Active spectroscopy: detecting what the eye cannot see
How configurable photonic chip technology enables affordable new spectroscopic solutions.
Active spectroscopy: detecting what the eye cannot see
When does the difference between what something looks like and what it is really like matter?
Take an example from agriculture: traditionally a farmer has relied on the sense of sight, alongside touch and taste, to estimate the ripeness of fruit in the orchard, but the results are often hit-and-miss. The science of spectroscopy replaces guesswork with measurement: spectral techniques allow the grower, and in fact the whole supply chain for fresh produce, to look beneath the skin of the fruit to analyze the crucial parameters of moisture and sugar content without any damage to the crop.
Measuring the ripeness of fruit is just one example of the value of using the science of light to discover more than the human eye sees. In fact, this optical technology for analyzing the composition and condition of materials has thousands of potential applications in industry and in the home, from quality control in food and pharmaceutical factories to automating the operation of home appliances such as washing machines and cooker hoods. But until now, cost, size and system complexity have largely confined high-performance spectral sensing to specialized laboratory and industrial equipment.
That is why innovators are growing excited about a new development: a technology called Active Spectroscopy which enables spectroscopic analysis of materials illuminated by a configurable assembly of standard, LEDs and sensors.
Low-cost LED-based spectral vision system
There are numerous potential applications for spectroscopy in industrial and consumer markets. But until now, cost, size and system complexity have largely confined high-performance spectral sensing to specialized laboratory and industrial equipment.
Active Spectroscopy is an approach to enabling spectral analysis at lower cost by using an array of specially selected standard LEDs to create an application specific, spectrally defined light source.
- Active Spectroscopy can be used to identify materials and measure specific properties
- for example, in automatic floor cleaners it can detect different flooring materials and measure their moisture level, identify fabrics in washing machines, and test the ripeness of fruit in agricultural and retail applications.
- Manufacturers can evaluate the general concept and configure component building blocks for their specific use case with a dedicated Optical Measurement Kit.
Spectroscopy: an established scientific technique
Spectroscopy is the science of detecting and understanding materials and states of matter through the distinctive behavior of light at a specific wavelength:
- When the light is transmitted through or reflected by the material
- Or when the material is stimulated to emit light by the incidence of light on it
The spectrometer, an instrument which performs spectroscopy, is a familiar part of the scientist’s toolkit. Under a reference illuminator, materials will yield a distinct ‘spectral signature’ – the shape of the curve in the spectrometer’s output – which depends on the materials’ specific transmission, reflection or absorption characteristics. It enables phenomena that are invisible to the human eye to be clearly detected and analyzed.
This is why benchtop spectrometers are used in the laboratory to analyze chemical compounds and to detect molecular structures. These instruments, which include laser light sources tuned to emit light at a tightly specified wavelength, and an array of specialist light sensors, are large and extremely expensive.
A similar instrument, the hyperspectral camera, analyzes spectral characteristics of materials across a broad spectrum which can stretch from ultraviolet to infrared wavelengths. First used for geological surveying by mining and oil companies, hyperspectral cameras are now deployed in agriculture, healthcare, food processing, surveillance and more. But like the spectrometer, the hyperspectral camera is a specialist instrument costing thousands of dollars, which limits the range of uses for which it is viable.
High-value use cases for low-cost spectroscopy
The ability to see beyond the visual appearance of objects and to analyze their composition offers many benefits outside the lab.
In the home, for instance, spectroscopy could analyze the concentration of steam and smoke from a stovetop to allow automatic adjustment of a cooker hood’s fan speed, or to indicate to the user when the filter requires cleaning or replacement. In a washing machine, spectral intelligence could be applied to the washing water to automatically adjust detergent dosing and the length of the washing program. Robotic floor cleaners can use spectroscopy for material inspection, and so apply the right kind of cleaning tool and program to different flooring materials. Spectroscopy can also identify foreign objects such as pet droppings, so that the floor cleaner can avoid brushing contaminated material into the fibers of a carpet.
Spectroscopy has many potential uses elsewhere, including in industrial production and quality control. Pick-and-place operations can be made more reliable and efficient when spectral analysis supports or replaces the output from a vision camera. Many types of industrial operation are set to be transformed by the application of machine learning to enable automatic recognition and handling of objects. The distinctive data points produced by spectroscopy make it easier for a neural network’s inference engine to distinguish two visually similar objects than if the network works with the output from a camera alone. This means that in edge AI applications such as the hands of a humanoid robot, spectroscopy can greatly reduce the cost and sophistication of the inference engine required to perform object detection functions.
Also in the industrial sector, the spectroscopic technique can for instance detect damaging partial discharges in electric motors or high-voltage power grids, or in production settings, identify objects by their material type, not only by the shape and color that a vision camera sees.
