light, colour & photography

the dimension of light & dark

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The aim of this introductory article is to explore just one of the dimensions of colour space: the polarity of light and dark. We shall learn that the light-dark axis for illuminants (the white-grey-black axis for surfaces) is independent of the scale of colours and that there are significant differences when speaking about illuminants and surface colours. For an exploration of other dimensions of colour-space you should refer to the section on Colour Space.

the polarity of light and dark

Imagine a vertical axis. At the top is the whitest white you could possibly imagine; at the bottom is the blackest black. In between we have all the shades of grey ranging from the palest, softest greys nearest white to the darkest, deepest greys as we approach black. No matter how dark or light each grey is they all have one thing in common - not one of them contains any hint of colouredness. No redness, no blueness, no yellowness. They are all neutral.

I accept there are some who regard grey as a colour in its own right - painters and decorators and artists particularly so - and I don't take issue with this point of view. But this is nothing to do with semantics and whether you regard grey as a colour or not is irrelevant. The fact is: the scale of greys can follow a line from pure white to pure black without having any hue whatsoever. Further, pure white and pure black represent the end-points of a continuous scale; it is a true polarity. There is nothing beyond white, nor anything below black. They are the limits, the extremes, of a one-dimensional neutral axis.

illuminants

So far we have restricted our discussion to surfaces, but what of illuminants or sources of light? Here the same one-dimensional scale applies but the limits are different. To imagine this we can picture a single lit candle in a room. We add another candle and light it. The room gets a little brighter, Add another lit candle and the room gets brighter still. With every addition the room gets brighter and brighter.

Apart from the physical limitation of so many candles we can imagine no upper limit to the number of candles we can add. This is true for any light source - whether it be the rather yellowish light from a candle or the more white light from modern electric lamps.

For illuminants there is only one definite fixed point: total "darkness" - when there is no light at all. At the other extreme we can imagine no upper limit, the light can get brighter and brighter, limited only by the amount of energy taken to produce the light itself.

As with surfaces there is a single dimension with "darkness" at one end and something we can best describe as "brightness" at the other. These are the subjective terms to describe what we experience. Of course, there are coloured light sources too - and for a discussion of coloured light you should refer to the separate section on Colour.

surfaces

Surface are passive materials, they do not produce light of themselves but rely on some other light source to make them visible. They are said to reflect light. Exactly how they reflect light determines their appearance - at least in part. Surface colouring is the property of pigments, paints, certain minerals, plants - indeed every physical object in the world.

In this section we are restricting our discussion specifically to surfaces that are neutral: black, white and grey. The distinction between these is a single one: the amount of light reflected by the surface. White reflects the most, black hardly any and the greys progressively more or less depending on whether we are going from black to white or vice versa.

Two terms are important here: reflection and absorption. White reflects most and absorbs the least; black the opposite. It is important to realise that white, even the whitest white, does not reflect all the incident light. Black, even the blackest black, does not absorb all the light that falls on it. The whitest white is about 99% reflective; the blackest black will still reflect about 6% of the light that falls on it. These are the practical limits of the physical world itself. It's how nature is. The situation is further complicated by the angle at which light falls on a surface as well as the surface roughness and smoothness.

Obtaining a 99% reflectivity is not easy and doing so across the entire spectrum is more difficult still. "Standard" white surfaces are manufactured specifically for calibration purposes and you can expect to pay hundreds of dollars to obtain one. The following table provides some useful examples of whites:

There is a similar problem with black, but the other way round: no surface will absorb 100% of the incident light.

terminology

The difference between illuminant and surface allows us to use an appropriate terminology. We speak of white and we speak also of "white light". We speak of black, but never of black light - we call it darkness. In similar vein we never refer to grey light - it is simply dim. Grey therefore is truly an attribute of a surface material rather than an attribute of light itself. We know from the previous discussion that white is a property of a surface - so should we be using it to describe an illuminant? Strictly we should not - white, grey and black are descriptors for surfaces. So how should we describe light that has no colour? The most appropriate descriptor would be "neutral". Neutral light accurately describes all intensities of light from the the dimmest light to the brightest.

It could be said, albeit perversely, that white, grey and black are all the same "colour". In one respect this is true - they all reflect exactly the same colour of light as the ambient illumination. If all three share a common name for this colour it could only be "neutral".

• what's special about grey? >>