Colorimetry and multispectral imaging in the shortwave infrared

Abstract

The typically used shortwave infrared spectral range (SWIR) between 900 nm and 1700 nm is a spectrally broader wavelengths range than the visible range. Available SWIR cameras generate a gray level image using the intensity over the entire spectral band. However, objects can exhibit completely different spectral behavior in this range. Plants have a high reflection at the lower end of the SWIR range and liquid water has a strong absorption band around 1400 nm, for example. We propose to divide the SWIR range into an appropriate number of spectral channels to extract more details from a captured image. To extract this information the proposal follows a concept similar to color vision of the human eye. Analog to the three types of color receptors of the eye four spectral channels are defined for the SWIR. Each point of the image is attributed now by four “color values” instead of a single gray level. For a comprehensive characterization of an object, a special SWIR colorimetry is possible by selecting appropriate filters with suitable band width and spectral overlap. The spectral sensitivity, the algorithms for calculating SWIR-color values, the discrimination of SWIR-color values by Noise Equivalent Wavelength Difference (NEWD) and spectral coded false color image display is discussed and first results with an existing SWIR camera are presented.