THE ACCURATE REPRODUCTION OF SKIN TONES

This is a topic your editor first wrote about five years ago in Digital Photographer magazine. The subject continues to generate much discussion and we are frequently asked for advice and article reprints on the features from Professional Imagemaker. It is therefore timely to revisit the subject and add the mass of information gleaned in the intervening period. There have been a number of changes. The use of RAW files has increased and calibration has become more common for cameras, monitors and printers. Our discussions for this feature have been wide-ranging and include instrument makers, camera makers, dermatologists, artificial limb makers, medical illustrators and a number of photographers. SKIN TONES

Why Accurate?

As humans we are tuned to recognise certain colours with more discrimination - the so-called known colours. Skin is a known colour and its pleasing portrayal in an image is of paramount importance. There is a difference between an accurate colour (as in an image depicting a medical diagnosis) and a pleasing colour. Here we move in cultural preferences. Whist it is generally true that lighter skin is favoured by many cultures, very pale skin is only prized by certain ethnic groups. Let’s not beat about the bush, we are talking sexual attraction here – that is the point of wishing to look flattering in a portrait. One of the saying we came across in our research is “wealth lightens the skin” This is true for the darker skinned ethnic groups but most Europeans favour a more “tanned look”. The crossover between looking tanned and looking tacky is fine and movable, especially with the current campaigns to reduce exposure to damaging uv light. Today people are far more aware of the danger of skin cancer and a heavy, induced tan is looked upon as a sign of ignorance, a bit cheap. At the other end of the scale the dark-skinned ethnic groups stay out of the sun to stay as pale as possible. You will not see students walking about under parasols in a London university but you certainly do in Kuala Lumpur. Also recall that when the Khmer Rouge were running riot, the advice to academics was to get out into the fields and get a tan – pale skinned people were targeted as being part of the upper classes and more ruthlessly murdered.

Film manufacturers have endlessly pursued the accurate reproduction of skin colour. Starting with the move from Ortho Chromatic to Pan Chromatic film the topic always features in advertising literature. Here are some examples from a 1994 Fuji catalogue “reproducing natural skin tones with subtle shades”; “accurately reproduces a broad range of flesh tones”; “ideal for weddings, portrait and general purpose people photography”; “the most vivid colour reproduction and rich skin tones”; “sensuous flesh tones”

Note the key words here; natural, sensuous, rich, accurate, subtle. What does all this mean?

The arrival of digital has eased the mud off the bottom of the flesh pond. While digital is demonstrably more accurate at rendering most skin tones it also carries the risk of drifting further from optimum in unskilled hands. In addition there is the problem of un-natural reddening of certain complexions when shot digitally as compared to negative-silver halide reproduction – more on this later.

What is Skin?

The skin is the largest organ in the human body. It keeps all the bits inside that should be and all the bits outside that should be. It is waterproof when it needs to be, so that we don’t dissolve in the bath but not waterproof when we need to sweat to lose heat.

Whilst it is not appropriate to delve into medical science here, we do need a few pointers. The skin consists of a three layered structure, –subcutaneous tissue, dermis (cortium), and epidermis. The surface of skin itself is effectively matte because the uppermost level of skin is covered with dead cells causing no regular reflection. The glossiness of skin can be due to sweat, skin oil or an applied layer of cosmetic cream.

The matt skin colour appearance is influenced by the light filtering capabilities of three main colouring agents: melanin in epidermis, carotene in dermis and subcutaneous fat, and blood capillaries across the dermis. Melanin is a brown pigment and carotene gives an orange tint. Haemoglobin (an element of blood) can produce two different tints: if the haemoglobin is oxygenated (oxyhemoglobin), the tint is reddish or pinkish apart from when it is deoxygenated (reduced haemoglobin) then the tint is bluish. Like most natural objects, the skin has variability, which in this case is mainly due to amount, density, and distribution of melanin. Under the surface there are colourant particles which interact with light, producing scattering and colouration. The variation within a person can be quite marked, freckles being the classic example. There are also differences around the body. Production of melanin darkens the skin of the person to protect them against the harmful effects of the sun’s uv rays. For this reason the soles of the feet and palms of the hand contain less melanin and are pinker. The quantities of melanin vary with ethnicity and roughly correspond to the following variations:

Light Skinned Caucasians 1-3%

Well-Tanned Caucasians and Mediterranean 11-16%

Darkly Pigmented Africans 18-43%

melanin

L The lightness usually expressed as a percentage

a the redness to greenness of a colour

b the yellowness to blueness of a colour

There are huge advantages to separating the lightness values of a colour from its hue and saturation, which is what you can do by using Lab values. Ignoring the variations in different parts of the body, the values of a and b remain more constant around an image which may be lit is cross lighting. In this case the dark side of a face may have different L values but near identical a and b values to the lighter side.

There is another advantage to Lab. The concentration of melanin governs the “tan” of a skin colour by changes to the a value and the L value. The blood flow and concentration of blood vessels close to the surface effect the Lab b value. However just to confuse matters there are several types of melanin; a red and yellow sort. Red melanin is higher in Irish, Danish and Dutch populations which also colour red hair red! Within these variations, the presence of carotene (which is orange) influences the tone where it is stored in the subcutaneous fats and this underlying variability gives skin its variable texture and tone. Carotene concentration can be affected by diet – you may recall the little girl who turned orange after drinking way too much of a proprietary orange drink.

The problem which faces us as photographers is that we have to distil all this colour variability and conjure up a pleasing picture. To do these we need practical guidelines of how colour should be adjusted to achieve a particular “look”.

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