# Are Lumen and Lux the same in a bulb light?

You know how important is to pick up a bicycle light when you leave home for riding in the dark. But how to chosse the best? How to make diference between lumen an lux, when both of them are characteristics of the light.

In this article we explain what lux and what lumen means and say about the light?

##### WHAT LUX MEANS

Lux is the amount of light on a certain area. It is measured on a distance of 1 or 10 meters.

Lux is used as an unit of illumination in the International System of Units (SI).

One lux (Latin for “light”) is the amount of illumination provided when one lumen is evenly distributed over an area of one square metre.

This is also equivalent to the illumination that would exist on a surface all points of which are one metre from a point source of one international candle (candela). One lux is equal to 0.0929 foot-candle.

##### WHAT LUMEN MEANS

Lumen is the total amount of light (in all directions) emitted by that lamp, in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd.

The level of lumen from the lamp itself is lower, because the light has passed the lens, resulting in loss of light. This is the amount of light you really see in front of your bicycle.

The lumen is a small unit. One lumen is the equivalent of 1.46 milliwatt of radiant electromagnetic (EM) power at a frequency of 540 terahertz. Reduced to SI base units, one lumen is equal to 0.00146 kilogram meter squared per second cubed.

Other interesting caracteristincs of the light are candela and sreadian.

The **candela** is the SI base unit of luminous intensity. This is, luminous power per unit solid angle emitted by a point light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source’s spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths).

A common candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

Originally, luminous intensity was measured in terms of units called candles. This expression arose from the fact that one candle represented approximately the amount of visible radiation emitted by a candle flame. This was an inexact specification because burning candles vary in brilliance. So, for a time, a specified amount of radiation from elemental platinum at its freezing temperature was used as the standard. Late in the 20th century, the current definition and terminology were adopted.

The **steradian** or square radian is the SI unit of solid angle. It is used in three-dimensional geometry, and is analogous to the radian which quantifies planar angles. The name is derived from the Greek stereos for “solid” and the Latin radius for “ray, beam”.

There are 4 pi, or approximately 12.5664, steradians in a complete sphere.

A steradian is defined as conical in shape, as shown in the illustration. Point *P* represents the center of the sphere. The solid (conical) angle *q*, representing one steradian, is such that the area *A* of the subtended portion of the sphere is equal to *r*^{2}, where *r* is the radius of the sphere.