Light Meter

Light plays a major role in most living organisms and in the operation of certain chemical processes. The light radiation occurring at a particular location can be described by the values for brightness determined with a light meter and by the colour of light. However, the term light meter or photometer is used to describe not only the devices for measuring the properties of light, but also the devices that use light sensors to determine quantities that affect the light transmission of the substance under inspection. In the following text, only the devices used to determine the values for light properties of light sources or of illuminated surfaces will be discussed.

Explanations of luminous flux, luminous intensity and illuminance
In light measurements, the brightness is specified either for the light source or for the illuminated surfaces. In relation to the light source, a distinction can be made between the total emitted luminous flux in lumens and the luminous intensity in candela, which also takes the beam angle into account. One candela corresponds to one lumen per steradian, i.e. per solid angle unit. In the luminaires and lamps currently available, the luminous flux is almost always specified in lumens. To determine the luminous intensity in candela, the number of lumens must be divided by 4 π for a light source radiating uniformly in all directions, and by 2 π for a light source radiating only in a hemispherical way.

The term candela refers to candlelight. Thus, it is easy to remember that a candle flame shines with a luminous intensity of one candela. In contrast, a bright firefly has a luminous intensity of only 0.004 candela, and the daytime running lights on motor vehicles have a luminous intensity of at least 400 candela. The high beam of all headlights on a road vehicle may not shine brighter than 225,000 candela, and beacons used as orientation aids for seafaring may emit 35,000,000 candela, for example. The luminous flux and luminous intensity of luminaires and lighting elements are generally determined in the course of product development and during quality control at the manufacturers' with a special light measuring device.

While luminous flux and luminous intensity can be used to compare the light output of different light sources, illuminance describes how much light is incident on an illuminated surface. Illuminance is specified as an area-related quantity in lux, where one lux is defined as one lumen per square meter. Apart from the sun as a very strong light source, it is easy to see, even without a light meter, that the brightness of the light hitting a surface decreases with increasing distance from the light source. However, it is not only the distance from the light source that affects how much light arrives, but also additional light sources, shading, attenuation, and reflections from surrounding surfaces. The luminous intensity of elements for artificial lighting can also be weakened by aging or soiling. It is therefore advisable to check the illuminance at the relevant points on site not only immediately after installation, but also at regular intervals later with the help of the suitable light meter. In the case of technical systems for the use of sunlight, it is recommended to use the light meter before the installation to check which arrangement is the most favourable. After the installation, it can be checked whether the system achieves the promised performance for the amount of the incident light.

Selection criteria for the light meter to measure illuminance
First, the light meter must be suitable for the purpose of the measurement. This is not only a question of whether the luminous flux, luminous intensity or illuminance is to be determined. It is also important whether the light is to be used primarily for people, for plants or for technical purposes. Most light measurements are intended to measure the illuminance achieved for the particular light use at specific locations. Even if in each case the illuminance is measured in lux, the measured values of different devices at the same position and with the same illumination can differ considerably.

In order for the light meter to provide the appropriate values for the application, attention must be paid to the following:

Should the illuminance be measured according to human visual perception or for other purposes?
In the standard DIN EN 13032 Part 1, Annex B describes many device-specific parameters that have an influence on the measurement result of the light meter. For the measurements of illuminance in the workplaces, in recreation rooms, playgrounds, sports facilities and traffic routes, it is important that the measurement is made according to the spectral sensitivity of the human eye. The light meter should be able to detect and evaluate the visible spectrum of different light sources according to this spectral sensitivity. In addition, it must also be possible to detect obliquely incident light. For other purposes, the sensitivity to ultraviolet and infrared light, the evaluation of polarized light and of time-varying light, among others, can play a major role.

Should the light meter also be used to determine the color temperature of the light?
The light meter with additional equipment can also be used to measure light colors. The distribution of the wavelengths or frequencies of the radiation is characteristic for the perceived light color. The color of light not only affects the perception of the illuminated surfaces, it also influences many cell activities in living organisms. The color coordinates of light can be specified as primary colors in three-part additive color models, for example, as red, green, and blue values according to the CIE-RGB color space or as X, Y, and Z values according to the CIE-XYZ color space. Many models that measure the light color not only give the numerical values for the primary colors, but also show the color distribution of the measured light graphically on the colored device display.

