LEDs and Lumens - Part 1

Light measurement basics

Both candela and candlepower correspond to the amount of light (i.e. the quantity of photons) produced by a standard light source. Originally, the standard light source was a real candle so it is also called candlepower. But like most measurement standards, it's now a theoretical construct having a specific wavelength of visible light at a specific power level. The good news is that the intensity of the standard candela is nearly identical to the original standard candle. All other quantitative measurements related to light are derived from the candela.

Candelas are a measure of the amount of light generated at a source. Lumens quantify how much total light flows away from the source. Foot-candles and LUX indicate how well the surface area of an object, wall or floor is illuminated at some distance away from the source (intensity of the light reflecting off objects like the floor, wall, or worksurface).

So what does this mean? If you have a light source of 1 candela, it's also 1 candlepower in intensity. If you measured the light on a 1 square foor surface, 1 foot away, it will have 1 foot-candle of illuminance over it's surface. In metric terms, if you measured the amount of light on a square meter that was 1 meter away, it will have 1 LUX of illuminance.So you could say that the light source is generating 1 Lumen in the direction of the 1 square meter that is 1 meter away. If the light source is in the middle of a sphere and generating light in all directions, you will find that the 1 cendella light source is generating 12.57 lumens as explained below.

Light Measurement Equivalents and Common Illumination Levels:

- 1 square meter (M²) = 10.7638 ft²
- 1 LUX is defined as 1 lumen/ M² of a surface; thus 1 LUX= 0.0929 FC,
- 1 foot candle (FC) is 1 Lumen/ ft² of a surface; thus 1 FC=10.7638 LUX
- There are 12.57 square meters on the surface of a 1 meter radius sphere.

In real life objects are illuminated and measured in LUX:

- Full Daylight is about 10,000 LUX (imagine 10,000 lumens each and every square meter!)
- Cloudy day is about 1,000 LUX
- A lighted parking lot at night is about 10 LUX (average)
- A full moon is about 0.1 LUX

OSHA Lighting Level Recommendations for Work Spaces:

- Public spaces with dark surroundings 30 LUX
- Simple orientation for short temporary visits 50 LUX
- Working spaces where visual tasks are only occasionally performed 100 LUX
- Performance of visual tasks of high contrast or large scale 300 LUX
- Performance of visual tasks of medium contrast or small size 500 LUX
- Performance of visual tasks of low contrast or very small size 1000 LUX

In LED specifications:

- lm is lumens - total luminous flux flowing from the LED. mlm = millilumens.
- cd is candelas - the luminous intensity at a specific forward current. mcd = millicandela.

Cd or mcd units are usually expressed as a value relative to the viewing angle. The larger the viewing angle, the more light flows given the same intensity. In other words, 1000 mcd over 130° viewing angle is a lot brighter (more lumens) than 1000 mcd over 20° viewing angle. Lumens (lm or mlm) usually refers to the total light output of the device at the rated current.