When designing lighting it is important to understand the physics, physiology, and psychology of how humans sense light, process it, and experience it. This chapter explores how we respond to light, how we adapt to intensity and changes in light levels, and how vision works. Equally important are the psychological aspects of lighting, including mood and cultural preference.
When designing lighting
The human body has many ways of understanding its environment. It has a multitude of specialized systems designed to be sensitive to both internal and external changes. Sound and light are two kinds of stimulus that the body is designed to respond to. They are external stimuli and are transmitted through the environment as waves. Waves can be described in terms of their wavelength (the distance between wave peaks), or by their frequency (the number of wave peaks that pass in a certain period of time).
Frequency and wavelength are just different ways of describing the same information about waves. Light is a wave (of electromagnetic energy) that can be described in terms of frequency. Blue light has a frequency of around 660 trillion hertz. However, light is traditionally described in terms of its wavelength rather than its frequency. Blue light is therefore described as having a wavelength of around 470 nanometers (a nanometer is a billionth of a meter).
Receptors are specialized cells that send signals to the central nervous system when there are changes in the body’s internal or external environments. There are different types of receptor for different functions. For example, olfactory receptors respond to the chemical signatures of different odors and trigger our sense of smell, while taste receptors signal flavor to our brains. All receptors act as transducers, converting one form of energy (for example chemical, mechanical, or electromagnetic) into another form that is used to communicate with the brain.
Receptors can only be on or off; they have no amplitude or scale of trigger. To communicate the intensity of a signal of a received stimulus (such as the volume of a sound), receptors fire more frequently for a strong stimulus and less frequently for a weaker one. If a receptor is stimulated for a prolonged time by the same stimulus it begins to decrease its rate of firing, and becomes desensitized to the continuous stimulus. This is called adaption.
All receptors display the ability to adapt to a constant stimulus. Walk into a garden and initially the smell of freshly mown grass can be very strong, but it seems to fade even though the smell is still present. When we are adapted to a particular stimulus we only become aware of it again when there is a change: perhaps we go indoors (where we become adapted to the indoor environment), then return to the garden and smell the grass anew. Although we are rarely aware of it, the same process of adaption affects our sense of sight.
Our visual system becomes adapted to the colors in our surroundings when we wear tinted sunglasses, and we are surprised at how different the world looks when we remove them. The process of adaption also allows the visual system to become more sensitive. In a dark space, we become adjusted to lower light levels over a period of time and the space seems to become brighter. During this process of adaption to darkness, the visual system becomes much more sensitive to light, matching its range of sensitivity more closely to the surroundings.
We have a remarkable visual system that performs consistently over a wide range of lighting conditions, but there is one thing it cannot do: it cannot enable us to measure quantities of light just by looking. We often talk about “brightness” as if it is some form of measurement, but the best that can be said is that brightness is a perception, not an absolute. A single candle flame in a dark room appears to be very bright, but can hardly be seen in daylight. The sensation of brightness is also subjective. A person who has spent a lunch hour in a dimly lit restaurant may perceive some areas of the space as being quite bright. Meanwhile, another person walking in from the sunny street outside will see the whole restaurant as being dark.
What we think of as our built-in brightness scale is a contrast measurement, a relative assessment based on the surrounding light conditions and the conditions we have recently experienced. What is most remarkable about this is that our built-in assessment is constantly adjusting to suit our surroundings. This allows us to move between very light and very dark spaces, but prevents us having any real sense of measurable quantities of light. Luckily, there are standardized measurements of light that do not rely on personal judgment. Unfortunately, they are standard physical units and the definitions can be quite complex.
All lighting units are interlinked, so despite the simplifications some definitions can be difficult to decipher without reading other terms. Hopefully, a couple of readings will make things clear, and further information can easily be found in printed and online dictionaries.