Atelier Bonryu(E)

infrared photography


Laboratory: Infrared Photography

1-3 Features and Applications of Infrared Photography


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Features of Infrared Photography: The infrared light which has longer wave length in comparison with the visible light shows various features different from those of the visible light.  These features are utilized for various purposes.  These features are summarized as follows.

  1. 1)We, the human beings, cannot perceive the infrared light directly.

  2. 2)In various situations, the infrared light has higher penetration power than the visible light.

  3. 3)Surfaces of the objects have broad-ranging reflectivity for the infrared lights with different wavelength, and therefore a colorful infrared photograph is finished like a visible color photograph by assigning proper colors to color channels.

In the following we describe how these features affect the infrared photography.

We Cannot Perceive Infrared Light: Not only a human being but also most creature cannot perceive the infrared light (though some kinds of snakes can perceive the infrared light).  Therefore, it is convenient to observe night behaviors of animals unnoticed by using an infrared camera.  Cameras specialized for this purpose (a trail camera) are placed in the market.  As the human being cannot perceive the infrared light this kind camera is utilized also as a security camera.

Infrared Light Has High Penetration Power: Because the penetration power of the infrared light depends, of course, on the wavelength and a penetrated substance the high penetration power is not always realized.  But there are various application of the infrared photography making use of this feature.

At first, it is pointed out that the landscape in the dim distance can be clearly photographed because the infrared light of long-wavelength is scarcely scattered by water drops in haze or fog and can arrive at the camera from a long distance without large attenuation.  Figures 1-5, 1-6 are, in the respective order, the visible light and the infrared photographs of the peaks of the Tateyamas taken from the Amaharashi-Kaigan (coast) of the Noto Peninsura, about 50 km apart from the peaks.  On a bright day we can see the peaks clearly but I took the photographs on a foggy day and the peaks were almost invisible (Fig.1-5).  In the infrared photograph, however, the peaks are clearly seen (Fig.1-6).

Fig.1-5 The peaks of the Tateyama photographed at the Amaharashi-Kaigan (coast) of the Noto Peninsula (a visible light photograph).

Fig.1-6 The peaks of the Tateyama photographed at the Amaharashi-Kaigan (coast) of the Noto Peninsula (an infrared photograph).

The high penetration power of the infrared light is utilized and studied actively also for the medical application.  For example, by using the feature that the infrared light interpenetrates deeply into the skin, photographs of foreign matters or veins under the skin are taken for the medical researches.  Relating to the research on such problems the penetration of the infrared light through a cloth covering a skin is also studied.  According to such studies it turns out that the reflectivity, absorptivity, and penetration efficiency of the infrared radiation do not depend on the color of the cloth itself but on the material of the cloth.  For the infrared light to penetrate through a substance it is necessary the substance must be thin and its penetration efficiency of the infrared light is high enough.  As for a leaf of a plant it is often the case that there appears the infrared light which penetrates a leaf and reflects at the surface of another leaf closely contacted the leaf on the front.  Figures 1-7, 1-8 are, respectively, the visible light and the infrared photographs of lily pads in a water bowl.  Lily pads are uniformly thin over a large area and the surfaces of overlapped pads are appressed closely, and moreover, the reflected infrared light from the pads on the back is very intense because of the chlorophyll in the pads.  Though in the visible light photograph (Fig.1-7) only pads on the front are seen, pads on the back are also seen in the infrared photograph (Fig.1-8).

Fig.1-7 Lily pads in a water bowl (a visible light photo )

Fig.1-8 Lily pads in the water bowl (an infrared photo )

Reflectivity of Infrared Light Depends on Photogenic Object: All the infrared photographs in this web site were taken by calling this feature of the infrared light into play.  Then how this feature emerges in an infrared photograph?

The most characteristic appearance of an infrared photograph is that leaves of trees or green grasses glow brilliantly and become snow-white as if snow flies.  This is due to the fact that the infrared light is reflected intensely by chlorophyll in the leaves, and this phenomenon is called as Wood’s effect.  The Wood’s effect was named after the prominent American physicist, Robert William Wood (1868 - 1955, Remark #2).  Generally, leaves of a plant reflect a green light with the wavelength of 500 - 600 nm but absorb lights with other wavelength in the range the visible light.  But in the range of the infrared light, reflectivity of lights with the wavelength of 750 - 1300 nm are large.  It is the matter of course the reflectivity depends strongly on the kind of the plants.

On the one hand, concerning water, absorption of light starts from the wavelength of about 800 nm and at 900 nm the reflectivity becomes almost null.  Therefore, water looks black in an infrared photograph with long wavelength (Fig.1-9).

This feature of the infrared photography is of assistance to investigate vegetation on the ground by a satellite photograph.




Fig.1-9 Pottery hexagram on the bottom of a water-filled bucket, photographed by visible light (a), infrared light of wavelength more than 850 nm (b), and  infrared light of wavelength more than 950 nm (c).


This feature of the infrared photography is of assistance to investigate vegetation on the ground by a satellite photograph.  Figure 1-10 shows the visible light photograph (left) and the infrared photograph (right) of a sheet  of paper on which character strings “Infrared Photography” are printed by various colors (black, red, green, blue) using an ink jet printer.  Moreover, the  character string printed black is concealed partly by different “filters”.  The “filters” are, an exposed and developed negative color film, the visible-cut infrared-transparent filter IR680, and the UV-IR cut filter.  Some features derived from these infrared photographs are summarized as follows.

At first, in the infrared photograph the character string printed by the red ink does not appear but character strings of other colors can be seen.  This is because the reflectivity to the infrared light for the red characters is very large and almost the same as that of the white paper of the background.  In the next place, we can understand that there are various materials with different wavelength-dependent transparencies.  In the case of Fig.2-10, for example, the exposed and developed negative color film completely blocks the visible light but transmit the infrared light to a certain extent.  This means that something hidden by a visibly opaque materials can be seen by utilizing the infrared photography.  Therefore, this feature of the infrared light is used for investigations or researches of cultural assets and antiques.  On the other hand, the visible-cut infrared-transparent filter, as a matter of course, blocks completely the visible light and transmit completely the infrared light, and when the visible-transparent UV-IR cut filter is put on the character s we cannot see the character string in the infrared photograph.

Fig.1-10  Effects of color and inserted materials on an infrared photograph.  (Left) A visible light photograph, (right) an infrared photograph.  The materials put on the sheet (filters) are, from left to right, a developed color negative film, a visible-cut infrared transparent filter, and a UV-IR cut filter.

Among the above features of the infrared photograph it is often found that the feature that a certain material reflects the infrared light very well and the corresponding image become white and cannot be distinguished from the surrounding shining objects.  Figure 1-11 shows an example of this phenomenon.  This is a photograph of the Japanese national flag of the Rising-Sun which was fluttered in the sky of  the Izumo-Taisha shrine.  We can see the Rising-Sun in the left subfigure of the visible light photograph, but in the right infrared photograph we cannot see the Rising-Sun because the red Rising-Sun reflect the infrared light very well and shines similarly as the white part of the flag.

Fig.1-11  The Japanese National Flag of the Rising-Sun fluttering in the sky of the Izumo-Taisha shrine. (Left) a visible light photograph, and (right) an infrared photograph.