Light properties, and why it is important to know them

Light is a complex medium with which to work. While light itself is understood by science, how it is perceived by humans is still not fully comprehended. Different people react to light in different ways, and it is important to be sensitive to what people like and dislike with certain properties of light.

Light travels in straight lines, and so it is easy to predict where light will fall within a room. Standard mathematics can be used to determine if a light source will be visible and have a high glare, or if light will fall on a particular surface. When light travels through different materials, such as air and glass, then the direction of the light will change. Most of the time this is not relevant, but when lighting glass it is important to remember the principle of total internal reflection. If the light contacts the glass at a very shallow angle, almost parallel with the surface of the glass, then it will not penetrate it, but reflect off it and away, similar to a mirror. This is important to note, since in some cases it will not be possible to light through a piece of glass, such as a step, if the light is being transmitted next to it.

Light properties are usually discussed in terms of the following:

Luminous Intensity
Luminous intensity is the measure of visible light in a particular direction per solid angle. The SI unit for luminous intensity is the candela (cd). This gives a good indication of the intensity of the light emitted from a lamp. Most lamps with a beam angle (generally up to 60°) will have a peak intensity, or candela, value. This allows comparison between different lamps with a beam angle and gives an idea of the maximum light output.

Luminous Flux
Luminous flux is the measure of the visible light output of a light source (a lamp). The SI unit for luminous flux is the lumen (lm). Whereas luminous intensity deals with visible light emitted in a particular angle, luminous flux is the light emitted all around a light source. The lumen value is generally given for unidirectional lamps, such as fluorescent tubes and standard incandescent lamps. This is another way of comparing the light output between different lamps to see which is brighter.

Illuminance
Illuminance is the measure of luminous flux per unit area. The SI unit for illuminance is lux (lx). One lux is equal to one lumen per square metre. In most homes an illuminance level of between 100lx and 500lx is required, depending on the different areas in the home. A house would not be lit with 500lx through its entirety, as this may only need to be achieved on some work surfaces. Most residential lighting designs do not need or want measured illuminance levels, but it may be helpful to compare different lamps and the actual light output. Most lamp manufacturers produce a light cone, which shows the peak illuminance at different distances. These provide a quick comparison of the actual light output from a lamp, and show which is brighter.

Power
Power in lighting refers to the electrical power used by the lamps in the system. The SI unit of power is the watt (W). Almost all equipment will have a maximum wattage that it can control. Some will have a minimum wattage as well. It is important that these limits are observed, since if they are exceeded then it could mean early failure of either the light fittings or the equipment. For example, many dimmer switches have a maximum wattage of 250W. If the lighting circuit carries more than 250W, for example three 100W incandescent lamps, then this could cause the dimmer to overheat. It is normally acceptable to have less than the maximum wattage on lighting equipment, unless it is a fluorescent or metal halide lamp, in which case it must be matched. Efficacy Efficacy is similar to efficiency, but is the ratio between two figures with different units. In lighting terms, efficacy relates to the lumen to watt ratio. This is a common way of measuring how low energy a light source is. It is the number of lumens emitted divided by the number of watts of power consumed, and is noted with the unit lm/W. There are different variations of the lm/W ratio. Some take into account the power losses of transformers and ballasts used by a light fitting (commonly noted as a circuit watt), and some also take into account the light lost when a lamp is fitted into a light fitting (the luminaire lumen value as opposed to the lamp lumen value).

Electric Potential
Electric potential is measured in Volts (V). An electrical circuit will have a voltage associated with it. In most cases this matches the national voltage, provided by electric companies to homes. In the United Kingdom this is 230V AC, whereas the United States uses 120V AC. There are variations across the world, so if light fittings are being purchased from other countries, it is important to ensure that they will work in the installation.

Current
Current is the flow of electric charge. The SI unit of current is the amp (A). There is a maximum amount of current that can be taken into a home, and this in turn limits what can be used for lighting. There is a maximum amount of power that a dimmer switch can handle, which is related directly to the maximum amount of current that can flow through that dimmer switch. Current, Voltage and Power are all inherently related to one another, and if two values are known then the third can be calculated using the formula Power = Voltage x Current. In some cases, such as with on/off switches, there will not be a maximum wattage but rather a maximum current, commonly 10A. It is worth being aware of the different values required and how to find them out.

LEDs generally require a constant current to make them emit the maximum possible light. Rather than a specified voltage, like most other lamps have, many LEDs have a specified current, such as 350mA or 700mA. It is important to obtain an LED driver that matches this to enable them to work at maximum efficiency, and avoid damaging the LEDs.

Correlated Colour Temperature
The correlated colour temperature (CCT) of a lamp is the measure of the warmth of the light emitted from it. It is measured in Kelvin (K). Incandescent lamps have a colour temperature of 2700K, whereas fluorescents can have colour temperatures ranging from 2200K (orange/white) to 8000K (blue/white).

Ensuring that the correct colour temperature is used is essential to any lighting design. If cool colours are used in rooms for relaxation then they will look harsh and uninviting. If warm colours are used exclusively then the rooms may appear dirty and old. Finding a balance between different colour temperatures can be quite difficult and it is worth experimenting with different lamps of different colours to find the right combination.

Colour Rendering Index

The colour rendering index (CRI) is a measure of how well a light source matches a particular spectrum standard called a black body radiator. An incandescent lamp will match the black body radiator, and has a CRI of 100. It is used as a guide to how well light shows colours on a surface. A good example of a very low CRI lamp are the sodium lamps used in some street lighting, giving an orange/yellow light. It is almost impossible to discern different colours below one of these lamps, since they all look the same, and these have a negative value CRI. The CRI of a lamp is a good measure of how well the lamp will show colours. If the CRI rating is in the 90s, then this is exceptionally good; if it is in the 80s it will be good, but not amazing. Anything below 80 is not really suitable for residential use.