Infra® SWIR Lens Application Note
Short-wave infrared (SWIR) light is a second of the electromagnetic spectrum typically in the 0.9μm to 2.5μm (900nm to 2500nm) wavelength range and is therefore invisible to the human eye.
However SWIR light interacts with objects in a similar manner to visible wavelengths in that photons are reflected by the object itself. As a result of this reflective nature, SWIR light has contrast and shadows in the images it produces which are a necessary requirement for high-resolution imaging.
SWIR is unlike Mid-Wave Infrared (MWIR) and Long-Wave Infrared (LWIR) light, which is emitted from an object as thermal IR (heat) and therefore has lower resolution and shows less detailed images.
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.
Operation Principle
To achieve these high-resolution images, it is essential to use a lens that is specifically designed and coated for the SWIR wavelength range.
Using a lens designed for the visible spectrum will result in lower-resolution images and higher optical aberrations. Since SWIR wavelengths are transmissible through glass, then lenses and other optical components (optical filters, windows, etc.) designed for SWIR can be manufactured using the same techniques used for visible components. This leads to lower manufacturing costs and enables the use of protective windows and filters within the optical system.
Applications
A large number of applications that are difficult or impossible to perform using visible light are possible using SWIR. For example, monitoring the water content within agricultural products is a key aspect in measuring the maturity and health of the produce. Using a SWIR camera, crops can be inspected and examined for quality. The camera detects moisture within the product by highlighting water-saturated areas. Areas with higher water content will appear darker in comparison to the drier sections. These images show bruising below the surface of the skin of an apple that is undetectable by the eye but can be seen using SWIR.
