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Basic knowledge about LED lights


Light Quantity

The distribution of light on a horizontal surface. The purpose of all lighting is to produce illumination.


A measurement of light emitted by a lamp. As reference, a 100-watt incandescent lamp emits about 1600 lumens.


A measurement of the intensity of illumination. A footcandle is the illumination produced by one lumen distributed over a 1-square-foot area. For most home and office work, 30–50 footcandles of illumination is sufficient. For detailed work, 200 footcandles of illumination or more allows more accuracy and less eyestrain. For simply finding one's way around at night, 5–20 footcandles may be sufficient

Energy Consumption

The ratio of light produced to energy consumed. It's measured as the number of lumens produced divided by the rate of electricity consumption (lumens per watt).



The percentage of total lamp lumens that a lighting fixture, luminaire, or system emits, minus any blocked or wasted light

Light Quality

Color temperature
The color of the light source. By convention, yellow-red colors (like the flames of a fire) are considered warm, and blue-green colors (like light from an overcast sky) are considered cool. Color temperature is measured in Kelvin (K) temperature. Confusingly, higher Kelvin temperatures (3600–5500 K) are what we consider cool and lower color temperatures (2700–3000 K) are considered warm. Cool light is preferred for visual tasks because it produces higher contrast than warm light. Warm light is preferred for living spaces because it is more flattering to skin tones and clothing. A color temperature of 2700–3600 K is generally recommended for most indoor general and task lighting applications.


Color rendition
How colors appear when illuminated by a light source. Color rendition is generally considered to be a more important lighting quality than color temperature. Most objects are not a single color, but a combination of many colors. Light sources that are deficient in certain colors may change the apparent color of an object. The Color Rendition Index (CRI) is a 1–100 scale that measures a light source's ability to render colors the same way sunlight does. The top value of the CRI scale (100) is based on illumination by a 100-watt incandescent lightbulb. A light source with a CRI of 80 or higher is considered acceptable for most indoor residential applications.


The excessive brightness from a direct light source that makes it difficult to see what one wishes to see. A bright object in front of a dark background usually will cause glare. Bright lights reflecting off a television or computer screen or even a printed page produces glare. Intense light sources - such as bright incandescent lamps - are likely to produce more direct glare than large fluorescent lamps. However, glare is primarily the result of relative placement of light sources and the objects being viewed.


Glare occurs when people complain about visual discomfort in the presence of bright light sources, luminaires or windows. Discomfort glare is quantified by the Unified Glare Rating (UGR). Glare calculations are complex and involve summing all the light from all sources at a particular angle entering the eye at a particular location. The UGR simplifies this by providing a rating for a luminaire which enables us to determine if it is likely to cause discomfort in specific types of work situation. In offices used for normal (clerical) type work, the LUX level will be around 500 and at this lighting intensity a UGR of 19 is specified as the maximum glare rating above which discomfort may occur.

Glare is subjective and sensitive to many factors including positioning of workspaces and relative positioning of luminaires. In a small (4m x4m) office glare is unlikely to be noticed even if the luminaires are > UGR19, as the visual angle from the eye to the luminaire will be well above the normal sight line for working and no discomfort is likely to arise. This will not be so in a shared workspace (say 15mx15m) where distant luminaires could easily cause discomfort in the working sight line or by reflectance from a Computer screen. Here luminaires with UGR 19 or better ensure that in most situations, stray light cannot enter the sight line and cause discomfort.

UGR19 (in old terms CAT2 lighting) limits the emission of light at shallow angles from the luminaire which might otherwise interfere with office tasks.

In corridors and circulation spaces, where people are standing and the typical LUX levels are closer to 200, a UGR of 22 is acceptable.

Lighting Uses

Ambient lighting
Provides general illumination indoors for daily activities, and outdoors for safety and security.


Task lighting
Facilitates particular tasks that require more light than is needed for general illumination, such as under-counter kitchen lights, table lamps, or bathroom mirror lights.


Accent lighting
Draws attention to special features or enhances the aesthetic qualities of an indoor or outdoor environment.

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