Light meters, commonly referred to as lux meters, are precise instruments designed to quantify the amount of light. Though both natural and artificial light, or the
absence of it, is a natural part of our lives, being able to measure it is important for a number of reasons. Light affects the way we see the world. Too much light, however,
can damage sensitive materials, cause sleep disruption, contribute to light pollution, and waste electricity and money. Too little light can harm plants, cause eye strain,
conceal dangers from us, and affect mood. It's no wonder controlling light is so important.
Lighting comes in all shapes and sizes. From large industrial scale lighting to the tightly designed illumination of a museum piece. Lighting also has a wide variety of sources
including incandescent, LED, fluorescent, and many other types. Overall, lighting accounts for about 17% of electricity usage in the United States. The potential for savings is
enormous, especially for large facilities. As tempting as it might be to dim the lights to save money, workplace regulations require minimum lighting standards to provide safe
working conditions.
Light meters are useful tools in building design and operation. They help inform lighting requirements prior to building commissioning and, once the building is inhabited, they
allow routine auditing of lighting systems for energy efficiency and lighting performance making it easy and convenient to optimize light levels to reduce the energy burden of
buildings and fulfilling compliance with OSHA regulations.
Measuring Light
Understanding light intensity and, therefore, being more capable of accurately measuring it is much easier with the knowledge of a few important terms.
Luminance: Luminance is the amount of light reflecting off a surface. Generally refers to brightness as the human perception of brightness is due to reflected light.
Luminance is measured in candelas per square meter.
Illuminance: Illuminance is the total luminous flux on a unit of surface area. It is a measure of how much incident light illuminates the surface, weighted according
to ability of humans to perceive the wavelengths of light. Illuminance is measured in lux or lumens per square meter.
Luminous flux: Luminous flux is the measure of the perceived power of light. It is evaluated according to its ability to produce a visual sensation, thus favoring visible
light over infrared or ultraviolet light. Often used as an objective measure of the "useable" light emitted from a source, it is measured in lumens.
Luminosity function: Luminosity function describes the average spectral sensitivity of human visual perception of brightness. While not perfectly accurate in every case,
it is a good representation of visual sensitivity of the human eye.
Candela: The candela is a unit of light intensity. It includes light emitted from a source in a particular direction and weighted according to the luminosity function. A
common candle emits light with an intensity of roughly one candela.
Lumen: The lumen is a unit of total visible light (luminous flux)emitted by a source. A lumen includes light radiated in all directions. A light source that uniformly radiates
one candela in all directions has a total luminous flux of 12.57 lumens.
Lux: The lux is a unit of light intensity. Measuring luminous flux per unit area, is it equal to one lumen per square meter.
Foot-candle: The foot-candle is a unit of light intensity equal to the illuminance cast on a surface by a one-candela source one foot away. One foot-candle is equal to one
lumen per square foot or approximately 10.764 lux
Light Meter Technology
Most industrial light meters utilize silicone photodiode sensors as the measuring element. Photodiodes are photovoltaic cells which generate a current proportional to light exposure. As photons
of light strike the diode, electrons are created which flow towards the cathode creating a photocurrent. More photons striking the diode results in more total current. The current is converted
to measurement units of light (lux, foot-candles, etc.) and displayed on a needle galvanometer or on an LCD screen.
Light measurements, of course, are susceptible to a number of types or errors. Light at the point of measurement, for example, should be proportional to the cosine of the angle at which the
light is incident. The design of many light meters, though, forces the light to enter the meter at the incorrect angle which can cause measurement errors of up to 25%. Some meters include a
"cosine correction" feature which consists of a diffuser which is placed over the sensor and filter which corrects the angle of collection.
The human eye also perceives wavelengths and colors of light differently. The human eye is more sensitive to yellow and green light than red or blue. We are also blind to certain wavelengths
of light such as ultraviolet or infrared. For light meters to measure light in the way it is perceived by humans, special filters need to be incorporated into the meter to match the CIE photopic
response (human eye response). As different sources of light have different color palates, they may be perceived differently by human eyes even though they are of equal illuminance. The spectral
response filters correct for this phenomenon.
Things to Consider When Purchasing a Light Meter:
- What accuracy is required for your application?
- Do you need a data logging feature or computer interface?
- What range and measurement units do you require?
- Does the unit have color correction (spectral response filter)?
- Does the unit have cosine correction?
If you have any questions regarding light meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
