To study the construction of an ancient structure, you cannot simply chip away large chunks and take them back to the laboratory. You might not be tracked down by a fedora-wearing, whip-toting archaeologist, but damaging buildings with obvious cultural significance nonetheless will earn you scorn.Figure 1. Investigators from Italy’s National Research Council in Florence and from the Soprintendenza Archeologica di Roma, along with their colleagues from Lund University in Sweden, used two hyperspectral fluorescence lidar imagers to determine attributes of the Colosseum in Rome. They especially wanted to distinguish original building materials from those used to repair and restore the amphitheatre since its construction during the first century of the Common Era. Shown here are the locations from which data were acquired; the numerals refer to the number of each arch. Images reprinted with permission of Optics Express.Analysing photographs of an edifice can provide some information, but if more detailed information is necessary, then advanced techniques are required. Now a group of researchers from Italy and Sweden has demonstrated that hyperspectral fluorescence lidar imaging enables a penetrating look at culturally interesting buildings, such as the Flavian Amphitheatre — aka the Colosseum — in Rome.The group describes its efforts in the 12 May 2008 issue of Optics Express.Figure 2. Fluorescence data acquired from 65 m away helped the researchers determine which clamps — used by 19th-century restorers to support the stone blocks — were made solely of iron (original) or were coated recently. The clamp at A presented spectra indicating that it is coated with an anticorrosive material. The clamp at B does not have this coating.Figure 3. The researchers used two lidar systems, one based on a 308-nm excimer laser developed in-house and the other based on a 355-nm Spectra-Physics Nd:YAG laser emitting at 20 Hz. The systems utilized Newtonian telescopes — 25 and 40 cm, respectively — to collect fluorescence emissions from the targeted areas on the Colosseum, and spectrometers made by Acton Research and Oriel, respectively, for spectral analysis. The image and hyperspectral data map shown here — taken from area D in Figure 1 — indicate which travertine blocks are reworked or unaltered originals (dark-coloured pixels) and which are replacements (light-coloured pixels).Figure 4. The researchers also quantified differences among the mortar-based (left) and cement-based (right) joints between the blocks.