The use of LEDs in sensing and inspection applications has grown significantly, particularly where an application must rely on short-wavelength infrared (SWIR) wavelengths. Traditional halogen light sources are extremely effective and have comparable radiation efficiency, but LEDs hold some unique advantages over them. Longer operational lifetime, an extremely accurate & customizable spectral range, and fast pulse modulation capabilities are just some of the benefits. In this article, we outline some of the most popular applications and find out what role Ushio’s Epitex SWIR LEDs play in them.
SWIR LEDs for Liquid Level Inspection
There are many instances where the accurate measurement of liquid within a container is required. When the container is transparent and the process is fairly slow, it may be enough to use human visual confirmation; however, on modern production lines, containers are often opaque and the automated process takes place at high-speed.
Ushio has mastered the art of placing up to three different chips, measuring only a few square millimeters, into one package. The chips can be an assortment of LEDs and sensors, so each package effectively reduces its occupied space on a circuit board by two-thirds. With this technology, we can analyze various material components in great detail by cycling through different SWIR wavelengths. Independently pulsed light-emitting chips illuminate the subject, while a sensor, or high-speed camera, captures the desired spectroscopic images.
Ushio’s 1450 nm LEDs match the spectral absorbency of water, which means very little of the light transmits through it, but other substances can be identified by SWIR wavelengths too. By switching between the precise wavelengths absorbed by each substance, the individual components of a liquid mixture can be identified. For example, LEDs commonly used for sensing ethanol emit 1500–1600 nm, while oils and fats are easily detected with wavelengths at around 1700 nm. While 1450 nm is mostly absorbed by water, it has a high transmittance through the resins and glass commonly used for making bottles. As a result, the transmitted image shows a strong shadow caused by the water and its clearly defined top surface.
The liquid level information extracted from the transmitted image can be used to automate a number of actions. For example, it is possible to continuously observe the liquid level while pouring liquid into the bottle and halt pouring when the pre-set liquid level has been reached.
For more information about Ushio Epitex SWIR LEDs in liquid level inspection applications, please visit https://swir-led.com/liquid-level-inspection/
SWIR LEDs for Silicon Wafer Defect Inspection
There are various types of silicon defect inspection, such as detection of cracks and internal defects in the wafers themselves, or the detection of defects in circuits formed with different materials on silicon substrates. All of these applications rely on the transmittance of SWIR wavelengths longer than 1100 nm through silicon.
Typically, defects are detected by acquiring a transmission image of the area to be measured. This could be an image of an entire wafer, or a small area measuring only a few microns. When the area of interest is that small, microscopic optics are an essential part of the detection system, while an InGaAs area sensor can be relied on to detect even the smallest defects.
SWIR LEDs illuminate the target area of the Si wafer and the transmitted image is captured by a photodetector, typically an InGaAs area sensor. In principle, any SWIR wavelength longer than 1100 nm will reliably transmit through silicon, but longer wavelengths may be used depending on the impurities in the silicon and the different materials that form the circuit.
A closer look at Ushio Epitex SWIR LEDs and their role in silicon defect inspection applications can be found at https://swir-led.com/silicon-defect-inspection/
LEDs for Optical Sorting Systems
Optical sorting is a system that detects the quality and/or presence of foreign substances based on spectroscopic information. Most importantly, manufacturers of optical sorting systems are interested in the properties of light absorption, transmittance, and reflectivity.
In order to obtain spectroscopic information, illumination using a combination of LEDs with multiple wavelengths, from ultraviolet (UV) to visible and infrared (IR) is used. In particular, SWIR is used for the quality inspection of foodstuffs because this band of wavelengths is mostly absorbed by organic materials. A list of the key absorption wavelengths for organic materials is available at https://swir-led.com/optical-food-sorting/
Typically, for the measurement of transported objects, a large number of LEDs are arranged in a line perpendicular to the conveyor belt, or other transportation mechanism. The LEDs, each emitting a different wavelength, illuminate the object sequentially. The object is captured as a series of images, each contributing spectroscopic analysis. The image capture is achieved using a line image sensor, specifically an InGaAs line sensor when SWIR measurement is required, which is typically also installed perpendicular to the transport direction.
Based on predetermined quality attributes, the transported objects are assessed and selected for retention or rejection by the mechanical sorter, should they fail to meet the required standards.
SWIR LEDs for Road Surface Condition Sensing
Multiple SWIR wavelengths are emitted at the road by LEDs, with the reflective intensity of each wavelength measured by photodetectors. The photodetectors can be integrated into the same Ushio Epitex package as the LEDs or positioned nearby in their own package. The most important wavelengths in this application fall within the absorption bands of water and ice, such as 1450 nm and 1500 nm.
Both 1450 nm and 1500 nm are absorbed by liquid water, but 1450 nm has a greater absorption level. In the case of ice, however, both 1450 nm and 1500 nm are absorbed, but this time, more of the 1500 nm wavelength is absorbed. Based on the relationship between the strength of the absorption, it is determined whether the target is water or ice.
A third wavelength, neither absorbed by water or ice (e.g. 1100 nm), is required to serve as a reference point for the amount of light reflected from the road itself. The relative intensity of the reflected light is used to accurately determine the condition of the road surface. To measure the intensity of each wavelength, multiple Ushio Epitex photodetectors are fitted with wavelength-specific band-pass filters.
Alternatively, a single photodetector can do the job, but only if each wavelength is emitted sequentially, rather than simultaneously. The timing of the lighting sequence can then be used to identify which wavelength is being reflected in each sensor image. Once the data has been successfully captured, an algorithm is used to process the received information, with the end goal of determining the road surface condition.
More information on the role of SWIR LEDs in road surface condition analysis applications is available
Contact Ushio about SWIR LEDs for Sensing and Inspection Applications
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