## 1 Introduction  
A virtual TV set allows the composition of real actors and/or objects with computer generated elements or even full environments, in real time. Nowadays, this technique is used in most of the broadcasting TV channels (even in live programs) and cinema (in post-production) in order to locate actors in places where they are not and even making them interact with virtual objects or actors (Figure 1). The origin of this technology dates back to the 1930's when a patent presented by [PERSON] [1], described the process of \"inserting\" the information from one television camera into the signal from another camera in order to produce a composite result. The term \"inset\" was used to describe the process where an area in a live captured or recorded video signal (generally an actor) is used to delete a corresponding area from a second camera image (generally a real or synthetic background), so insetting the first camera's picture, precisely, in the \"hole\" created in the second camera's signal. At the beginning, television was monochrome and brightness variations in the signal were used to delimit the areas to be erased. With the advent of color television this technique evolved towards chroma-keying [2], which uses hue differences to discern if an area is part of the background or the foreground of the image (Figure 1). It is a very efficient approach that works by comparing pixel by pixel its color with the chroma key (reference hue) taking into account a variation threshold that must be accepted in order to erase all the similar tones and not only the identical ones. The main disadvantage of this method is that, being based in color, the chroma key hue can only be used in the elements that will be deleted in the final composition, being impossible to preserve the actor's cloths or the objects with the same tone. Due to this problem, since its beginnings, there have been many attempts to substitute or improve chroma-keying using different approaches. For example, [PERSON] et al. [3] proposed the use of striped cycloramas using two different colors, [PERSON] et al. [4] tried using a checker pattern and the enterprise LEDchromaKEY uses a reflective screen that does not require additional illumination as it works with a led ring located around the camera lenses [5]. Different approaches make use of depth information to separate background and foreground using multiple cameras [6], or time of flight IR cameras [7]. However, the bigger complexity of these approaches along with their limited advantages over chroma-keying, have produced that it remains as the most common approach for composite real-life productions. Figure 1: Camera captured image (**top**), computer generated image (**middle**) and final composition (**bottom**). In a virtual TV set, the quality of the final integration comes both from the chroma-keying quality and other elements such as the interaction between the presenter and the virtual environment. Therefore, in this paper, we focus on improving and simplifying the chroma-keying process by ensuring the homogeneous illumination of the cyclorama surrounding the stage keeping the hue or color variation along its surface as minimum as possible. The differences in color may be originated in the painting, such as humidity, wrong painting technique, etc. (in which case the best solution is to repaint the cyclorama); or a wrong illumination, causing the appearance of shadowed or over-lighted areas, that requires the, recalibration of the lights. This last case is the most common, as the lamps tend to degrade with time and the light position may be modified by the day-by-day work on the set. For this reason, the light configuration must be re-calibrated every so often. The calibration of the cyclorama illumination is generally done by an expert that makes use of a photometer to take several samples along its surface and check that the results are consistent.