How Much Light is Enough?

When you start to think about lighting for your space, one of the first questions you should answer is how much lighting you will need. In other words, how much lighting is enough lighting?

The short answer is: that depends. At home, the lighting you’d want in your living room or patio for entertaining friends and family would be different from the lighting you’d want in your home office for reading or working. You’d also want brighter lighting in the bathroom. Lighting needs also vary for commercial spaces, be they industrial, commercial, retail or office spaces. Of course, the lighting required for outdoor areas will differ from the lighting you’ll want indoors.

Before deciding how much lighting you will need, you should have a basic understanding of the nomenclature. Light level, or luminance, is measured in foot-candles or lux. One foot-candle equals one lumen of light density per square foot, while one lux equals 1 lumen per square meter.

To better understand lighting levels and what these figures mean, think of some typical natural lighting situations. Full daylight typically equals about 1,000 foot-candles, while an overcast day would have only 100 foot-candles of light. Twilight produces just 1 foot-candle, while a night with a full moon has 0.01 foot-candle and an overcast night has only 0.00001 foot-candle.

The kind of light you will need for your situation depends on many factors, including the space to be illuminated, what the activity will be in the space and who will be using it. As humans age, for instance, their eyesight declines, and they need more light to see. Thus, a classroom for elementary school children would need less illumination than an office for working adults.

The requirement for light also depends on activity. Though elderly retirees would need more light to see than working adults and children, they are less likely to need high foot-candles unless they happen to be doing a hobby that requires intense light.

Likewise, the space itself will help dictate lighting requirements. An outdoor public area surrounded by darkness will require less lighting than an office or classroom, for instance, and a typical office or classroom will need less lighting than a laboratory in which minute, detailed measurements and experiments are being carried out.

Each lighting situation will differ, of course, based upon the above factors, but here are some basic guidelines. Public areas surrounded by dark areas will generally require 2 to 5 foot-candles, or 20 to 50 lux, while typical offices and classrooms without highly detailed work will need 10 to 20 foot-candles, or 200 lux. Offices with more intensive use and showrooms will need brighter lighting provided by 50 to 100 foot-candles, or 500 lux. Very intricate, exacting and detailed work may require 1,000 to 2,000 foot-candles, or 10,000 to 20,000 lux.

Generally, exterior spaces will require more and better lighting than interior spaces, with the requirements for exteriors generally falling between 100 and 10,000-plus foot-candles and the requirement for interiors usually falling between 10 and 100 foot-candles.

Of course, other factors of lighting itself beyond luminance will come into play when estimating how much lighting you will need. These include the lights’ direction, distribution and color temperature.

In general, if you’re going to make a mistake when deciding how much lighting to provide, it’s best to err on the side of providing too much rather than too little lighting. That will ensure that everyone in a room has enough light. There are exceptions, however, including lighting of valuable artwork and artifacts that may be damaged by too much lighting.

Table 1: Common Light Levels

Activity	Category	Illuminance Range		Visual Plane Reference
		Lux	Footcandles
Public spaces with dark surroundings	A	20-30-50	2-3-5	General lighting throughout spaces
Simple orientation for short, temporary visits	B	50-75-100	5-7.5-10
Working spaces where visual tasks are only occasionally performed	C	100-150-200	10-15-20
Performance of visual tasks of high contrast or large size	D	200-300-500	20-30-50
Performance of visual tasks of medium contrast or small size	E	500-750-1000	50-75-100	Illumination on local task
Performance of visual tasks of low contrast or very small size	F	1000-1500-2000	100-150-200
Performance of visual tasks of low contrast and very small size over a prolonged period	G	2000-3000-5000	200-300-500	Illumination on extremely difficult visual tasks
Performance of very prolonged and exacting visual tasks	H	5000-7500-10000	500-750-1000
Performance of very special visual tasks of extremely low contrast and small size	I	10000-15000-20000	1000-1500-2000

Table 2: Weighting Factor

Room and Occupant Characteristics	Weighting Factor
	-1	0	+1
Occupant ages	under 40	40-55	over 55
Importance of speed/accuracy (for Categories D-I)	Not Important	Important	Critical
Reflection of room surface/task background	Over 70%	30%-70%	Under 30%

Figure Your Needed Light

Identify what kind of space you have or what kind of task is being performed. Then located that space or task in Table 1 to find the range of illuminance values to use.

Table 2 will determine which of the three illuminance values to use. Add up all the weighting factors. If you have a total factor of -2 or -3, use the low illuminance value from Table 1. If you have a factor of +2 or +3, use the high illuminance value. Otherwise, use the middle illuminance value.