Hi All,
the only solution to do low-cost multispectral imaging at high spatial resolutions is to use two reflex or mirrorless cameras. Whatever LDP LLC (MaxMax) or other companies claim, the current Bayer patterns that cover the silicon sensor make it impossible to create a decent (i.e. spectrally pure) NIR - Red - Green camera.
Some years ago, such an approach was tried by the Eastman Kodak Company with their now discontinued Kodak DCS-200 CIR, DCS-420 CIR and DCS-460 CIR. Although their spectral response approximately matched Kodak’s NIR-sensitive emulsions, both these film emulsions and these digital cameras acquired spectral information in very broad wavebands, hence taking significant portions of the other spectral bands into account: the red sensitive photosites (and to a lesser extent also the green channel) take large portions of the NIR into account, a drawback Kodak counteracted by subtracting the NIR’s Digital Numbers (DNs) from the initially captured green and red DNs. However, this approach could never yield pure spectral information.
Because the spectral response curves do not even coincide on the long wavelength side, it is impossible to remove the precise NIR-contributing part of the green and red channel. Moreover, the NIR and red channel still took a significant portion of the red edge region into account, which made these broad band imagers less suitable for quantitative spectral analysis as they masked unique spectral features to a large extent. As also the analogue media were characterized by a low spectral fidelity, Kodak’s NIR-enabled digital cameras emulated the Kodak CIR film rather well, and were therefore often used in several vegetation studies.
Current digital sensor do, however, suffer from the same spectral overlap (see the figure of a bare Nikon D80 sensor that I measured several years ago). A two-camera system based on a NIR-modified with an unmodified camera can, however, deal with these issues. In this solution, the spectral fidelity of the acquired information is, obviously, superior to the CIR film and any current, full-spectrum modified camera.
A possible solution to capture CIR imagery with only one DSC was presented in the United States patent 20060066738, in which Hershey and Zhang proposed a Colour-Filter Array pattern consisting of four different coloured filters, three passing the blue, green and red visible bands, while a fourth is dedicated to transmit pure NIR or UV radiation. It is, however, highly questionable whether any company will ever market such a device, in which case a multispectral imaging system utilizing a multitude of cameras would still offer a higher resolving power and more possibilities in choosing spectral combinations.
Cheers,
Geert