3.1)

Vegetation and Wood effect

In the description of the first recording of NIR photos of natural scenes in 1910, Wood already noted that NIR photos are quite different in appearance from ordinary photographs, in particular due to the extreme brilliancy of the trees and grass. This often gives the impression of seeing snow-covered landscapes. In honor of Wood’s pioneering work on NIR photography, the effect of green vegetation appearing extremely bright is called the Wood effect [9]. Fig. 3 compares visible (a) with NIR (b) photos of a scene containing houses, sky, clouds, and trees with green leaves. The most obvious difference between the two photos is indeed much brighter vegetation.

Fig. 3:

Urban scene in VIS (a) and NIR (b) photos showing the bright scattering effect of vegetation in the NIR (Photos K. Mangold). In addition enhanced sky contrast and also the differences of reflection in the windows of the right-hand-side house are obvious. c) Reflection spectra of two green and one red leaf measured with an integrating sphere detector that gathered light from the complete backward hemisphere. The minimum thickness of the leaves was around 100 µm for the green and 200 µm for the red leaf.

Although Wood described the phenomenon of bright NIR radiation scattering by vegetation, he did not explain it. This was done later by others. Most probably, light enters the epidermis of leaves and is scattered in deeper sections where there are many cellular walls of interior cells separated by air inclusions. This model is strongly supported by ray tracing models [10]. Multiple scattering events finally give rise to very strong backscattering, similar to VIS light back-scattering from snow-covered ground, where multiple scattering also leads to effective backscattering that yields very bright natural scenes.

It is quite easy to verify the strong NIR backscattering in leaves by recording transmission and reflection spectra of sunlight through or from green vegetation. Fig. 3c shows some examples of reflection spectra (R) for light and dark green, as well as dark red, leaves. These spectra were recorded with a UV-VIS-NIR Lambda 900 spectrometer (Perkin Elmer).

It is obvious that the absorption features of chlorophyll, which are responsible for photosynthesis, are only important in the VIS spectral range. At NIR wavelengths, there is much less absorption and the high reflectivity is due to diffuse scattering of radiation from index of refraction inhomogeneities within the cells, such as interfaces due to cell membranes, water accumulation, etc. For leaves of 100 to 200 µm thickness, transmission can still be of the order of 50% in the NIR range and a similar amount is scattered backwards. Interestingly, the NIR behaviour appears to be independent of the visual colour of the leaf (Fig. 3b).

Because the difference of vegetation reflectance at VIS and NIR wavelengths depends strongly on chlorophyll content and plant health, it is readily possible to use NIR images to distinguish between healthy and unhealthy vegetation – even before such differences become apparent by eye (more details and refs. see [4]).

3.2)

Colored Liquids

Many liquids that have very dark colors in the visible spectral range can appear colorless and highly transparent in the NIR. Fig.4 depicts an example of a bottle and a glass filled with a red wine. Three different images were recorded, one with normal IR cut-off filter (a: for λ=400 to 700 nm), another with an IR-transmitting filter (c: for λ = 830 nm to 1100 nm), and one recorded with the whole spectral range from 400 to 1100 nm (b). The VIS image looks familiar, showing a deep red color for the wine. In contrast, the NIR image makes the impression of a photo of water in a glass and clear bottle recorded in black-and-white mode, i.e. both the red wine and the green glass have turned completely transparent in the NIR range.

Fig. 4:

Images of a bottle and glass filled with red wine recorded for VIS light only (a), whole range VIS and NIR (b) and IR only (c). Photos K. Mangold.

Detecting VIS plus NIR radiation simultaneously leads to a mixture of both: the green bottle still looks a bit green and the wine in the glass still has a red tint, though not very pronounced; i.e., the IR contribution dominates the appearance in the photo. This is an excellent example of why IR cut-off filters are used in digital photography: any observer of the middle photo would be extremely puzzled and at best expect to have a wine mixed with water. Similar images can also be recorded for other liquids, e.g. diet Coke or coffee.

The explanation of this change from nearly opaque to transparent liquids is simple when studying transmission spectra of water compared to red wine (Fig. 5). The spectra were recorded for liquids in 1 cm glass cuvettes, hence the maximum transmission amounts to about 91% because of reflection losses. Red wine shows strong absorption features in the VIS range, with a steep increase of transmission for longer wavelengths. Similar to water, it is transparent in the NIR up to about 1400 nm. Thicker liquid samples, such as 5 cm for a glass or bottle, of course show appreciably lower transmission.

Fig. 5:

Transmission spectra of red wine and water for thickness of 1cm.

