• Energy & Power

Lighting Principles and Terms



light output by bulb type

In choosing the best options, you must understand what basic is. This page explains the terminology used in the industry, which will help you better understand the language that you might hear in a store.

Illumination

A lumenis a measurement of the light of a lamp, often a tube or a lamp. All lamps are rated in lumens. For example, a 100-watt light bulb produces about 1750 lumens.

The distribution of light on a horizontal surface is called its illumination. Illumination is measured in footcandles. A footcandle of illumination is a lumen of light distributed over a 1-square-foot (0.09-square-meter) area.

The amount of lighting required varies depending on the difficulty of a visual task. Ideal illumination is the minimum footcandles necessary to allow you to perform a task comfortably and proficiently without eyestrain.  The illuminating Engineering Society says that the lighting of 30 to 50 footcandles is sufficient for most homes and offices.  Difficult and lengthy visual tasks—like sewing for extended periods of time—require 200 to 500 footcandles. Where no seeing tasks (i.e., tasks whose speed and accuracy of completion are affected by quality and quantity of light) are performed, lighting systems need to provide only security, safety, or visual comfort—requiring from 5 to 20 footcandles of illumination.

Another term relief you hear is efficacy. This is the ratio between the amount of light from a lamp to the electrical energy it consumes and is measured in lumens per watt (LPW).

Lighting Uses

Experts divide lighting applications into three categories: ambient, task and accent lighting. Ambient lighting provides security and safety, and general lighting to perform daily activities. The goal of task lighting is to provide enough illumination so that tasks can be completed accurately but not so much light that whole areas have been illuminated. Accent lighting illuminates walls so they blend more closely with naturally bright areas like ceilings and windows.

Light quality describes how well people in a lighted space can see to do visual tasks and how visually comfortable they feel in that space. Light quality is important to energy efficiency because spaces with higher quality lighting need less illumination. High-quality lighting is fairly uniform in brightness and has no glare.

For example, direct intense sunlight streaming through the windows of a room with chocolate brown carpets and dark wall paneling will likely give too much contrast in brightness. The pupils of your eyes will constantly adjust to the differing brightnesses. Making this area visually comfortable would involve using lots of artificial lighting with a high illumination level.

On the other hand, in a pale-colored room bathed in soft light, you can hardly tell where the light is coming from because no one area of the room appears much brighter than another. The walls, ceiling, floor, and work surfaces are relatively the same light hue. People can perform tasks faster and with fewer mistakes with this type of high-quality lighting. Also, lighting such a room requires far less artificial lighting than the previous example.

Glare

Eliminating glare (i.e., excessive brightness from a direct light source) is essential to achieving good lighting quality. Types of glare include direct glare, reflected glare,and veiling reflections.

Direct glare results from strong light from windows or bright lamps shining directly into your eyes. Reflected glare is caused by strong light from windows or lamps that is reflected off a shiny surface into your eyes. Veiling reflection is a special type of reflected glare that can obscure contrasts and reduce task clarity. Veiling reflections occur when light is reflected into your eyes from a work surface, such as a printed page or a computer screen.

Light Color and Color Rendering

Lamps are assigned a color temperature (according to the Kelvin temperature scale) based on their “coolness” or “warmness.” The human eye perceives colors as cool if they are at the blue-green end of the color spectrum, and warm if they are at the red end of the spectrum.

Cool light is preferred for visual tasks because it produces higher contrast than warm light. Contrast is the brightness difference between different parts of the visual field, which is the expanse of space you can see at a given instant without moving your eyes. Warm light is preferred for living spaces because it is more flattering to skin tones and clothing.

Keep in mind, though, that artificial light sources vary widely in their color rendering indexes (CRI). The CRI is a measurement of a light source’s ability to render colors the same as sunlight does. For example, incandescent lamps are rated at a CRI of 100—nearly equal to sunlight—while some high-pressure sodium lamps have a CRI of 22, which means they render colors very poorly.

However, a light’s color-rendering ability is not related to whether it is a cool or warm color. For example, blue light from the northern sky, white light at noon, and red light from a sunset all have perfect color rendering (a CRI of 100) because our eyes are designed to read the colors of objects illuminated by sunlight.