Lux Meter

Lux is the unit for illuminance in the international SI system of units. A lux measuring device or lux meter is therefore a device for determining illuminance. The illuminance is a value that does not refer to the light source, but to the illuminated area. The lux meter can be used to determine how much of the luminous flux emitted by one or more light sources reaches a specific surface. One lux is defined as one lumen per square meter, which corresponds to the illuminance produced by the light of a candle at a distance of one meter. When comparing different measuring points and evaluating the measurement results, it must be taken into account that the impression of brightness can differ even for the surfaces illuminated in the same way if they have different colors or structures.

Selection criteria for a lux meter
In order to find a suitable model from a large number of the lux meters available, the requirements for the device should first be clarified. For this purpose, it is important to know under which conditions and with which aim the illuminances are to be measured. The selection can be narrowed down using the following points:

- For continuous measurement of lighting conditions at the same position, a stationary lux meter should be selected.
- For single measurements and time-limited series of measurements at different locations, mobile lux meters are suitable.
- The measuring range of the device must match whether measurements are to be taken indoors, outdoors or under special lighting conditions.
- The required accuracy of the measurements should be determined in advance.
- Other features important for the measurement and additional functions that are necessary or desired should be evaluated.

Measurements with a mobile or stationary lux meter
In the darkness, human vision is significantly limited. Certain lighting conditions are necessary to recognize details, colors and spatial structures. To ensure that dangerous situations can be perceived visually and that the completion of work and learning tasks is not made difficult by insufficient lighting, minimum illuminance levels are prescribed for many areas. This applies to both private and public areas.

Compliance with the illuminance values specified in accident prevention regulations, occupational health and safety directives and technical standards is checked with the lux meter at the level relevant for the use. For example, the measurements are taken on the table surface in the case of work tables or workbenches, on the panel surface in the case of wall panels, and in the case of traffic routes – on their surface. In order to determine whether the desired target values are achieved on all the relevant surfaces with an existing or just installed lighting system, a mobile lux meter is usually used. Since such measurements often involve testing many individual measuring points or even complete grids, handy lux meters with battery operation are generally used for this purpose.

For the measurements of illuminance in the workplaces, traffic routes and in occupied areas, these lux meters must be adapted to the spectral brightness sensitivity of the human eye and also correctly evaluate obliquely incident light. If the illuminance is not sufficient at all points, the mobile lux meter can also be used to quickly and easily test different options for improvement. In some cases, it is possible to determine directly on site whether sufficient lighting conditions can be achieved by replacing the lamps with lamps with a higher luminance efficiency or by installing additional luminaires.

It is not always a question of adhering to minimum values, but in some areas also of the targeted use of different illuminance levels. This applies, for example, to lighting in hospital, sick and nursing rooms, but also to lighting for the cultivation of plants and for the rearing and keeping of animals in stables. In other areas, it is a matter of falling below maximum values, since light is not desirable for certain processes because it shortens the shelf life of certain substances or causes other undesirable reactions. The lighting of ordinary recreation rooms is usually checked with a mobile lux meter only when there is a specific reason to do so. In the areas where too little or too much light can lead to damage, or where only a narrowly limited range of illuminance values leads to optimum results, stationary lux meters are used for continuous light measurement. This applies, for example, to photographic laboratories and certain facilities in food and drug production or in plant cultivation. Such a stationary lux meter is usually equipped with relay or control outputs that can be used to switch the artificial light sources or shading systems when specified values are reached, or to trigger visual or audible alarms outside the affected area.

Measuring range and accuracy of the lux meter
Depending on the measuring task, not only the upper but also the lower limit value of the measuring range should be considered when comparing different lux meters. The larger the measuring range, the more possible applications there are. If the lux meter is only to be used for testing the lighting situation in common indoor spaces, the measuring range of up to 2,000 lux or 10,000 lux is often sufficient. For the measurements in the workplaces where a lot of light is required and outdoors where also high illuminance levels from sunlight must be correctly recorded, the measuring range should be at least 100,000 lux.

