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Hyperspectral imaging technology

Shrimps see what we, humans don’t see. Mantis shrimps have a compound eye which allows them to see in the ultraviolet down into the infrared whereas human eyes can only detect the visible portion of the electromagnetic spectrum. This allows mantis shrimps to detect preys which would appear transparent to our eyes. This is just an example of what lies beyond what human eyes can see.

Similar to the way how these shrimps scan the world across different wavelength bands, the hyperspectral imaging(HSI) technique collects information of a target scene across the electromagnetic spectrum, from ultraviolet to long-infrared. This enormous information facilitates the identification of objects. For example, HSI in the Shortwave Infra Red (SWIR) spectral region provides detailed information about the chemical composition of tablets, which is not possible using normal imaging. The HSI technology has the potential to transform early detection and treatment of many life-threatening medical conditions. It can be used in dentistry to detect tooth decay without drilling or x-rays, help farmers to monitor crop’s health, detection of counterfeit goods and quality assessment of food products. Hyperspectral imaging is a non-destructive, non-contact technology which makes it ideal for a wide range of applications. Let’s look at the hyperspectral image in detail.

A hyperspectral image cube consists of a set of images layered on top of one another. Each image represents one particular wavelength band. This set of images form what is termed "the hypercube” ( Fig 1 ). Thus in a hyperspectral image, each pixel consists of a spectrum over an appropriate spectral region ( Fig 2 ). Every object has a unique characteristic across these different wavelength bands. This unique characteristic is referred to its ‘spectral signature’. For example, by comparing the spectral signature of the unknown target objects to that of known substances, the chemical composition of the target object can be identified. The property of HSI, which looks beyond the visible spectrum, enables not only the identification of objects but also their classification and quantification to a high degree of accuracy.


Fig1 : Hypercube – Pictorial representation


Fig2: HSI Wavelength regions

Gilden Photonics launched in 2005, is a leading manufacturer and supplier of hyperspectral imaging systems. Gilden Photonics offer a complete range of hyperspectral imaging solutions over the spectral range from ultraviolet to long wave infrared. The hyperspectral systems designed and built by Gilden Photonics employ push-broom imaging technique. In this technique, the spectral signature of one pixel to many thousand pixels in a spatial line (swath) across the target area is recorded at a time ( Fig 3 ). The second spatial dimension is built over time by sequentially recording one swath after another.


Fig 3: Capturing spatial dimension over time

How does push-broom technique work?

The push-broom technique allows simultaneous acquisition of all spectral bands. The three main components of a push-broom hyperspectral camera are a CCD camera (two dimensional detector array), imaging Spectrograph and objective lens ( Fig 4 ).


Fig 4: Hyperspectral camera

The fore optics, in a hyperspectral camera, image the light reflected from one line segment of the target sample onto the entrance slit in the spectrograph. This light is dispersed into different wavelengths by the dispersing element (prism / grating) in the spectrograph. This dispersed light is captured by the CCD sensor as a two dimensional image [X, λ] with X representing spatial position and λ represents spectral dimension. In order to capture the entire image, snapshots of every line across the target area have to be captured. A schematic representation of push-broom imaging is shown below ( Fig 5 ).


Fig 5: Schematic representation of push-broom imaging

Moving one of the following components in a perpendicular direction to a measured line can capture the images of different lines on the target area,
  1. Target object
  2. Spectral camera
  3. Scan mirror in front of the spectral camera

By synchronising the movement between camera and target object and the acquisition of the camera, one can capture different lines of the object and thus generate the hypercube [X, Y, λ] where Y represents the second spatial dimension. This technique works in a similar fashion to the pushing of a broom across the floor and hence the name. Gilden Photonics employs appropriate scanning systems in their hyperspectral imagers to facilitate the movement between the camera and the target scene.

Advantages of push-broom technique

The push-broom HSI systems are highly rugged and more stable. These systems allow rapid HSI acquisition. The complete spectral information of a line is acquired simultaneously along with the spatial information. This facilitates high throughput and hence faster hypercube generation. The push-broom technique also allows reduced illumination load. Only one line across the target area needs to be illuminated at a time. This reduces the heat load on the sample drastically compared to other imaging techniques. The push-broom HSI systems are ideal for online measurements with moving objects. These systems can be directly transferred from R&D labs to production line in industry with no changes to the system. For example, this can be applied for fruit sorting where the fruits are constantly moving over a conveyor belt.

Some Applications

Hyperspectral imaging is a non-destructive, non-contact technology which makes it ideal for a wide range of applications. HSI has a wide range of applications in the field of remote sensing, identification of tumours in biomedical, target detection in military, process monitoring, food processing and so on. Let’s take a closer look at some of the applications.

FOOD

HSI has been widely used in food quality assessment and safety control. The non destructive nature and rapid response of HSI aids the online quality control of food products. Researchers at Campden & Chorleywood Food Research Association (CCFRA) and Gilden Photonics Ltd. (GPL) developed a HSI system to determine the distribution of fat and moisture content in fried foods, distribution of fat in meat, freshness of fish and evaluation of baked goods. Conventional techniques such as NIR spectroscopy were able to provide only the average fat content in food. HSI facilitated the analysis of non- uniform distribution of fat in doughnut. A doughnut has been measured in NIR region. The fat and moisture shows separate characteristic absorbance bands in NIR region ( Fig 6 ).


Fig 6: Frying – Fat and water content in a doughnut (Image from CCFRA, Campden, UK )

PHARMACEUTICAL

Hyperspectral Imaging in Near-Infrared (NIR) and Shortwave Infrared (SWIR) region provides information about the spatial distribution of chemical components in pharmaceutical samples. The pharmaceutical applications of HSI range from R&D, to production line in industries for qualtiy inspection. The picture below ( Fig 7 ) shows how different ingredients and homogeneity in their distribution in tablets is detected in the NIR image (left) based on different spectra of the ingredients (right).


Fig 7

FORENSICS AND COUNTERFEITING

HSI has been increasingly applied in forensic industry such as Inks examination on questioned documents like altered date on contract signature, examination of samples like fibers and paint cut offs for trace evidence, detection of counterfeit samples and so on. The following picture demonstrates identification of alterations in a forged cheque ( Fig 8 – 9 ).


Fig 8: False colour hyperspectral image of the cheque created with three different wavelength bands. This shows that the number ‘1’ has been altered to number ‘4’


Fig 9: Reflectance spectra of the horizontal line (green plot) and vertical line (red plot) in the number ‘4’. The difference in spectra shows that different inks were used.

In 2010, Gilden Photonics launched a Hyperspectral fluorescence lifetime imaging system which combines the advantages of spectral imaging with time resolved measurements. This unique multi dimensional measurement and analysis technique provides enormous amount of information about the sample. This technology would take the object classification and quantification to an entirely different level.

Hyperspectral imaging technology
 

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Gilden Photonics Ltd.
Unit 13 Telford Court
9 South Avenue
Clydebank Business Park
Glasgow G81 2NR
UK

tel: +44 (0)141 952 9475
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