How Does Shortwave Infrared Imaging Enhance Visual Functions?

Most developers of machine vision and image processing systems have widely used cameras that work in the visible light band for quality control, process control, or applications that need to distinguish objects. Traditional visible light imaging uses broadband white light illumination, which allows the camera to see all visible colors. In order to observe certain colors more effectively, developers can also choose specific wavelengths of illuminating light, such as red light, blue light, or green light.

For this type of visible light waveband application, lighting system developers for machine vision and image processing applications have done a lot of excellent work. Therefore, there are many commercial lighting products in various forms and colors for users to choose from on the market. However, as developers turn to image methods in the non-visible light band, lighting has become a more challenging proposition.

The imaging shortwave infrared (SWIR) band (defined here as 750~2600nm) beyond the visible light band is a non-visible light band. Many materials that look very similar under visible light will look very different under SWIR light. The following is an introduction to the specific applications of using short-wave infrared imaging to enhance visual functions.

In the visible light band, today's silicon chip cameras can image in the wavelength range of about 400~1050nm, just beyond the visible light band into the near-infrared (NIR) band, imaging uses ready-made illumination light sources, wavelengths of 850nm and 950nm. However, using reflected light to achieve imaging beyond this band requires cameras with other imaging detector materials, such as indium gallium arsenide (InGaAs), mercury cadmium telluride, or indium antimonide cameras.

Because different atomic structures have different reflection characteristics in the SWIR band, the uncooled InGaAs camera provides a simple method for imaging in the 400~1700nm band, which can easily distinguish many materials that look similar in the visible light band. However, in the SWIR band, they look different from each other, especially relative to the color of the substrate.

Imaging of wavelength as a function of temperature can be achieved by reflection, in which case the object is illuminated by an external light source; or if the object is hot enough, imaging can also be achieved by measuring the emitted light of the object. When an object is illuminated, the object reflects photons from an external light source to the camera.

The illumination angle of the external light source, the wavelength of the light source and its components will directly affect the reflection characteristics of the object. In other words, the object will emit light under the irradiation of light of all wavelengths, but the amount of light emitted depends on the temperature of the object. A black body is considered an ideal radiator.

As the temperature increases, the light radiation becomes stronger, and the wavelength peak radiation shifts to shorter wavelengths. Since the emissivity is a function of the wavelength and the physical properties of the object, when an object (such as coal) emits red light, the object is obviously very hot and cannot be touched. An ideal blackbody at 500°C emits enough energy in the red light band of 700~750nm to be seen by the human eye.

When the temperature is lower than 500°C, although the object may still be too hot to touch, the human eye can no longer see red light, because the combination of red light emission and emissivity makes the amount of luminescence too low to show red.

Using infrared cameras with a detection range of more than 750nm wavelength can easily detect hot objects. Usually, this is achieved with a long-wave infrared (LWIR) or mid-wave infrared (MWIR) camera, although it can also be achieved with a SWIR camera, depending on the temperature of the object.

In the visible light band, an external light source is required to illuminate the object to make the object hot enough to emit the number of photons required by the visible light camera to detect it. During imaging, the photons emitted by the external light source are either reflected by the object or absorbed by the object.

Use external light sources of different colors to illuminate the object, the object is displayed as bright or dark, and the degree of brightness depends on the amount of reflection of the illuminating light by the object. In the bulb, the filament is heated to a high temperature of about 2800K, and emits many photons in the visible light range, so that people can see objects in the room.

The temperature of the soldering iron is lower than 300°C, and the hair and skin under SWIR light irradiation are different from those under visible light irradiation. Although the bulb has a luminous curve similar to that of the sun, its temperature is lower, so the overall amount of luminescence produced at any given wavelength is less. More importantly, the bulb produces more photons in the red band than in the blue and green bands.

This is why objects appear redder indoors than when illuminated by sunlight because the ratio of red photons to blue or green photons in the spectrum emitted by the sun and light bulbs is very different. This is why objects look different indoors than outdoors because the emission spectrum of the bulb is different from that of the sun, so the amount of light reflected by the object is also different.

Incandescent bulbs (such as products with a power of 60W) may no longer be used, and most modern machine vision lighting will be provided by light-emitting diodes. LEDs are semiconductor devices that can emit light with a specific wavelength, or light with a narrow emission spectrum, usually within ±50nm around a specific wavelength (±15nm for many high-end LEDs).

In the visible light area, there are already many types of LEDs that can provide the correct color lighting for the scene. The light source used for machine vision usually needs to specify the color or color temperature (such as 6000K red, green, blue or white light, etc.). 6000K white light lux or illuminance is used to characterize the brightness of light, or how many photons a white light visible light source emits.

For monochromatic or multi-color LED light sources, light is characterized by the unit mW/cm2, and lux is the measurement unit of white light. By combining multiple LED colors, white light or light of various color temperatures can be simulated.

LEDs try to simulate black bodies at these temperatures by changing the peak wavelength. However, for the new generation of SWIR machine vision cameras, visible light LED lighting is completely ineffective. Depending on the type of image sensor used, the sensitivity range of SWIR cameras is usually 400~1700nm or 1000~2600nm.

In the past, 6000K black bodies could be used as a very effective broadband light source for these cameras, but today's white LED light sources generally do not provide any light output exceeding 785nm.

The machine vision lighting industry has developed near-infrared light sources, usually using LEDs with a wavelength of 950nm or 850nm, and the imaging has exceeded the range of visible light. However, these light sources are limited to only one or two wavelengths.

These infrared light sources can make the imaging range of a silicon-based visible light camera beyond the visible light reach the near-infrared range. However, in order to make the imaging range beyond the NIR to SWIR range, a new light source that emits a longer wavelength than the existing visible light LED is required.

The new low-cost SWIR line scan camera manufactured with an InGaAs image sensor has a photosensitive range of 400~1700nm, which spans the visible light band and SWIR light. As mentioned earlier, incandescent lamps can provide illumination for these cameras because they are essentially blackbody sources. Depending on the glass that wraps the bulbs, incandescent lamps emit light in a wavelength range of 400~2500nm or more.

However, since the market has tended to LED light sources, the wavelength of most light sources is specific, usually with a wavelength bandwidth of ±50nm. Although people can think of these white light visible LED sources as broadband in the visible spectrum, they are probably not broadband in the NIR and SWIR bands because they are a combination of many LED colors.

The choice of light source in SWIR is critical to achieving a successful imaging solution. The SWIR band can distinguish and detect defects that are invisible to the naked eye. In order to image a given feature, the developer must understand how to choose the correct light source.

Standard visible light LEDs will fail, so it is important to understand the reflective properties of the material being analyzed in the image. This helps to select the correct light source and camera, thereby simplifying machine vision applications and eliminating the need for expensive and intensive processing algorithms in the visible light band.

The SWIR InGaAs camera designed and manufactured by JAVOL has the advantages and disadvantages of day and night imaging and good concealment, which can effectively enhance the visual function. If you want to know more about SWIR cameras after reading the above, you can get a comprehensive solution by contacting us.

With its leading R&D technology and excellent manufacturing process, JAVOL has become a leader in the infrared thermal imaging system manufacturing industry. We are committed to providing users with high-quality products and thoughtful services. We are equipped with a professional quality inspection system and a comprehensive management system, which can fully control the quality and details of the products. At the same time, we will also provide effective solutions according to the diverse needs of users. If you are interested in our SWIR cameras, please contact us immediately!