For the devices measuring the illuminance, the accuracy classes and their limit values are specified in DIN 5032 Part 7. This makes it possible to quickly and easily specify and identify whether the lux meter is only suitable for orientational measurements based on its accuracy class C, for lighting measurements in businesses and public areas with the classes B and A, or is even suitable for precision measurements with the accuracy class L.

Other important features and additional functions for the lux meter
Annex B of DIN EN 13032 Part 1 covers many other properties in which various models of the lux meter differ. These include, for example, deviations in relative spectral sensitivity from that of the human eye, sensitivity to ultraviolet and infrared light, and sensitivity to obliquely incident and polarized light. It also discusses differences in temperature dependence and in the evaluation of time-varying light.

For some measuring tasks it is advantageous if the lux meter has an external light sensor. The external light sensor can be easily positioned and, if necessary, fixed at the desired measurement point and sends the measurement data to the device, which can be conveniently read-out and operated without the user shading the light sensor.

Both mobile and stationary lux meters can be equipped with memory functions and interfaces for data transmission. Special software can be used to conveniently evaluate and document the complete measurement data. Stationary devices can also be equipped with analog or switching interfaces to automatically control the other devices depending on the measured illuminance. As combination devices for occupational safety or environmental measurements, some devices can measure not only illuminance, but also other values such as sound level, wind speed, air temperature and humidity.

The variety of lux meters on the market gives the users the possibility to select the model, suitable for the measuring task and for their budget as well. The measurement of illumination in the room, in the workplace, in the warehouse, even in the open air may be absolutely compulsory with the strict requirements to the accuracy of the measurements, or have spontaneous character and precise determination of the values is not a priority. The simplest lux meters are easy to operate and convenient, due to the LC display and clear menu. They do not have light filters as against to the most modern devices, but are still very appropriate for the measurement outside, for the gardeners in the greenhouses, inside for various measuring tasks, but it is not worth to expect high accuracy from such devices.

Modern lux meters often possess special corrective filters, which make the sensitivity of the photo element of the lux meter closer and similar to the sensitivity of the human eye. The photodiodes of the cheaper models of lux meters are sensitive to all types of the emission, IR and UV, which makes the accuracy of the measurement lower. Besides, more complicated models of the lux meters can be equipped with the accessories like various nozzles. They are needed for the specious illumination characteristics or measurement under the conditions when light incidence occurs at different angles. The nozzles help to compensate for this and achieve accurate results.

During the application of the lux meter, it is necessary to pay attention to the type of the source of light, in other words, type of the lamp, since emission spectrum from the diode lamp will be different from the one coming from the fluorescent lamp etc, thus, the coefficient for different types of lamps is introduced.

Lux meters capable to measure the UV emission are applicable for more complicated tasks, like sanitary control, technical control, solarium and different types of researches and experiments.

The most modern devices carry out a wide range of measurements, due to the possibility of combination with different sensors: from UV sources to various artificial light sources. Such devices may be called professional, and they are suitable for the most demanding measurements in the various laboratories, including medical. There is a possibility to connect the devices to the computer and not only store, but also to process and analyze all the measurements later.

Lux measurements are required both in private and public life, since life without life is impossible. Growing conditions for plants, mood of people, level of comfort, convenience and favorable conditions for performing numerous tasks – all depends on illumination. Industrial sector stands in this list quite ahead, because it goes not only about comfort, but often about safety.

ISO standards describe the requirements to the illumination in the workplace, in order to avoid accidents and damage to health of people caused by insufficient illumination. Correct distribution of the lighting device in the company: offices, corridors, meeting-rooms, warehouses etc. helps to sustain a good productivity level and creation of the most optimal working conditions. Lux meter replaces the visual control and provides exact numbers that indicate what and where should be changed. The choice of the light source may be wrong, the lamps may turn out to be dirty and thus block the light, or there may be just not enough lighting devices. All these problems can be easily eliminated and lux meter is a perfect auxiliary tool for this purpose.