**1. Introduction**

The colour and appearance of an object depend on the physical and chemical properties of the object itself, the source of visible electromagnetic energy that illuminates it, and the observer who detects the energy reflected by the object [1]. Taking into account these three factors, it is possible to think that by projecting adequate lighting on an object, its appearance could be altered.

From this idea, the term Mixed Reality arises, in order to alter the perception of reality including virtual content. Milgram and Kishino defined the concept of mixed reality as a subset of the technologies related to virtual reality, which includes Augmented Reality (AR) and Augmented Virtuality (AV). Thus, they defined Milgram's Reality–Virtuality Continuum (Figure 1), which is a line on which the virtual space is located at one end, real space at the other, and AR and AV in the center. AR was defined by Milgram and Kishino as the technology that improves the visualization of a real environment through virtual objects and AV was defined as the opposite of AR. Therefore, in Figure 1, AR is closer to reality and AV closer to virtuality [2].

Taking these factors into account, it is possible to think that by projecting adequate lighting on an object, its appearance could be altered in a virtual and therefore reversible way. Thereby, the technique of Spatial Augmented Reality (SAR) or projection mapping arises, which uses optical elements and video projectors, holograms, radio frequency labels and other tracking technologies to display virtual information directly on an object altering their appearance [3–5].


**Figure 1.** Milgram's Reality–Virtuality Continuum [2].

Azuma, in 1997, completed the definition of Milgram and Kishino, defining AR as 'a technology that allows the user to see the real world, with virtual objects superimposed or compounded with the real world' [6].

In recent years, AR has become more important in fields such as education, entertainment, conservation, restoration and adequate exhibition of cultural heritage, among others [3,7].

The projection mapping technique uses cameras to capture the object to be illuminated and projectors to project the desired image onto the object. It has the advantage over other AR techniques that it doesn't require viewers to use any type of glasses or instrument to appreciate the effect, allowing multiple users to appreciate the effects of projection mapping technology at the same time [5].

Section 2 of this article collects those studies which use the projection mapping technique in order to alter the appearance of an object, with special emphasis on those studies that use the technique as a method of displaying cultural heritage.

For the development of the projection mapping technique, a meticulous calibration method is required that allows adequate synchronization between the instruments used. Then, it is necessary that the image be correctly located on the object, avoiding unwanted effects. Section 3 of this article lists the main calibration techniques used in projection mapping in recent years.

The complexity of implementing this technique in the field of conservation of cultural heritage objects consists in the development of a lighting system that allows a safe display of the artwork without producing alterations in its appearance with respect to when it was made by the artist. This is due to the photochemical and thermal effect of light, causing degradation of the materials it illuminates. Section 4 of this article collects the most significant techniques to address this difficulty.
