How to organize a modern photometric laboratory Part 3 – introduction to luminance measuring devices

Luminance is the most important quantity in lighting design and the most important measure in
road lighting and architectural illumination. It is also the measure that is probably the easiest to understand and notice by the user, because it determines the “brightness” of the observed objects and surfaces. The 21st century, revolutionized by the widely available, increasingly cheaper to buy and maintain LED sources, contributed to a huge increase in the number of all lighting investments – using luminescent diodes.

LED is a source of high intensity, which is characterized by high luminous efficacy too. Unfortunately, a specific side-effect of LED construction is the high luminance and annoying effect of glare. The assessment of glare is dependent on the luminance value in relation to the background luminance level.
Designers and lighting companies often use design software and the available visualisation functions for lighting installations, where luminance plays a key role. However, there is a big discrepancy between the assumptions of the designer and the design and the actual implementation of the lighting system.
As the technological possibilities of LED luminaire construction and lighting control increase, the need for more reliable verification of lighting quality at the stage of completion of construction works increases. The client, the customer or the ordering party more and more often demands a lighting audit report and a declaration of conformity of the installation with the design and the order.

 

Fig. 1. Software for ILMD solutions.

Tradition vs. modernity
Traditional luminance meters using a single lens optical system and appropriately adjusted photodiode, the so-called spot luminance meters, allowed for precise targeting and precise focusing at a small point. They are fast and reliable, but have two main disadvantages due to their technical characteristics. Firstly, they usually have a very narrow viewing angle, i.e. from a given distance we can measure an area (point) with relatively small dimensions, which in the case of luminance distribution measurements forces the user to make multiple measurements and calculate the luminance distribution of a given area manually. Secondly, the matching class of a photodiode with a V(lambda) filter may cause measurement errors depending on the type of light source’s spectral power distribution or the colour temperature. These meters were perfect for measurements of broad band light sources i.e. light sources with a wide range of radiation. On the other hand, in the case of some white LED sources, especially RGB systems, the errors can be up to 20% even for a good class meter.

New solutions available on the market, using CMOS or CCD sensor technology, commonly used in digital cameras, offer far greater  possibilities to use this technology both for measuring and testing the luminance of illuminated surfaces, as well as backlit elements and the luminaires and light sources. With the use of a camera luminance measurement system, the so-called Imaging Luminance Measuring Device (ILMD), it is possible to perform measurements and compare luminance values on the basis of the so-called image analysis.

Fig. 2. Luminance distribution in the interior with LED luminaires.

These meters are equipped with a high-resolution sensor and optical system consisting of a lens and a V(lambda) filter adjusting the sensitivity of the sensor to the sensitivity of the human eye. In this way, the image recorded by the sensor is subject to computer analysis, and the recorded luminance (brightness) level corresponds to the impression received by the human eye, i.e. it is the level of absolute luminance value. Unlike an ordinary camera, where the level of brightness in different points of the image is a relative value, in the case of a imaging luminance meter we have an image showing the distribution of luminance, and on its basis we can analyze, compare and measure the level, uniformity, changes in values, etc., for each point individually or for a given area of the image.

More about the luminance camera (ILMD) from GL Optic HERE

Thanks to this technology, with the use of a 2D image sensor in several million pixels, we can record the entire image of a given surface, a lighting component or the entire interior of a building. The recorded images can be analyzed in detail with the supplied software. With image analysis tools it is possible to quickly mark the areas or points of interest and easily assess, both visually and metrologically, the connections between different measurement areas. Dedicated software for such systems allows for additional presentation of luminance levels in Pseudo-colours, isocandels, 3D charts, in the form of histograms, to create reports and contains many other useful functions.

When it is necessary to determine luminance distribution uniformity throughout the scene, a traditional, spot meter seems to be an uncomfortable solution. Point by point measurements would have to be made, which would be very time-consuming and would not be applied in practice. Similarly, the measurement of small objects like indicating LEDs or backlit icons cannot be carried out with such a luminance meter because the measuring angle is constant and usually not small enough.

Fig. 3. Sample of display and controls (automotive industry).

The measurements made with ILMD meter compared to a spot meter allows to capture an entire image of the scene. All the luminance information in the frame can be stored in a single image, so this method takes much less time. Repeatability is also an important advantage compared to a point by point measurement of luminance, as the measured image can be recorded and re-assessed at a later date.

Using V (lambda) filters in the measuring devices may cause problems with accuracy of readings for different types of light sources or colour temperature. To overcome this a mismatch correction can be applied by combining a ILMD system with a spectroradiometer. This configuration increases the accuracy of the luminance measurement and additionally allows for colorimetric analysis, e.g. of colour temperature, colour rendering index and other colour measuring functions. In advanced systems, this is part of an integrated measuring instrument. It is also possible to use a two-stage measurement procedure for this purpose, where we first measure the luminance with an ILMD meter, and then with a spectroradiometer we measure the spectral distribution of the lighting. So with this software it is possible to combine measurement data and present consistent reports including both luminance and colour assessment.

Fig. 4. Luminance meter scheme.

Advanced technology
Moreover, very technologically advanced meters are already available on the market. Some allow the luminance distribution to be measured with a single frame in the field of view, and the number of spots collected is equivalent to 1,000,000 or more synchronously measured with a traditional meter.

Imaging meters are characterized by very high accuracy. They are usually equipped with a high Class A optical system correction (DIN 5032-7) for V (λ) function. These can be very high-resolution measurements thanks to CMOS or CCD sensors, which can have up to 8 million pixels. A wide range of measured values enables accurate measurement of objects with large variations in luminance. The dynamic range of imaging meters can be from 0.001 cd/m2 to 200 kcd/m2, using appropriate neutral density filters or high dynamic range images techniques. The most advanced models use special cooling technology of the measuring system for superior stability and repeatability too.

Excellent image quality is achieved with a high-quality lens. Some measuring systems allow the lens to be changed however, it is recommended that you buy one from the manufacturer to ensure that it is included in the calibration procedure.

The use of imaging meters is very wide as they are used to measure the luminance of widely interpreted LED products: from individual lamps, chips, modules, luminaires, car lamps, LED screens, to displays, various illuminated signs, indicators, signalling, up to the measurement of all kinds of installations: in road lighting, in architecture, etc.

Fig. 5. Sample of a luminaire with a LED light source

Inside and outside
Luminance measurement can be useful during the acceptance of lighting installations, both indoors, where the luminance image can be used to assess the uniformity of lighting, and outdoors, where we can measure luminance distribution, e.g. on the facade of a building, checking whether the levels and distribution of luminance are in line with the design or the expectations of the investor. It is particularly useful to use a ILMDs for road lighting measurements, where the luminance levels of illuminated road fragments are described in detail in EN 13201-2:2016-03 standard.

Light sources and luminaires
With this technology, luminaire manufacturers and installation companies can quickly and reliably verify the quality of their products. The high-resolution imaging meter can measure both the luminance of a single diode and the luminance distribution of large areas of low luminance.

Displays, screens and icons
The automotive and electronics industries make widespread use of illuminated signs, controls, switches and entire displays to provide users with convenient access to information and control options. Most household appliances are also equipped with displays and lights today. All such luminous elements can be easily measured and checked for quality and luminance levels.

More and more widely available and also cheaper Imaging Luminance Measuring Devices give hope that reliable measurements of lighting products and installations will improve the quality of lighting while reducing energy consumption. In qualitative terms, we should provide light where it is necessary, as much as is necessary and whenever it is needed.

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