Should the light meter be used continuously at the same position or at different measuring points?
Depending on whether the light meter is used for continuous monitoring of lighting conditions at a specific location or for the measurements at different locations, the requirements differ. In the case of the light meter that is to be used stationary in a light-sensitive production process, it makes sense to forward the measured values directly via interfaces to other devices for data evaluation or for controlling the lighting or shading equipment. In the case of devices that are used on as mobile devices at many different measuring points, attention should be paid to the storage options for the measured values. For certain measuring conditions, it is also advantageous if the light meter has an external sensor. The device can then be read-out and operated more easily without the risk of shading the sensor.

What measuring range and accuracy should the light meter have?
The measuring range is determined by the application. For many light measurements in recreation rooms, a measuring range of up to 2,000 lux or even 10,000 lux is sufficient. For work areas where a lot of light is required and when outside high illuminance levels from daylight must also be measured correctly, the measuring range should extend at least to 100,000 lux or beyond.

Based on its accuracy, the light meter can be classified in one of the accuracy classes according to DIN 5032 Part 7. These accuracy classes simplify the selection of device models. An instrument with the accuracy class C should only be used for orienting measurements. Class B and A instrument models can be used for measurements of lighting in businesses and public areas. For particularly precise measurements, on the other hand, the light meter of the highest accuracy class L should be used.

Can the light meter be calibrated and adjusted if necessary?
In the areas where too high or too low light intensity has a serious effect, it should be ensured that the measuring device used for testing works correctly. This is done by testing the accuracy of the measurements under specified conditions. Some instrument models can also be adjusted by the user. This means that these models can be readjusted if necessary to output the measurement results as accurately as possible .

Lighting and illumination have become essential in any sphere of life and they perform an auxiliary function when daylight is not enough or is absent. How much light is necessary? Is artificial light necessary if a natural light, both, inside and outside, is available? When is artificial light a huge help and when is it a complete waste?

A light meter is a device that helps to answer all these and other questions. It is a device which finds its application literally everywhere, from household to work places, streets, recreational and other facilities, illumination of the highspeed and other roads, all possible types of establishments and institutions. A very sensitive electrical device helps to monitor and control the level of illumination of any area. Lux is a standard, well-known unit, in which the measurements are expressed. Another possible term used in some systems may be foot candle (lumens per square foot).

Light meter is small in size, convenient to operate and since modern devices are equipped with a lighted screen (made of special glass), the reading of the measurements is significantly facilitated. Robust housing of the device protects the photocells from possible damages and excessive exposure to direct light of different origin (it may be natural sunlight, fluorescent light, high-intensity discharge lights, based on different gases).

The majority of people spend a big part of their day at work and it is natural that certain conditions should be created, so that the health and well-being of the workers was not damaged while they are performing their professional tasks. The quality of work and performance directly depends on how comfortable and safe the worker feels at the work place. Depending on the occupation, the amount of required light may differ tremendously. So, excessive light is as bad as scarce and this is quite a challenging task to create perfect working conditions for each particular task.

Besides, “comfort level” factor, which matters in general for any person, there is also “performance” factor. They now always coincide, due to a huge variety of jobs and tasks, but they should be kept in a certain balance. Educational establishments, hospitals, laboratories, factories etc. – these are the places where the level of light should be sufficient for 100% productivity. For example, if the light required is too bright or it should be too dim, the working process should be organized in such a way, so that the worker had enough breaks, allowing the eyes to relax, and was wearing respective eye protection. At the same time, it should not cause problems for the workers fulfilling other tasks, and whose comfort is totally disturbed by the improper lighting.

