Deciding how much light you need for a space requires looking at a few key factors. The recommended light levels depend on the purpose of the room and the tasks performed there. Understanding foot candles and lumens will help you choose suitable light fixtures and bulbs. With some simple calculations, you can figure out the number of fixtures needed to achieve your target light levels.
What is a Foot Candle?
A foot candle is a unit that measures light intensity. It is defined as the amount of light falling on a 1-square foot surface from a standardized candle at a distance of 1 foot. The foot candle is equal to 1 lumen per square foot and is denoted as fc.
Foot candles are very useful for measuring illumination levels on work surfaces and floors. If you know the recommended foot candle level for a specific visual task, you can calculate the number and types of light fixtures needed to provide that lighting.
What is a Lumen?
A lumen is a measure of the total amount of visible light emitted by a light source. For example, a standard 100-watt incandescent light bulb produces about 1,600 lumens. Fluorescent tubes and LED fixtures also have lumen ratings based on their light output.
When choosing light bulbs and fixtures, pay attention to their lumen output rather than just their wattages. A higher lumen rating means the light bulb or fixture provides more light. Lumens, along with beam spread, determine the intensity of light hitting surfaces (measured in lux or foot candles).
Recommended Light Levels by Room
The Illuminating Engineering Society (IES) publishes recommendations for appropriate light levels in various interior spaces. Here are some general IES guidelines for residential lighting:
- Living Rooms: 10-20 fc
- Kitchens: 30-50 fc
- Bedrooms: 10-20 fc
- Bathrooms: 30 fc
- Hallways: 5-10 fc
- Staircases: 10-20 fc
- Garages: 20-30 fc
- Laundry Rooms: 30-50 fc
- Home Offices: 30-50 fc
Higher light levels may be recommended for detailed work like sewing, reading small print, or doing artwork. Task lighting can supplement general ambient lighting as needed. For example, a sewing room would need 50-100 fc of task lighting over the work area.
Factors Affecting Light Levels
Several factors influence the actual amount of light reaching a surface in your home:
- Distance from the Light Source – Light intensity diminishes exponentially with distance. The inverse square law states that illuminance decreases by the inverse square of the distance from the source. This means that small changes in distance make a big difference.
- Room Surfaces – Light colored walls and ceilings reflect more light, brightening up a space. Dark paint and paneling absorb light, meaning you need more fixtures to achieve the recommended illuminance.
- Fixtures and Lamps – The type of light fixture, its position, and direction determine how efficiently it lights a room. Recessed cans, track lighting, pendants, and table lamps have very different lighting patterns. Bulb types like LED and incandescent also affect intensity.
- Room Size – Bigger rooms need more fixtures to provide uniform illumination. A 12×12 ft room needs far fewer light sources than a 20×15 ft room to achieve the same fc level.
- Furniture – Furniture placement impacts how the light gets distributed throughout a room. And task lighting may be needed in areas with dense furniture blocking ambient light.
Using Light Meters
A light meter is a handy tool for measuring the actual light intensity in a room. It can measure the illuminance hitting a surface in foot candles or lux. Taking readings around a room at the working height (e.g. 3 feet above the floor) will give you an accurate idea of existing light levels.
If the readings are much lower than IES recommendations, you’ll need to add more lighting. You can take new readings after installing fixtures to confirm you’ve hit the target light levels for each room’s needs.
Calculating Light Fixture Requirements
The lighting calculation or lumen method uses a simple formula to determine the number of fixtures needed:
- Target fc (foot candles) for the room x Room Square Footage = Total Lumens Needed
- Total Lumens Needed / Fixture Lumens = Min. Number of Fixtures
This helps you choose appropriate lamps and fixtures based on their lumen output. Let’s walk through an example:
Say you want to achieve 50 fc in a 160 sq ft living room. Multiply the two to get 8000 lumens needed. If you choose a fixture rated at 1200 lumens, divide 8000 by 1200. This gives you about 7 fixtures to provide 50 fc of illumination.
Spacing the fixtures evenly throughout the room will give reasonably uniform light distribution. You may need a couple more fixtures to brighten up darker corners or supplement with task lighting as needed.
Choosing Light Bulbs
Along with the fixture style, you’ll need to pick suitable wattage light bulbs. Here are some common types with their light output:
|Bulb Type||Typical Lumens|
|40W Incandescent||450 lm|
|60W Incandescent||800 lm|
|75W Incandescent||1,100 lm|
|100W Incandescent||1,600 lm|
|9W LED (60W equivalent)||800 lm|
|11W LED (75W equivalent)||1,100 lm|
|15W LED (100W equivalent)||1,600 lm|
LED and CFL bulbs provide the same brightness as incandescents but use far less energy. Where aesthetic allows, choose efficient bulbs to save on your electricity costs.
Dimmers allow you to easily control the light intensity in a room. By turning a knob or sliding a control, you can create brighter or dimmer ambiance. Dimmers work with compatible bulbs like incandescent and LEDs.
Installing dimmable fixtures gives you flexibility. For example, you may want brighter task lighting for working at a desk or lower, relaxing light for watching TV. Just tune the dimmer to the desired illumination level.
Occupancy sensors automatically turn lights on when motion is detected in a room and off after a set time with no movement. They help conserve electricity by preventing lights staying on in unoccupied rooms.
Bathrooms, closets, garages, and pantries are good places for sensor lighting. The lights turn on when you enter and switch off shortly after you leave. Look for wireless battery-powered sensors for easiest retrofit installation.
Smart LED lighting takes control to the next level. With smart bulbs and switches, you can set lighting schedules, change colors, adjust intensities, and operate fixtures remotely.
Scheduling lets you automatically have lights turn on to wake you up or turn off at bedtime. Smart technology works great for exterior lighting too. Geo-fencing can turn on porch and yard lights when you arrive home at night.
Tunable white smart bulbs let you shift from energizing daylight to relaxing warm white with the tap of an app. Whether you want simple scheduling or advanced features like voice and app control, smart lighting provides ultimate convenience.
Hiring an Electrician
Professional help is recommended if you need to install new fixtures or wiring. Licensed electricians have the expertise to handle electrical work safely. They can inspect your existing wiring and determine if any upgrades are needed.
Discuss your lighting goals and layout options with an electrician. They can advise suitable fixtures, switches, and controls to create your ideal lighting scheme. Many electricians also handle the entire process from layout to acquiring permits to final testing and certification.
Energy Efficient Lighting
Choosing energy efficient bulbs and fixtures can drastically reduce your home’s lighting electricity use. According to the U.S. Department of Energy, lighting accounts for about 15% of household electricity consumption.
Upgrading old incandescent bulbs to LED can cut lighting energy use by 75%. And features like dimmers, timers, and motion sensors help prevent waste from unnecessary use.
Energy efficient lighting provides substantial savings on your electricity bill while still meeting your lighting needs. And utilities or government agencies may offer rebates and incentives for installing qualifying energy efficient lighting.
Determining adequate light levels for a space involves considering the size, ceiling height, and tasks performed. Fixture and bulb lumens output, beam spread, and placement options all impact the resulting foot candles on room surfaces.
Use IES recommendations matched to room functions as a starting point. Adjust up or down based on specific needs and preferences. Taking light meter readings lets you fine-tune the lighting scheme and achieve your target illumination.
With the right light bulbs and fixtures arranged thoughtfully throughout each room, you can enjoy a comfortably bright and welcoming home lighting ambiance.