Basic LED Knowledge Course in 10 Minutes
A basic understanding of lighting fundamentals is essential for decision-makers who are evaluating lighting upgrades. This document provides a brief overview of design parameters, technologies, and terminology used in the lighting industry.
Quantity of Illumination
The most common measure of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. Similarly, a light fixture's output can be expressed in lumens. A 60 watt incandescent bulb produces 800 lumens. A 100 watt incandescent bulb produces about 1600 lumens. A LED bulb can produce those 1600 lumens with only an 18 watt bulb. A 10 watt LED bulb is the equivalent of a 60 watt incandescent. As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when the lamp is new).
Light intensity measured on a plane at a specific location is called illuminance. Illuminance is measured in foot-candles, which are workplace lumens per square foot. You can measure illuminance using a light meter located on the work surface where tasks are performed. Using simple arithmetic and manufacturers' photo-metric data, you can predict illuminance for a defined space. (Lux is the metric unit for illuminance, measured in lumens per square meter. To convert foot-candles to lux, multiply foot-candles by 10.76.)
A draftsman requires 75 foot candles to work effectively. Most office spaces are designed with 50 foot candles in mind.
Click here for the complete footcandle recommendations by Guth
Another measurement of light is luminance, sometimes called brightness. This measures light "leaving" a surface in a particular direction, and considers the illuminance on the surface and the reflectivity of the surface.
The human eye does not see illuminance; it sees luminance. Therefore, the amount of light delivered into a given space plus the ability of the surfaces within that space to absorb or reflect light affects your ability to see.
• Luminous flux is commonly called light output and is measured in lumens (lm).
• Illuminance is called light level and is measured in foot-candles (fc).
The most important factor with respect to lighting quality is glare. Glare is a sensation caused by luminescence in the visual field that is too bright. Discomfort, annoyance, or reduced productivity can result.
A bright object alone does not necessarily cause glare, but a bright object in front of a dark background, however, usually will cause glare. Contrast is the relationship between the luminance of an object and its background.
Although the visual task generally becomes easier with increased contrast, too much contrast causes glare and makes the visual task much more difficult.
You can reduce glare or luminance ratios by not exceeding suggested light levels and by using lighting equipment designed to reduce glare. A louver or lens is commonly used to block direct viewing of a light source. Indirect lighting, or up-lighting, can create a low glare environment by uniformly lighting the ceiling. Also, proper fixture placement can reduce reflected glare on work surfaces or computer screens.
Standard data now provided with luminaire specifications include tables of its visual comfort probability (VCP) ratings for various room geometries. The VCP index provides an indication of the percentage of people in a given space that would find the glare from a fixture to be acceptable. A minimum VCP of 70 is recommended for commercial interiors, while luminaries with VCPs exceeding 80 are recommended in computer areas.
Characteristics of Light Sources
Electric light sources have three characteristics: efficiency, color temperature, and color rendering index (CRI). Exhibit 4 summarizes these characteristics.
Some lamp types are more efficient in converting energy into visible light than others. The efficacy of a lamp refers to the number of lumens leaving the lamp compared to the number of watts required by the lamp (and ballast). It is expressed in lumens per watt. Sources with higher efficacy require less electrical energy to light a space.
Another characteristic of a light source is the color temperature. This is a measurement of "warmth" or "coolness" provided by the lamp. People usually prefer a warmer source in lower illuminance areas, such as dining areas and living rooms, and a cooler source in higher illuminance areas, such as grocery stores.
What Does Color Temperature Mean?
The technical definition of color temperature is full of terms like "black-body radiator" and "chromacity space" - in short, it's very confusing, very boring, and above all leaves you feeling even more baffled than before.
In layman's terms though, different light sources produce different colored light. For example, a candle emits a reddish light, while the midday sun's rays have a blue tint. These different colors’ can be expressed using a number, and this number is known as the color temperature.
Color temperature is measured on the Kelvin scale, which is denoted by the letter "K" or the word "kelvin" after the number. However, this is largely irrelevant - the only part we're interested in is the number.
Refer to the picture on the right for color temperatures of various light sources.
Color Rendering Index
The CRI is a relative scale (ranging from 0 - 100). Indicating how perceived colors match actual colors. It measures the degree that perceived colors of objects, illuminated by a given light source, conform to the colors of those same objects when they are lighted by a reference standard light source. The higher the color rendering index, the less color shift or distortion occurs.
The CRI number does not indicate which colors will shift or by how much; it is rather an indication of the average shift of eight standard colors. Two different light sources may have identical CRI values, but colors may appear quite different under these two sources.