What is Spectral Imaging?

Spectral imaging is an advanced technology that combines conventional imaging with spectroscopy to capture both spatial and spectral information from an object or scene. Unlike traditional photography that records only three-color channels (red, green, and blue), spectral imaging systems collect data across dozens or even hundreds of contiguous wavelength bands for each spatial pixel. This comprehensive approach creates a three-dimensional data cube where two dimensions represent spatial coordinates, and the third dimension contains spectral information.

The technology enables the detection, identification, and quantification of materials based on their unique spectral signatures – the distinctive ways in which different substances reflect, absorb, or emit electromagnetic radiation across various wavelengths. These signatures serve as “optical fingerprints” that can identify materials with greater precision than possible with conventional imaging. Applied spectral imaging has revolutionized numerous fields by revealing information invisible to standard cameras.

Spectral imaging techniques span various regions of the electromagnetic spectrum, including ultraviolet, visible, near-infrared, short-wave infrared, mid-wave infrared, and long-wave infrared. Each spectral region provides unique insights into different aspects of the materials being analysed. For a deeper understanding of the historical development of these techniques, the history of hyperspectral imaging provides valuable context.

How Does Spectral Imaging Work?

Spectral imaging systems operate by separating incoming light into its constituent wavelengths and measuring the intensity at each wavelength. This process creates a spectrum for each pixel in the image, resulting in a data-rich representation of the scene that reveals material properties invisible to conventional cameras. The analysis of a spectral image enables researchers and professionals to extract detailed information about the chemical and physical properties of the observed objects.

The fundamental components of a spectral imaging system include:

1. Illumination Source: Provides light energy across the wavelengths of interest, either natural sunlight or specialised sources like broadband lamps, LEDs, or lasers. The quality and consistency of illumination directly impacts measurement accuracy.
   
2. Optics: Lenses and mirrors that collect and focus on the light reflected or emitted from the target. These must be designed to minimise chromatic aberration across the full spectral range of interest, often requiring specialised materials and coatings.
   
3. Dispersive / Spectrally selective Element: Separates the light into its component wavelengths using prisms, diffraction gratings, tunable filters, or interferometers. This critical component determines the system’s spectral resolution and range capabilities.
   
4. Detector Array: Captures the intensity of light at each wavelength and spatial position. Different detector materials are used based on the spectral region: silicon-based sensors for UV to near-IR, InGaAs for extended near-IR and shortwave IR, and specialized materials like HgCdTe or InSb for longer wavelengths.
   
5. Processing System: Software that converts raw data into usable information through calibration, noise reduction, and analysis algorithms. Modern systems increasingly utilize AI and machine learning to extract insights from the complex multidimensional datasets.
   

There are several methods for acquiring spectral data:

• Point Scanning (Whiskbroom): Collects the full spectrum for a single spatial point at a time, then sequentially builds the complete image
• Line Scanning (Pushbroom): Captures spectral information for an entire line of the image simultaneously, suitable for moving objects or platforms
• Snapshot: Records the entire data cube in a single exposure, ideal for dynamic scenes

For a comprehensive comparison of different technologies in this field, resources like this guide to spectral imaging provide valuable insights into various imaging approaches.

Types of Spectral Imaging

Spectral imaging systems are typically classified based on the number of spectral bands they capture:

Multispectral Systems: These capture data in several (typically 3-15) discrete, non-contiguous spectral bands. Each band is relatively broad, spanning tens of nanometers. Multi spectral imaging systems are often simpler, more affordable, and sufficient for many applications where the spectral features of interest are known in advance.

Hyperspectral Systems: Hyperspectral imaging collects data in numerous (often 100+ bands) narrow, contiguous spectral bands, typically just a few nanometers wide. This high spectral resolution allows for more detailed analysis and the detection of subtle spectral features.

Applications of Spectral Imaging in Different Industries

The versatility of spectral imaging technology has led to its adoption across numerous industries:

Agriculture and Food Safety:

• Crop health monitoring and disease detection, as demonstrated by Living Optics’ portable field devices that detect plant stress up to two weeks before visual indicators emerge
• Precision agriculture applications including nutrient deficiency mapping
• Food quality assessment and contamination detection on processing lines
• Soil composition analysis, where Living Optics’ systems allow farmers to make data-driven decisions about irrigation and fertilization

Medical and Healthcare:

• Non-invasive diagnostics including early cancer detection
• Surgical guidance and tissue delineation during operations
• Wound assessment and monitoring of healing progression
• Pharmaceutical manufacturing quality control

Environmental Monitoring:

• Pollution detection in water, air, and soil
• Forest health monitoring and species identification
• Coral reef mapping and health assessment
• Algal bloom identification and monitoring

Industrial Quality Control:

• Manufacturing process monitoring and defect detection
• Recycling material sorting and identification
• Electronic component inspection and verification

Defense and Security:

• Target detection and identification, even with camouflage, using advanced systems like those developed by Living Optics
• Chemical and biological threat detection where remote sensing capabilities allow identification of hazardous materials without direct exposure
• Forensic analysis and evidence examination where applied spectral imaging techniques reveal details invisible to the naked eye

For specialised applications in this sector, Living Optics has pioneered imaging technologies for sensing at range and various national security applications.