Imaging the invisible using modified digital still cameras for straightforward and low-cost archaeological near-infrared photography

Abstract

Analogue near-infrared (NIR) photography has already been used a lot in both scientific and medical photography, in which cases near-infrared (NIR) radiation was mostly captured by InfraRed (IR) sensitive plates or film emulsions. However, its use in archaeology has remained rather restricted, most likely due to some ignorance and/or lack of knowledge about this kind of photography, while the critical imaging process also severely limited its use. This situation could be, however, changed completely, as the image sensors used in digital still cameras (DSCs) are very sensitive to NIR wavelengths, making the quite lengthy and error-prone film-based NIR imaging process obsolete. Moreover, modifying off-the-shelf DSCs even simplifies this digital acquisition of NIR photographs to a very large extent.

By starting with a general outline of the ElectroMagnetic (EM) spectrum and the specificities of NIR radiation, the base is laid out to tackle the possibilities and practicalities of archaeological NIR imaging, subsequently comparing the earlier film-based approach with the digital way of NIR shooting, showing how the latter can greatly benefit from modified compact, hybrid and small-format Single Lens Reflex (SLR) DSCs. Besides in-depth information on the technique of digital NIR photography, examples will illustrate its archaeological potential.

Introduction

Since the astronomer and composer Sir Frederick William Herschel (1738–1822) discovered in 1800 the InfraRed (IR) portion of the ElectroMagnetic (EM) spectrum, a lot of scientific disciplines have become fascinated by this kind of invisible radiation. This interest even increased after World War II, as Colour InfraRed (CIR) emulsions had shown their capabilities. These days, IR photography is used a lot in forensics, astronomy, aerial survey, (bio)medical and several other types of scientific photography, although its use in archaeology is rather disappointing. Because this can largely be attributed to a lack of knowledge about the technical specification of near-infrared (NIR) radiation and the concept of EM radiation, the specific character of NIR will first be explained before tackling the possibilities and practicalities of archaeological NIR imaging.

The light that makes the Human Visual System (HVS) perceive the world around us is in fact an EM wave, comprising two oscillating magnetic and electric fields perpendicular to each other as well as perpendicular to the direction of propagation (Slater and Frank, 1974, Waldman, 2002) and travelling at 299792.458 km/s in vacuum, a speed that decreases when light travels in air, glass, water or other transparent substances (Young and Freedman, 2004). This explanation, put forward almost two centennials ago by the Scottish physicist James Clerk Maxwell (1831–1879), was based on the findings of the Dutchman Christiaan Huygens (1629–1695), who already declared light to travel in the form of waves (Clegg, 2001, Waldman, 2002). Being a wave phenomenon, the wavelength (λ) is the most important characteristic of EM radiation.

The EM waves humans perceive – the so-called visible light – encompasses a very small portion of all EM radiation: only wavelengths between approximately 380 nm and 750 nm (Fig. 1), the absolute thresholds varying from person to person and specific viewing conditions. However, on both sides of this extremely small visible spectrum resides EM radiation the HVS is insensitive for, characterised by wavelengths smaller than 380 nm or larger than 750 nm. Just as visible light, these wavebands were divided into spectral regions and given names as gamma rays, X-rays and UltraViolet (UV) rays on the short-wavelength side, while IR rays, microwaves and radio waves can be found in the long-wavelength region (Fig. 1).

The aforementioned IR portion of the EM spectrum comprises wavelengths between 750 nm and 1 mm, hence spanning three orders of magnitude. As the width of this particular band is exceptional, it is often subdivided into several zones, for which the limits (and also the number of subdivisions) are to a certain extent dependent on discipline and largely varying through literature. In general, the following breakdown can be used (based on Daniels, 2007, Deutsches Institut für Normung, 1984):

• Near-infrared (NIR) from 750 nm to 1400 nm (1.4 μm);

• Short Wavelength InfraRed (SWIR) from 1.4 μm to 3 μm;

• Mid Wavelength InfraRed (MWIR) from 3 μm to 6 μm;

• Long Wavelength InfraRed (LWIR) from 6 μm to 15 μm;

• Far/Extreme-InfraRed (FIR) from 15 μm to 1000 μm.

Moreover, EM radiation can also be thought of as a travelling bundle of particles. Although this theory was contested by many since its launch by the Englishman Isaac Newton (1642–1727), Albert Einstein (1879–1955) finally demonstrated the existence of such discrete energy packets, now called photons (Clegg, 2001, Waldman, 2002). This wave-particle duality is still one of the key concepts in quantum mechanics, signifying EM radiation exhibits both wave and particle behaviours. Depending on the wavelength of the EM radiation, the energy of the photons differs. Applied to NIR, it means this type of radiation contains less energetic photons compared to visible light.

There exists a lot of confusion and misconception about NIR imaging, often linked with images like the ones displayed in Fig. 2.

However, these so-called heat images were yielded by electronic thermography, a technique based on a completely different part of the EM spectrum. Heat imaging uses the MWIR and LWIR (Richards, 2001), energy given off by all real-world objects (Barnes, 1963). As a matter of fact, all objects with a temperature above absolute zero (0 K or −273.15 °C) emit EM radiation, but the type and amount of the latter largely depends upon the temperature of the matter. A healthy human body with a temperature of 310 K (circa 37 °C) gives off wavelengths with a peak around 9350 nm (LWIR) and no detectable amounts of NIR energy. With a rising temperature, the amount of radiated EM radiation will increase and the wavelength of maximum emittance λ_max will be shorter (as described by Wien's displacement law). Practically, objects need to be heated to about 500 K before they start radiating in the NIR range, while a temperature of at least 800 K (e.g. an electric stove burner) must be attained before visible red light is emitted (Barnes, 1963, Ray, 1999). Because it is not possible to photograph IR radiation but the NIR portion with conventional film-based approaches or digital photo cameras, performing NIR photography basically boils down to capturing the particular amounts of reflected NIR radiation emitted by very hot objects such as the sun, incandescent light bulbs or specific extraneous NIR sources, rather than recording the ambient temperature variation. This misunderstanding is also fed by some of the older (archaeological) literature. Simmons (1969, p. 94) for instance literally says: “cool objects appear dark, warm objects appear light; hence green vegetation looks very whitish, while water is blackish”. Although water indeed appears black on an NIR photograph, this response has nothing to do with its temperature, but is due to a very high NIR absorption, indicated by its absorption coefficient (Curcio and Petty, 1951).

In NIR imaging, two techniques exist. The first, and by far most applied, kind of photography uses the already mentioned reflected/transmitted portion of the incident NIR radiation. Every object exposed to NIR radiation will absorb, reflect and transmit these incident photons to some extent. Recording these particular amounts is generally termed reflected (N)IR photography and should not be confused with IR reflectography, the latter using longer wavelengths up until around 2000 nm (van Asperen de Boer, 1966, van Asperen de Boer, 1969).

Secondly, there is NIR fluorescence photography (sometimes called NIR luminescence – Barnes, 1963, Bridgman and Gibson, 1963, Gibson, 1962, Gibson, 1963a, Gibson, 1963b), in which the NIR sensitive medium records in fact fluorescence, being radiation emitted by the subject under study in the NIR region. Rather than directly being emitted as in the case of very hot objects, these NIR photons are excited upon being exposed to incident shorter wavelengths (mostly UV or visible blue and green wavelengths). Although its application is mainly restricted to the forensic field, the very strong fluorescence of particular minerals and pigments (Barnes, 1958) makes this type of NIR photography worthwhile in certain archaeological case studies.