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What Kind of Light Do I Want?

With the numerous advances in lighting technology, it can be hard to chose the right light source unless you understand the basics of how light is described and measured.

Lighting measurements deal with:

  1. The quantity of light – How much light comes out of the light and how much hits the surface you want to illuminate, such as a desk or countertop.
  2. The quality of light – What is the color of the light itself and how does it affect what colors I actually see. Are the blues really blue or are they washed out? Can I distinguish one item from another on a dark night with this light?
  3. The fixture efficiency – How much light actually leaves the fixture and is directed towards the area or surface that needs to be illuminated? How does that affect my cost?

The Quantity of Light

Luminous Flux or Light Output is the quantity of light that leaves the lamp. This is measured in lumens.

Most people think of measuring light as how much light comes out of a traditional incandescent bulb. Everyone knows that a 100 watt bulb will pleasantly light a bedroom and that a 15 watt bulb is a good night light.

As new, more efficient lighting sources are developed, the general public will need to start learning how to measure light in lumens. For example, a 100 watt incandescent bulb is about 1700 lumens. An equivalent CFL (Compact Fluorescent Light) would be a 28 watt bulb at 1600 lumens. A 15 watt LED can output as much as 1620 lumens.

Illuminance is the amount of light measured on the work surface, such as a desk or counter in the lighted space. This is measured in lux (metric) or footcandles (English). A lux is one lumen per square meter. A footcandle is one lumen of light density per square foot. One footcandle equals approximately 10 lux.

The chart below shows the illuminance one can expect to see outdoors in different conditions.

  Footcandles Lux
Sunny Day 10,000 100,000
Overcast Day 1000 10,000
Very Dark Day 100 1,000
Twilight 1 10
Deep Twilight .1 1
Full Moon .01 .1
Quarter Moon .001 .01
Starlight .0001 .001

When determining acceptable light levels, it is usually more important to think of Lux or Footcandles rather than lumens as this more accurately measures the amount of light the human eye will see. Often lumens are “wasted” by throwing light into areas that are not perceived, such as the interior of the fixture or straight up into the night sky.

When buying a fixture, you can often get a spec sheet from the manufacturer that has a chart showing the lux levels (or footcandles) at various lengths away from the light source. The mounting height of the light source will affect these levels, so that is often specified on the spec sheet.

A Lux/Footcandle meter is a useful tool in understanding acceptable light levels. A simple meter can be purchased for less than $100 and is quite useful if you need to spec out or purchase a major lighting installation.

Lights can vary widely in their illuminance and descriptions of “Bright” or “Super-Bright” can be very misleading. A survey of several types of undercabinet lighting from a discount home improvement store revealed most had lux levels of less than 10% of the recommended levels. With the help of a lighting expert and a simple light meter, you can be assured to get the light output you really need.

The following light levels are recommended:

  Footcandles Lux
Ambient Light
(Home, Office, Classrooms)
15-30 150-300
Office Work, Reading 50 500
Supermarkets, Stores 75 750
Detailed Task Lighting 100 1000
Very Detailed Task Lighting 150-200 1500-2000
Poorly Lit Area Well Lit Area

Outdoor Lighting

The Lighting Research Center notes that in the early part of the twentieth century, when electric street lighting was beginning to be installed in many areas of the United States, moonlight levels were commonly used as a standard or reference point for outdoor lighting. In many instances, the visual quality of a street lighting design was measured against moonlight. When considering the needs of an outdoor lighting installation, it is still helpful to think in terms of moonlight levels.

In rural areas, moonlight, with an Illuminance of approximately .1 lux on the ground, often provides enough lighting for people’s basic needs such as walking or finding a house or a car.

Outdoor lighting is measured here as the Illuminance on the ground.

 

Lux Footcandle
Parking Lots 1 10 =100 full moons
Sidewalk, Home Driveways .05 .5 =5 full moons

The human eye is able to adjust to a wide range of light levels.

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