At the right price point and in the right form factor, spectroscopy has many more uses even than this, across fields as diverse as bioscience and personal health monitoring, agriculture and plant health, filter maintenance and replacement, water and waste treatment, and fermentation and brewing.
Mastering the physics of spectroscopy with standard semiconductor components
The cost of a conventional spectrometer or hyperspectral camera is too high to make it viable for widespread use outside an R&D lab, in industrial and consumer use cases such as robots and home appliances. This looks like a problem, but in the eyes of ams OSRAM engineers, it was actually an opportunity to create new value through innovation. Our idea was not to make a cheaper spectrometer. Spectrometers are expensive for a reason: it is not possible to make a cheap spectrometer that is not also a bad spectrometer.
The innovation was to rethink the category. A spectrometer is a general purpose tool, made from expensive components, which can be used to analyze almost any material or substance. What if we were to make instead not a general purpose tool, but a system of affordable and optically stable photonic components, all available from ams OSRAM, which the user configures for spectroscopic analysis in a specific use case? In other words, what if the user was able to build their own spectroscopic solution, limited to the few wavelengths of interest which are relevant for the application?
ams OSRAM is known for its broad portfolio of high quality LEDs and optical sensing products which are available in a wide range of options for wavelength coverage, package, and power output. This was the foundation on which the Active Spectroscopy solution was built: it strongly depends on the availability of photonic ingredients which offer predictable spectral stability over lifetime and temperature. Every Active Spectroscopy system uses a combination of LEDs, photodetectors or multi-channel spectral sensors, and an analog front end (AFE), an analog system management chip which performs both synchronized LED driving and embedded signal processing of the currents from photodetectors.
By pairing a chosen LED’s peak emission wavelength with a photodetector’s peak sensitivity wavelength, the system designer can target a narrowly specified portion of the ultraviolet, visible light, or infrared spectrum. With a configurable array of up to eight LED/photodetector pairs, an Active Spectroscopy system can provide multi-channel optical analysis, tuned to the wavelengths of interest to a specific application. The AFE’s embedded processing capabilities allow for spectral measurements over distances up to almost 1m, even in the presence of interference from ambient light.
It is a flexible solution, because the system designer can pick from among more than 880 LEDs and more than 70 photodetectors, giving a very broad choice of options for:
- LED peak wavelength across the spectrum from UV to near infrared, and on request, including the short-wavelength infrared (SWIR) portion of the spectrum
- Field-of-illumination and radiation angle
- Radiation power
- Photodetector filter characteristics and sensitivity
- Package characteristics (size, thermal behavior, robustness)
Even higher spectral resolution can be obtained by pairing a configurable array of LED emitters with a multi-channel spectral sensor. The mix of precisely characterized LEDs available in the Active Spectroscopy solution provides a spectrally diverse alternative to expensive broadband light sources such as miniature halogen or xenon light bulbs, while enabling precise analysis of targeted spectral signatures. The combination of LEDs and a spectral sensor provides a way to virtually scale up the number of measurement channels and increase spectral resolution compared to a system using photodetectors.
Whether a system uses spectral sensors or photodetectors with the LED illuminators, its total bill-of-materials cost reflects the competitive pricing of standard off-the-shelf optical semiconductor components.
The spectral behavior of the emitter and detector components in Active Spectroscopy is absolutely critical to the performance of the spectral analysis in the application: these components must maintain stable operation with small and predictable spectral drift over variations in operating temperature, and over the lifetime of the equipment. The performance and stability of products in the ams OSRAM portfolio are specified in datasheets and supporting documentation, providing users of Active Spectroscopy with confidence that spectral drift can be maintained at levels appropriate to the application and its typical operating conditions.
In addition, the Active Spectroscopy solution includes an AFE which provides a pre-processed digital output, which gives the system designer a familiar signal chain to integrate with a system’s microcontroller or application processor.
The significance of Active Spectroscopy for tech innovators
The new ability to embed spectral analysis at a low bill-of-materials cost means that product manufacturers can consider adding spectral intelligence into products where it has never before been viable, as well as in applications where there was not previously enough space, such as a humanoid robot’s hand.And the technology is ready for use now: engineers can experiment with Active Spectroscopy using the Optical Measurement Kit (OMK).
A modular development board, the OMK lets users plug in up to eight LEDs and eight photodetectors, and test for the best combination of wavelength specifications for their application. The flexible choice of components and configurations means that engineers can try different approaches to system integration and measurement, evaluate both reflective and transmissive set-ups, and see the effect of altering the spacing and radiation angle between emitters and detectors.
The kit provides a practical demonstration of the value of Active Spectroscopy, showing this new way to embed spectral intelligence in products without using a laboratory-grade instrument.