Fig. 6 depicts a similar scene as in Fig.4. Two glasses are partly filled from bottles of red wine (left) and water (right). These photos were recorded with a VIS camera for a λ-range of 400 to 800 nm (a), a NIR camera in the range of 830 to 1100 nm (b), and the InGaAs camera (c) from 900 to 1700 nm. The appearance of the liquids differs appreciably from one photo to the next. In the VIS, we have the expected behavior of transparent water but nearly opaque (i.e. very dark) wine. In the NIR, similar to Fig. 4 and explained by the spectra of Fig. 5, both water and wine look similar and are transparent. For the red wine this becomes more obvious if looking through 7.5 cm wine in a rectangular container (Fig. 7). The VIS image does not allow an observer to read the text behind the liquid, whereas this poses no problem for the NIR camera. Coming back to Fig.6c, the InGaAs image – if compared to the NIR image of Fig. 6b –shows both liquids as nearly opaque, which is surprising at first glance.

Fig. 6:

Scene similar to Fig. 4 with two bottles and glasses filled with red wine (left) and water (right). a) VIS photo, (b) NIR photo recorded with Canon EOS1000 for NIR filter at cut-on wavelength 830 nm and (c) InGaAs photo..

Fig. 7:

VIS (a) and NIR photo (b) of text observed through 7.5 cm of red wine.

According to the spectra of Fig.5 and the detector sensitivities (Fig.1), one may have expected a transmission similar to that of the NIR camera. The explanation is simple. The signal is due to the detector sensitivity, which for InGaAs is still rising for the small wavelengths where the 5 cm thick wine samples still have some transmission. As a consequence, the detector-waveband-integrated transmission signal of wine and water only amounts to a few percent of the transmission of a nonabsorbing specimen; i.e., it must appear very dark.

3.3)

Looking through paint

There are some objects, such as paints and varnishes, which are visually opaque but yet show high transmission in the NIR. This feature has already been used as a physics trick in demonstration experiments [11]. Fig. 8 depicts spectra of standard transparencies used for demonstrations on overhead projectors and others covered with black overhead markers. The ink from overhead transparency markers makes it impossible to look through a transparency or – if marked on paper – to see the underlying text in the visible spectral range (Fig. 9a). In contrast, this is possible in the NIR, due to its transmission above 800 nm (Fig. 9b,c). These examples can be demonstrated nicely in the classroom.

Fig. 8:

Spectra of clear transparency and the same transparency partly covered with the same ink.

Fig. 9

VIS (a), NIR (b) and InGaAs (v) photo of black text from a laser printer on regular paper observed through various colored text markers. In the VIS, one may just have a chance to see through the blue color. In contrast the NIR, and even better, the InGaAs photo allows an observer to read the text very clearly.

3.4)

More examples

Besides the Wood effect, Fig. 3 also showed partly cloudy and hazy skies. Apparently, there is a much higher contrast between the clear sky and the clouds for the NIR image. This is due to the scattering characteristics of molecules and the much larger haze or cloud droplets. Molecular scattering by atmospheric gases follows Rayleigh’s law, i.e. a rapid (1/λ)⁴ decrease with wavelength. In contrast, much larger cloud droplets scatter radiation according to Mie scattering, which has a less pronounced wavelength dependence. The consequence of higher NIR contrast between dark clear sky and brighter clouds was discussed more quantitatively from spectra of scattered sky light and diffusely scattered cloud light (see [4]).

Other spectacular effects of NIR photography become apparent when analyzing photos of people. The skin behaves quite different in the VIS and NIR ranges (see Figs. in ref [4,5]). Human tissue is a highly inhomogeneous material containing chromophores, blood vessels, and light-scattering particles and fibres whose sizes differ appreciably. Therefore the optical properties of biological tissue in general, and human skin in particular, must take into account a large amount of multiple scattering events within the various layers of the skin. As a result, the 1/e penetration depth of optical radiation can become much larger for NIR radiation than for VIS light. For example, penetration depth values can range between 90 µm for blue light (λ = 400 nm) and 1.6 mm for NIR radiation (λ = 1000 nm). As a consequence, veins may be visualized even if they are a few mm below the surface [12]. Image contrast is due to a combination of NIR scattering in the tissue and absorption by hemoglobin in the blood vessels. This phenomenon has been applied [12] to enhance localization of subsurface blood vessels needed for blood withdrawal from small children. The blood vessels were usually located at depths from 1 to several mm and the method worked successfully in hands or wrists and also at the inside of the elbow of children. High-power NIR LED’s around λ=850 nm were used underneath the puncture sites and images were captured from above.