Arranging proper illumination for the workers performing their tasks outside is even more complicated, since it is difficult to predict and count on permanent stable illumination by natural light sources. That is why auxiliary lighting sources must be provided if the level of the natural light is not sufficient. The same relates to the professionals working in the dark time of the day or having late shifts. The range of workplaces and illumination requirements to them is extremely wide, but all of them have been classified and organized by the official authorities. Health care organizations pay a big attention to this issue, and numerous rules, standards and regulations are continuously discussed and monitored. The requirements to lighting at workplace may be found in the guidelines of International commission on Illumination, ISO 8995, Society of Light and Lighting (SLL) in Britain, European Norms, Illuminating Engineering Society of North America (IES) and similar organizations.

Humanity has developed a variety of man-made light sources to illuminate the dark and to light indoor spaces. Recommendations for indoor lighting depend on consumer preference and the location of the light source (e.g., home, office, school, parking lot, street, theater or hospital examination room, operating room, recovery room, etc.).

One of the most popular achievements in modern lighting technology is the light emitting diode (LED). LED lights can be found almost anywhere, inside and outside, and are available in different sizes and colors. The LED's greatest advantages are low energy consumption and long service life. These attributes make LEDs very attractive to consumers. LEDs do not emit as much heat as incandescent lights. Even when an LED light is on, there is no risk of burning the skin if the light bulb is touched. In addition, thanks to their epoxy lenses, LEDs are not too fragile. Though LEDs are quite popular nowadays, other types of indoor lighting still are widely used.

The aforementioned incandescent light has been used for decades in numerous indoor lighting applications. Incandescent light bulbs produce a soft and pleasant glow. This is why incandescent lighting still can be found in residential homes, even though incandescent light bulbs consume much more energy. Incandescent lamps get warm over time, and though the extra heat can help keep indoor temperatures warmer during the winter months, careful handling of recently or currently illuminated bulbs is required, because touching the hot light bulbs can burn the skin.

Another alternative to incandescent lighting is halogen lighting. Sometimes halogen light bulbs are selected for use in table lamps, inside and under cabinets, and in different wall-mounted fixtures. Halogen lights are small in size, contain halogen and inert gas, and offer a higher level of brightness and a longer service life.

Integrated compact fluorescent light (CFL) bulbs are reminiscent of incandescent light bulbs, because both kinds of bulbs feature the same base. Integrated CFLs can be used in many standard incandescent light fixtures. The difference between integrated CFL and incandescent light bulbs is that the service life is much longer (in average 8 – 13 hours) and the energy consumption is ¾ lower for integrated CFL bulbs. Aesthetically, the twisted-tube design of the integrated CFL bulbs looks much different than the egg-shaped design of the incandescent light bulbs. For this reason, some consumers prefer to use incandescent light bulbs in clear glass light fixtures.

Traditionally, when one thinks of fluorescent lighting, one envisions the long tube-shaped bulbs frequently used in industrial-looking overhead light fixtures. Since these large fluorescent bulbs produce a very bright light, they are often referred to as daylight lamps. Because there is a certain amount of mercury in these kinds of light bulbs, the bulbs should be handled and disposed of carefully.

Lastly, neon lighting can be found in some indoor environments. Neon lighting is comprised of electrified glass tubes filled with rarefied neon or other gases. The type of gas used in the tube determines the color of the neon light. Neon lights can be used for decorative lighting in bars and nightclubs as well as for small indicator lights in electronic displays.

Therefore, indoor light bulb selection largely depends upon the intended application. However, whether a light bulb is needed for ambient lighting, spotlighting or task-specific use, factors such as service life, energy consumption, aesthetics and price are almost always considered. In addition, special functions like dimming often are added to the equation. Only after analyzing all the required (and desired) lighting parameters can the best (most suitable) indoor lighting solution be identified.

A standard light meter is a useful tool for evaluating indoor lighting conditions, especially those involving incandescent and fluorescent light bulbs. When evaluating LED lighting indoors, an LED-specific light meter is recommended.

Should you have any questions about light measurement using a light meter, please do not hesitate to contact the PCE technical sales and support team at  or +44 (0) 2380 987 035 for PCE Instruments UK, or at  or +1 (561) 320-9162 for PCE Americas Inc.