Atelier Bonryu(E)
infrared photography
Laboratory: Infrared Photography
I. Digital Infrared Photography
1-1 Infrared Light
Fig.1-2 Wavelength of light and corresponding color
Wavelength of light: Human eyes are sensitive to electromagnetic waves with wavelength between about 400 nm (violet light) and about 700 nm (red light), where 1 nm is 0.000,000,001 m. Therefore, these electromagnetic waves are called visible lights (Fig.1-2). The electromagnetic waves in the wavelength region next to the long wavelength boundary of the visible light (from about 700 nm to 1 mm=1,000,000 nm) are called infrared lights. On the other hand the electromagnetic waves with the shorter wavelength (from about 400 nm to 10 nm) are called ultraviolet lights. Both the infrared and the ultraviolet lights are invisible. Boundaries of these lights are not strictly defined. According to the definition described in the JIS Z8120, for example, a visible light or a visible radiation is defined as an electromagnetic wave with wavelength from 360 - 400 nm to 760 - 830 nm. The reason why eyes of living beings are sensitive to the visible lights is considered as the solar radiation includes most the electromagnetic waves with the wavelength in the above region. In this site we define conveniently the wavelength range of the visible lights as from 400 nm to 700 nm.
Infrared light: Our surrounding is filled with invisible infrared and ultraviolet lights originated from the solar lights. Usually the infrared or the ultraviolet lights in the solar lights are reflected at surfaces of some objects and such reflected lights are observed. If human eyes can perceive these invisible lights we can see an image constructed by these reflected infrared or ultraviolet lights, and we will see a considerably different surrounding scenery because objects around us reflect different quantity of lights depending on the wavelength of the incident lights. Though, in reality, we cannot perceive directly such “invisible” lights, a sensor of a digital camera can detect the infrared or ultraviolet lights (Fig.1-3) and, therefore, by using such a digital camera we can take a photograph of photogenic objects reflecting the infrared or ultraviolet lights. Though the difference between the images taken by the infrared lights and the visible lights may not be necessarily very large, the human eyes make the slight difference out without thinking and we feel a very funny-peculiar impression from an infrared photograph.
Complete Guide to Digital
Infrared Photography
(English Edition)
Handbook of
False Color Infrared Photography
(Japanese Edition)
Infrared photographs have been taken since a century ago by using an analogue camera. It was rather messy to take an infrared photograph by a film and this kind of a photograph was used for specialized purposes rather than creating an artistic product. By using a digital camera , however, it became rather easy to take an infrared photograph and to retouch it as one pleases with the advance of computers and softwares for the image processing. In this web site I would present basic knowledge on the infrared light and the image sensor, photographing method of the infrared photograph, and the post-processing methods.
Light and Electromagnetic Waves: Though a light behaves both as a particle and as a wave, it may be rather easy to understand the photographic image formation by thinking that the light is a wave. This wave is an electromagnetic wave which propagates in a vacuum without any material unlike a wave of water or a sound wave in air. The electromagnetic waves are classified by the magnitude of the wavelength (Fig.1-1) or the oscillation frequency (=the light speed/ wavelength) and there are various kinds of electromagnetic waves with different wavelengths which belong to different classes and have different features. Radio wave, light wave, X-ray, and gamma ray are kinds of the electromagnetic wave. In these electromagnetic waves, the longest wavelength of ELF(extremely low frequency) radio wave (100 Mm: mega-meter) is 20th power of 10 times the shortest wavelength of gamma ray (1 pm: pico-meter). Among these electromagnetic waves, especially, the waves with the wavelength from about 10 nm to about 1,000,000 nm (= 1 mm, nm: namo-meter) are called as the light, and the light wave is further subsectioned as the ultraviolet , visible, and infrared lights.
Fig.1-1 A wavelength
As described above the wavelength range of the visible light is from about 400 nm to 700 nm, which means the maximum wave length is twice as large as the minimum wavelength. As opposed to it the wavelength ranges of the ultraviolet and the infrared lights are 10 ~ 400 nm (40 times) and 700 ~ 1,000,000 nm (about 1,500 times), respectively. Because of these large ranges of the ultraviolet and infrared lights these ranges are subdivided further to smaller ranges as the vacuum ultraviolet region (10 ~ 200 nm), the middle ultraviolet region (200 ~ 300 nm), the near ultraviolet region (300 ~ 400 nm), the near infrared region (700 ~ 2,500 nm), the middle infrared region (2,500 ~ 4,000 nm), and the far infrared region (4,000 ~ 1,000,000 nm).
The infrared light used for the infrared photography described in this site is a lower wavelength part of the near infrared region (700 - 1000 nm). It should be noted the infrared thermography is different kind of the infrared photography and it will not be described in this site. For this thermography the middle infrared light emitted from the object itself is used. On the other hand an object of the infrared photography in this site does not emit the infrared light and for the photography the solar light reflected at the surface of the object is used.
In Japan there are not many photographers who find a pleasure in the digital infrared photography at the present time but in US and European countries it is more popular among photographers and there are published some good books for introducing the digital infrared photography written in English, which will be described later in the References page. Infrared photographs leave us “bizarre”, “weird”, or “unearthly” impressions. One of the reasons bringing such impressions is because unnatural colors of photogenic objects are introduced because there are no “valid” color corresponding to the “invisible” lights with wavelength outside the range of the visible lights. On the other hand, I think, in an infrared photograph only slight difference of distribution of light intensity which is too slight to be sensible except for a special object as a shining foliage by infrared lights, but anyway different from the familiarized color and shape of an object may cause the funny-peculiar impression by contrast.
Fig.1-3 A schematic graph of dependences of sensitivities on light wavelength of sensors of digital cameras (CCD: blue and CMOS: red) and a human eye (green). Though the human eye is sensitive to the light with wavelength of about 400 to 700 nm, CCD and CMOS sensors are sensitive to the light with wavelength from about 300 nm to more than 1100 nm.
