Atelier Bonryu(E)
infrared photography
Laboratory: Infrared Photography
False Color Infrared Photography
3-2 Filters and False Color
Infrared Light, Sensor of Camera, and Filter: When we take a photograph by fitting an infrared filter (transparent to the infrared light and opaque to the visible light) in front of a lens, a something dull colored photograph is obtained. This dull colorization is considered to be due to the camera itself and/or the setting for the photographing such as white balance setting. Therefore, if photographs of a same object is taken by two cameras different color development of the two photographs may be observed. This phenomenon occurs because infrared lights entering the R-, G- and B- channels are taken, respectively, as red, green, and blue lights, and the colored photograph is reproduced from the combination of these signals according to the setting of the camera. Though the filters of the R-, G-, and B- channels pass the red, green, and blue light very well, they pass also the infrared light a little (Remark #5). Therefore, for the incident light with wavelength within the infrared light finite amount of R-, G-, and B- signals are observed.
As the sensitivities of the R-, G-, and B- sensors decrease with increasing incident light wavelength from the visible light wavelength and also the differences among the sensitivities of these three sensors become small, which means the photograph composed of three images from these three sensors becomes monochromatic for the infrared light with longer wavelength. Therefore, a colorful photograph can be finally obtained for the infrared lights with shorter wavelength.
Infrared Filter and False Color: Within the wavelength range of the visible light the characteristics of the three filters R, G, and B of the Bayer filter are completely different each other. For the infrared light the difference of the sensitivities become smaller with the increasing wavelength as described above. Therefore, in order to change the impression of an infrared color photograph to change the hue of the color to make a final false color photograph is often very effective. Consequently, the channel swapping in the RGB color space and the tone inversion in the CIELab color space are promising methods to finish a beautiful false color photograph.
