Light is electromagnetic radiation. Light acts as a wave which has a frequency and a wavelength. The wavelength of light is the distance between corresponding points in two adjacent light cycles, and the frequency of light is the number of cycles of light that pass a given point in one second. Wavelength is typically represented by λ, while frequency is represented by ν, the lowercase. Wavelength has units of length (meters, centimeters, etc.), while frequency has units of per second, written as s−1 and sometimes called a hertz (Hz).

Spectrum is the term used to describe the distribution of electromagnetic waves. Only a small part is visible to humans in the range between 380 and 780 nanometres (nm) – this is what we call light. Colloquially, we talk about “visible light”. The spectral distribution of a light source determines its light colour and its colour rendering. Depending on the method of generation of the light, basic types of spectra can be distinguished between: the continuous light spectrum (daylight, thermal radiators and white LEDs), the line spectrum (low pressure discharge lamps) and the band spectrum (high pressure discharge lamps).

The electromagnetic spectrum is a range of wavelengths of electromagnetic radiation. From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma rays.

The above shows the wavelength range for each region we talk about in Light as 400nm -700nm Visible Light, 700nm to 1000nm Near Infra Red, 1000nm – 2300nm SWIR, 3um to 5um MWIR, 8um to 14um LWIR.

The above image shows the importance of knowing how these IR wavelengths behave in terms of reflection and Thermal.

Short-wave infrared (SWIR) light is typically defined as light in the 0.9 – 1.7μm wavelength range but can also be classified from 0.7 – 2.5μm. Since silicon sensors have an upper limit of approximately 1.0μm, SWIR imaging requires unique optical and electronic components capable of performing in the specific SWIR range.

Silicon-based CCD or CMOS cameras have excellent sensitivity over the UV-to-NIR range, the bandgap properties of silicon prevent these sensors from having sufficient sensitivity over 1100 nm. To detect light beyond the visible spectrum, their photon-sensitive area has to be made of materials such as indium gallium arsenide (InGaAs).

SWIR wavelengths can only be seen by sensors such as Indium Gallium Arsenide (InGaAs). Images from InGaAs camera sensors are comparable in detail and resolution to visible light images but are not in color. This is one of the many advantages of SWIR as it allows objects to be more easily recognized and individually identified.

Unlike Mid-Wave Infrared (MWIR) and Long-Wave Infrared (LWIR) light, which is emitted from the object itself, SWIR is similar to visible light in that photons are reflected or absorbed by an object, providing the strong contrast needed for high resolution imaging. To achieve these high-resolution images, it is essential to use a lens that is specifically designed and coated for the SWIR wavelength range.

SWIR is the bandwidth most widely used for industrial and scientific applications.

As SWIR is about reflected light, it is important to have a proper SWIR Lighting on the objects based on the project need and object’s nature to see the invisible contents to our naked eye.

Short-wave infrared cameras enable various new applications or enhance current machine vision systems by imaging beyond the visible spectrum.

Examples are seeing through non transparent to human eye surfaces to visualize underlying features such as fill levels, hidden moisture or tamperproof security codes etc.

Additionally, using filters, wavelength-optimized optics or monochromatic light sources is convenient for capturing a distinct and a measurable contrast of the inspected object.

A popular SWIR application is moisture detection. Water is transparent to visible light, but it absorbs strongly at wavelengths of 1450 and 1900 nm, which makes it appear black in the image. With a corresponding filter or lighting, this ability can be used for various inspection tasks in the food and beverage, woodworking, textile or automotive industries: Examples include verification of coatings or dryness uniformity in bulk material, fill-level detection through non-transparent containers, detection of damaged or bruised fruit, and gauging relative water content in plants.

Other materials are opaque in visible light but transparent to infrared radiation. This property can be used for the non-destructive inspection of products. For example, SWIR cameras are used by syringe manufacturers to automatically check the presence of the needle by imaging through the protective cap.

Sample Image taken Visible camera and SWIR (SWIR shows the fill inside)

SWIR Range SWIR or Short-Wave InfraRed LED lights.

The SWIR is the spectral band typically defined between 1 and 2.5 μm. On the boundary between visible and thermal imaging, this light field has unique properties.

As it has the proximity to the visible range, this gives it all its imaging qualities in terms of contrast and resolution, and at the same time its infrared spectrum allows it to reveal phenomena and elements invisible to human eye like  transmission of materials like Plastics, silicon, etc., show elements like water, liquid etc., see through the smoke, fog and dust, day and night.

The SWIR range of Lights are now available for both Line Scan and Area Scan camera based  industrial and scientific applications.

Optics:

The purpose of the optic is to gather light, transmit as much as possible in the desired spectral band, and focus the rays on the correct points of the sensor in the camera. Many optical properties inherent to visible lenses can have severe consequences on image quality with a SWIR imaging camera.

SWIR optical components generally utilize borosilicate glasses, fused silica, MgF2, or sapphire. These materials exhibit high internal transmission of visible, SWIR, and even SWIR wavelengths for out-of-band applications. A typical SWIR broad-band AR coating (BBAR) can reduce reflection of the base material on a single surface from 4% down to <1% across the spectral band.

Many SWIR applications necessitate a single wavelength of light to maximize performance. Filters are commonly used to reject all other wavelengths of light, considered noise, and increase the signal-to-noise ratio (SNR) of the desired wavelength.

By ensuring the lens has been color-corrected for an application, the sharpest and most accurate images can be obtained.

For a successful project completion, understanding the object and project need towards usage of SWIR camera is the key. Even if SWIR Camera is preferred, it is important to get proper SWIR Lighting with needed wave lengths suitable for the project and also the exactly coated Lenses. Filters to focus only on single of band of wavelengths would help the project to eliminate unwanted being imaged. It should be always kept in mind that unlike MWIR and LWIR that works as temperature Imaging, SWIR works on light reflection.

Even in factories like in Hot steel, SWIR cameras can be used for monitoring the process where smoke and Fog hides the actual process along with visible color cameras. Through software, an integrated solution to analyse both visible and SWIR images can be done to arrive at conclusion on defects or scene or any needed result. Fruit, Vegetable and other food sorting would need such setups where both visible and SWIR setup used connected to a single software to analyse.

We Online Solutions (Imaging) Pvt. Ltd., Chennai India can provide solutions based on SWIR and Visible spectrum for any kind of projects. We represent  many companies across the globe for different product line in terms of cameras, lenses, lighting, software, accessories etc., With 3 decades of experience in the field of Vision, we can provide apt solutions.

Note: The above write up has collection of information and data from different web sites along with our contents. Objective of this blog is to provide a fair understanding about SWIR Imaging related components and some applications. This blog is not aimed at any commercial earning.

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