Verification Procedures for Indoor Visual Environment According to the EN Standard

Keywords: design criteria, visual environment requirements, lighting calculation methods, light-meters.

Introduction

The criteria and requirements reported in the Standard EN 12464-1 [1] must be fulfilled during the design process and verified after the installation. This is made through data provided by manufacturers, calculation tools and on field measurements.

For example, most of requirements are reported in Table 1, for general activity in classrooms (educational buildings). In this case, if the positions of the workplaces are not known, or they can be changed, as in the case of Figure 1, where desktops can be moved, the entire area is to be considered as the task area.

Table 1. Requirements for classrooms in educational buildings from [1].

Figure1. Example of classroom where task areas can be moved.

Data provided by manufacturers

Some of the requirements can be verified through the documentation provided by the luminaires’ manufacturers, they are:

·         Unified Glare Rating (UGR)

·         Colour Rendering, Ra

·         Luminaire Luminance

UGR glare rating is an index for assessing the entity of discomfort glare caused by electric light. For each activity it should be lower than RUGL and depends on the luminance of luminaires, on their position in the visual field, and on the average luminance of background. Despite the Standard requires that “authenticated UGR data produced by the tabular method shall be provided for the luminaire scheme by the manufacturer of the luminaire”, it must be noted that the tabular method does not cover all the possible configurations that can be presented. Furthermore, at present, no reliable procedure for assessing discomfort glare from the simultaneous presence of daylight and electric light is available. UGR can also be calculated more accurately by the use of software or measured, in this case a calibrated video-luminance meter would be the preferred choice despite the very expensive costs of the instrumentation. Indeed, measurements through luminance meters are time-expensive and require accuracy in pointing the device. For these reasons, in most cases, lighting designers present only tables provided by manufacturers.

As for colour rendering, this is provided by the manufacturer, in most of the application it should be higher or equal to 80, but for special applications, where colour recognition and discrimination is of paramount importance, it must be higher or equal to 90. The Standard does not consider effects due to very selective materials, as coloured glasses (for the entrance of daylight) or coloured surfaces that could modify the spectral content of light and consequently affect the Ra (colour rendering index). It’s important to notice that Ra assessment is based on the colour difference of only 8 colour samples under the examined light source and under a “reference” spectrum, so in very specific cases, for example in museums or galleries, a more accurate analysis must be carried out.

Another parameter linked to the colour is the correlated colour temperature (TCC), which represents the colour appearance of light. It is provided by the manufactures as well, and its choice is defined by the design characteristics.

In presence of display screen equipment (DSE), the luminaire luminance for different directions is provided in order to avoid visual impairment or discomfort effects. It should be provided by the manufacturer but can also be measured.

Calculated data

Other requirements are average illuminances on surfaces and cylindrical illuminances. Their evaluation requires complex calculations due to the multiple reflections among all the interior surfaces. Currently, several simulation and calculation tools are available to calculate both illuminance and luminance values on surfaces, supporting practitioners in their choices during the design process. Often, software users are not aware of the adopted calculation method and about the degree of accuracy, so in many cases they believe that obtained results are quite coincident with the real ones. Indeed, to assess the accuracy, it is fundamental to be aware about the numerical model, the calculation procedure and possible assumptions regarding materials and luminaires.

Differences among software results depend on the specific calculation method and, as a function of it, different details for input data. The two main typologies of calculation are the radiosity method and the inverse ray tracing method. The former is a finite element method and assumes that all surfaces are perfectly diffusing. So, if some or all surfaces present a good diffusing behaviour, results will be more accurate and, inversely, if they show some glossiness, the predicted values will diverge from reality on going far from the perfect diffusing behaviour. The latter method, named inverse ray tracing, is based on statistical calculation and aimed at evaluating the path of light beams from the arriving point coincident with the observer’s eye to the starting points corresponding to the light sources. This method is more accurate but requires as input the knowledge on how the materials interact with light, i.e. how much light is reflected in each direction for every incidence direction. This is described by means of bidirectional reflectance functions BDRF, that are not so easy to be measured. Despite the fact that some catalogues are available, the practitioner is not always aware of exactly what material corresponds to those reported in the catalogue. Furthermore, results depend on the point of view (observer’s eye), whereas for the radiosity method, being the surfaces perfectly diffusing, luminance values are constant in every direction for each point of view. Both of these methods can be based on photometric quantities, i.e. considering the light globally, according to the spectral sensitivity of human visual system and neglecting the spectral composition of the light emitted by the sources and spectral selectivity of materials. In this case, a photopic total reflectance value is assigned for each surface and results will be more accurate if materials are quite neutral (grey colour or poor saturated ones). In presence of selective surfaces (for example saturated colours, total reflectance for light will change even significantly on changing the spectral distribution of the light incident on it. So, on changing the spectral distribution of the light source, with the same luminous flux and spatial distribution, results can change, even significantly. What is important is to assess the accuracy of the adopted model, even considering specific cases (for example presence or not of diffusing and neutral surfaces), because, according to the Standard, assumptions including degree of accuracy shall be declared. Furthermore, a relevant input for results is the “maintenance factor”, that could be accurately calculated as a function of several parameters. This can be done only defining in detail a schedule for maintenance, but in many cases, practitioners don’t provide all the necessary information which results in a bad estimation of the maintenance factor (and final results) and maintenance issues during the lighting system lifetime.

Measured data

In order to verify the compliance with the Standard’s requirements, on field illuminance measurements must be carried out. In some cases, also luminance measurements, through luminance meters or video-luminance meters can be carried out, for assessing UGR. First, the installation and the environment must be checked against the design assumptions. To do this, measurements of spectral reflectance of indoor surfaces would be useful, but instruments are expensive, and they are not required. For this reason, it is not always possible to accurately compare the design assumptions made about surfaces’ reflectance with the real values.

It must be noticed that the electric light installation alone must comply the requirements, by the way the presence of daylight must be favorited, and it allows to reduce electric light contribution through proper control systems, but, given its variability, it is not object of verification according to the Standard. For some applications (educational buildings, residential buildings, hospitals), a certain daylight provision is required, and this can be assessed through specific calculation methods [2].

When verifying conformity to the illuminance requirements, the measurement points must coincide with any design points or grids used. Account should be taken of the calibration and of the technical characteristic of the of the light meters used, but the standard does not refer to further characteristics or specifications about the measurement instrument.

The average illuminance and uniformity shall be calculated from the measured values, particular care should be devoted to the maintenance factor, verifying that it is greater or equal than the specified value.

References

[1]     EN 12464-1:2021 “Light and lighting - Lighting of work places - Part 1: Indoor work places”.

[2]     EN 17037:2018, Daylight in buildings.