Calculating the Required Amount of Lighting for a Room

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There are defined methods for calculating the exact amount of light needed for a particular living space if one is attempting to illuminate a room. To begin with, one needs to consider several important facts about light bulbs, light bulb types (e.g., incandescent, LED, fluorescent), and light bulb power use (wattage). This is because lighting is not simply a matter of light bulb number or wattage but also light intensity, distance of the light to its target object, and the energy efficiency of the lighting fixture. Such considerations determine how many light bulbs will be required to light a room, how far away the light fixtures will be spaced, and whether or not it will even be possible to light a room to its required illuminance (a physical parameter that defines how much light lands on a surface). In essence, lighting a room requires more than just installing a large number of light bulbs.

Light measurement is usually conveyed in units of lumen, lux, and footcandles. Different light bulb types will produce different amounts of illuminance as defined by these units. Furthermore, total illuminance is not additive in nature; increasing the amount of light bulbs does not proportionally increase illumination. This is because illuminance is not only a function of light intensity, but also of the distance of a light source to its target.

A lumen (lm) is the measure of luminous flux (light in transit), which is defined as light power as perceived by the human eye. When the metric area over which luminous flux is spread is taken into account, the SI measurement of illuminance, or lux (lx), is used. Alternately, when non-metric (foot) area is calculated, the non-SI footcandle (fc) unit is used to measure illuminance. Mathematically, one lux equals one lumen per square meter, or 1 lx= 1 lm/m2. Conversely, one footcandle equals one lumen per square foot, or 1 fc = 1 lm/ft2. Finally, one footcandle is approximately equal to 10.764 lux.

When calculating either the lux or footcandle units for a fixture from its provided lumen information, keep in mind that while lumens are always discrete values, lux and footcandles vary depending on distance. Also, lux and footcandles are inversely related to lumens as the square of the distance between the light source and the object being illuminated.

This means that as one moves away from a given light fixture, illuminance decreases drastically. For example, consider an incandescent light bulb that is rated at 300 lm. Standing one meter away from this light bulb, one obtains an illuminance of 300 lux (300 lx= 300 lm/1 m2). However, if one moves away from that light bulb an additional 2 meters (i.e., 3 meters total), illuminance changes to just 33 lux by the inverse square law (33 lx= 300 lm/3 m2).

Because illuminance changes as a function of distance, lighting an object located at a shorter distance, such as a kitchen counter, is much easier than lighting an object located far away, such as a box placed on a warehouse floor. Whereas the kitchen counter may easily achieve an illuminance of 600 lux from a single fluorescent fixture of 12,000 lumens, achieving the same value for the warehouse floor box will require either additional lights or a light containing a higher lumen value.

Building codes usually require that a room, walkway, or building be equipped with a given minimum number of lux or footcandles. For example, the Illuminating Engineering Society of North America requires a minimum illuminance of 30 fc for kitchens. Most light fixtures are provided with information about their total lumen capacity as well as their wattage (W). Just because a lighting fixture has a higher wattage does not automatically mean that it also has a higher lumen capacity. Wattage is a unit of power, i.e., the amount of power required for the light fixture to provide its specified number of lumens. Lumens, meanwhile, remain a static quantity.

A standard incandescent 15W light bulb generates 122 lumens, or 8 lm/W. A CFL (compact fluorescent light) bulb of almost the same lumen capacity (125 lm) requires only 3 watts of energy, producing 42 lm/W. A metal halide light bulb is even more energy efficient, with some halide bulbs generating as much as 115 lm/W. Light bulb energy usage is imperative when considering how much illuminance is required for a room versus how many lighting fixtures are possible. Furthermore, as wattage is increased, so is electrical resistance, as illustrated by the Ohm's Law derivation R= P/I2 (where R= resistance (Ohms), P= power (Watts), and I= current (amperes)). When calculating the amount of lighting required for very large rooms, therefore, or rooms with high ceilings, these basic physical laws must be accounted for.

Overall, it is not impossible to calculate the absolute exact amount of lighting that is needed for a room. One should consider the necessary illuminance, desired number of light fixtures, ceiling height of the room, as well as room size. Finally, using energy efficient light bulbs can help reduce electrical resistance, save on installation time, and reduce energy costs.