In this paper, the author presents current work undertaken by the BBC to look at the representation of human skin tone in ITU-R BT.2100 High Dynamic Range video systems. Specifically, the author presents luma values for a range of skin tones, categorised by the Fitzpatrick Scale, which can be used as a line-up guide by cameramen, racks/shader operators and colourists. The effect of make-up is also covered.
When creating video in live situations or for pre-recorded shows with minimal postprocessing, cameramen rely on knowing the correct video luma level to place objects within the scene.
Traditionally, television cameramen have relied on knowing the correct level to place grass or skin tone under varying lighting conditions and some cameras have had visual aids (known as zebras) to help with this placement. This paper presents results for skin tone tests undertaken to understand how skin tones should be displayed in ITU-R BT.2100 HLG.
Regional differences in skin tone exist due to evolution - skin tone is optimised for the local UV radiation levels. UV levels change throughout the year, but are generally highest between the Tropics of Cancer and Capricorn.
Local effects such as shade, vegetation canopies, altitude and presence of bodies of water effect the average annual amount of UV an individual receives. The skin tone reflectance is directly related to the amount of a pigment called melanin present in the skin.
In the skin, melanin acts as an optical and chemical photoprotective filter, reducing the penetration of all wavelengths. The optimal amount of melanin for a given environmental average amount of UV light is small enough to allow adequate production of Vitamin D (needed to produce healthy bones – a lack of vitamin D causes Rickets) and large enough to prevent the UV radiation altering a sufficient amount of skin cell DNA to cause cancer.
Research suggests that persons at either end of the range of human reflectances are quite uniform across the body and are unaffected by the process of tanning, but in the middle of the range it is expected that significant differences may exist between areas exposed to the sun regularly and the covered torso. Where differences do exist, these change over the course of the year as atmospheric UV levels change.
A second reason for differences in reflectance across the body is perspiration and oily secretions. For a given, fixed skin tone, damp areas of skin reflect more light than dry areas. It can be seen that, for a person with a uniform skin tone, we would expect the forehead to reflect more light than the cheek, which in turn will reflect more light than the forearm. These reflections (from damp skin) may be specular in nature.
Television make-up is applied for the following reasons:
To combat the effect of strong television lighting which alters the perception of facial colours and contours (luma and chroma contrast), and to aid in the creation of a character by changing physical appearance, e.g. making an actor appear older.
We can therefore expect that make-up will be used to reduce specular highlights caused by perspiration and to enhance contours and colours on the face.
It should be noted that the use of make-up alters the perception of facial colours and brightnesses.
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