AGM plus guest lecture by Prof Patrick Callet (Paris)
Spectral Simulation for Cultural Heritage -- A scientific Methodology and Some Examples
University College London W.S. Stiles Lecture
Wednesday 02 May 2012
City University, London

After the AGM chaired by Prof Andrew Stockman (NEAR RIGHT), Prof Patrick Callet (FAR RIGHT), Ecole Centrale, Paris, described the development and use of his Virteulium software that from first principles simulates optical properties of materials.
	This software starts with photons and calculates where they will go and what the material will look like. For example, it calculates the caustics (see above left for the caustics formed by a small sherry glass of water) and the surface appearance of objects. He showed predictions for a small bronze horse when illuminated by directional light or diffuse ambient light (BELOW) and also demonstrated the predicted appearance of the horse if made with tin, copper or various mixtures of the two.
He had had a series of horses made and the one that was of tin is seen being examined by a member of the audience (ABOVE) whilst the simulations are shown below, right.
Prof Callet showed several other examples of what the software would do, but concluded with his work on the pigmentation and the simulated appearance of the west front of Notre Dame de Paris (BELOW LEFT). This was originally bright and polychromic, garish to our eyes. He concentrated on the row of statues on the Kings' Gallery that runs across the front at the first level. Each king stands in a niche or alcove, the top of which is formed by a three centred arch - see the inset below right.
The concave surfaces of this complex arch were originally gilded and the later afternoon sun would have been reflected into a caustic as shown above. He finished by showing the simulation made with the software of the whole west front at different times of the day. Shown on the right are two of the kings at different times in the evening. The simulation was obtained with Virtuelium using the photon-mapping algorithm, the optical model for gilts and the polychromy measurement made by his team. The lighting was estimated on the basis of a solar emission spectrum progressively modifiy by a Rayleigh scattering model. The computation, involving 120 Intel processors, for a high resolution rendering was made at 3minute intervals between 20:58 to 21:58 for the clear sky illumination on 21st June 2012. The computation time was about 40 minutes. The upper image is for 20:58 hrs and the lower image for 21:55 hrs. Notice the the caustics are shining just behind and above the heads in the lower image. A spectacular end to a summers day in the Parvis Notre-Dame. It is hoped that a fully illustrated paper based on this work will be available in a few months as the third of the Colour Group Occasional Publications distributed free to members. It is to be especially prepared by Prof Callet.

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