EVK HELIOS cameras are Push-Broom hyperspectral cameras that are capable of high speeds and real-time measurements, as well as real-time chemometric data evaluation. Thus, they enable real-time process decisions based on the chemical properties of fast-moving objects.
The Structure of a
Push-Broom Hyperspectral Camera
A typical push-broom hyperspectral camera consists of an objective lens, an entrance slit, a collimator, a dispersive element, the focusing optics, and an imaging sensor. The arrangement of these components is shown in the following figure.
The scenery is projected by the objective lens onto the entrance slit. The collimation optics collimate the centerline of the scene, meaning that the light rays of each point in the scene are parallel to each other. The dispersive element separates the radiation into different angles for different wavelengths, while the focusing optics reconstruct the image on the sensor plane. This ensures that that the full information of the scanning line is on the sensor plane at any instance, such, that movement of objects while recording, leads to motion blur at high exposure times but not to any reconstruction artifacts.