The light given off by any source, whether it's a lamp, a computer screen or the sun itself, carries an intensity and brightness as defining features of it. Calculating the lux levels can give you a greater idea of how powerful a light bulb is or how effective a light source is in using energy. There are straightforward formulas for doing so.
Lux Level
Lux is a unit for measuring illuminance, the amount of light that hits an area, for a particular surface. Because light spreads in all directions from its source, the "surface area" for light at a particular point in space may seem confusing.
In calculations of lux, you imagine a spherical surface area through which the light travels and use the point of interest as a point on the surface area. Other units of illuminance that scientists and engineers use include phot or foot-candle, with 1 Phot equal to 10000 lux and 1 foot-candle as 10.7639 lux.
You can also measure illuminance as E with the equation
for luminous flux "phi" Φ (in lumens) and surface area through which light travels A in m2. This means you can calculate lux from lumens if you know the area of a particular surface over which the luminous flux occurs. Illuminance uses lux as units, and luminous flux uses lumens as units. Don't get "flux" and "lux" mixed up!
You can then use luminous flux Φ in determining intensity I and candela "omega" Ω using
in which the candela measures the amount of light emitted in the range of an angular span that connects the light source to the point of interest in units of steradians (sr).
If the light source extends in all directions and you want to measure a point on an imaginary surface area that extends from the light source, you use 4 π steradians as candela Ω because a sphere is defined to have 4π steradians. Take into account the angle over which a particular surface area extends to figure out what proportion of a sphere's surface area over which a given light source extends.
Experimentally Measuring Lux Level
Make sure that, if you use equations involving lux of a light source, that you account for the distance between the light source itself and a given point. This means using the tungsten filament of a light bulb or the center of the empty space in a light bulb instead of only stopping at the light bulb or light source's case itself.
Though calculations of theoretical examples can tell you hypothetical values of lux for given arrangements of light sources, in practice there are more straightforward ways of measuring lux.
The formula
for illuminance E (sometimes denoted as I), average lumens value from a light source F (sometimes Ll), coefficient of utilization UF (or Cu) and light source maintenance factor MF (or LLF) and area per lamp A. The coefficient is also referred to as the utilization factor, and it accounts for the coloring of the surfaces of the light source. The maintenance factor, or light loss factor, describes how the lamp lets the level of light fall over time.
Using a lux Measurement Chart
Light meters measure the intensity of light and can tell you the illuminance. You may also consider using sources such as an online lux measurement chart. EngineeringToolBox offers tables on illumination values for common light sources in lux. Other examples of online lux measurement chart values can tell you about the recommended illuminances in various environments. Banner Engineering offers one that tells you this.
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About the Author
S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.