Verifying indoor LED lighting for classrooms. Standard requirements, available instruments and new quality considerations

By Mikołaj Przybyła IES Member CIE Member| GL Optic 

Proper lighting plays a key role in creating the right lighting conditions for learning and working in schools and other educational institutions, providing visual comfort, safety and energy efficiency. However, it is not enough to replace old luminaires with modern ones. What should be done to check whether the modernization of lighting has been carried out correctly? How to verify if after a long period of use it still meets the minimum requirements and assumptions made by the designer during the preparation of the lighting design?

Before making a decision to modernize lighting at school, it is advisable to conduct a lighting audit, which is a process that helps in the development of various investment variants.

Nowadays, energy efficiency is the main argument and motivation to replace lighting in schools. The audit will also make it possible to estimate a possible reduction in the cost of electricity used for lighting. It should be remembered, however, that adequate lighting in schools must meet the quantitative and qualitative requirements to ensure comfort at work, as well as meet specific additional requirements.

How can the quality of lighting be determined?

The key parameters of lighting, which should be verified by measurements, are:

1. Luminance distribution [cd/m2] – it should be adjusted to the intended use of a given interior and the type of work performed in it, in which the following are essential: visual acuity, differentiation of relatively small differences in luminance and improvement of visual function
efficiency. Too high luminance that causes glare, high luminance contrasts that cause fatigue, as well as too low luminance and its contrasts that make the working environment monotonous and do not motivate to work should be avoided.

2. Illuminance [lx]– it is one of the most important factors influencing work safety. It determines the speed, safety and comfort of performing visual tasks. Illuminance should depend primarily on the level of difficulty at a given visual workstation – the amount of light should be higher wherever activities requiring precision and accuracy are carried out.

3. Glare [UGR] – it describes the feelings of inconvenience or reduction in the ability to recognize objects in the field of vision, which result from too high luminance in the room or its inefficient distribution. Since glare can cause errors of workers, as well as fatigue and often leads to accidents, it should be limited.

Example of an LED luminaire with glare-reducing cover

4. Flicker [TLA] – like glare, flicker has a negative impact on the well-being of employees, causing deconcentration and even a headache. Stroboscopic effects are dangerous, especially in fast-moving work establishments, where fast moving machines and elements are used because they may cause serious accidents.

5. Energy efficiency of lighting – work place lighting should have recommended parameters while maintaining energy efficiency. To achieve this, it is necessary

The design principles for indoor lighting are defined by European standard EN 12464-1:2012 Light and lighting – Lighting of work places – Part 1: Indoor work places.

This document sets out the requirements for indoor lighting in work places in order to ensure visual comfort and efficiency, taking into account typical visual tasks, also in rooms with monitors. The standard contains requirements for lighting solutions in most indoor work places, taking into account the quantitative and qualitative features of lighting. Additionally, recommendations concerning the so-called good practices in the field of lighting, i.e. generally accepted design principles, are given.

Lighting in schools and classrooms is a special case of work place lighting due to the typical longitudinal arrangement of the room with large windows on one side, specific arrangement of tables and whiteboard, which requires skilfully ensuring the uniformity of horizontal intensity of
lighting on the tables and the vertical component on the whiteboard and in its surroundings while reducing the level of glare. In addition, it should be remembered that children and teenagers studying in schools may be particularly sensitive to some effects of lighting
installation, such as flicker, and require special working conditions that are conducive to concentration, such as uniform distribution of luminance without excessive contrasts.

What should be considered in order to create optimal lighting conditions?

  • Appropriate distribution of luminance through adequately designed and manufactured lighting installation with the use of suitable luminaires and their skilful arrangement. It should be remembered that the old lighting installation, including the arrangement of the luminaires, took into account the old recommendations and completely different lighting systems than the currently used LED technology, so it is not always enough to replace the old luminaires with new ones. The luminance distribution depends on the type and colour of all surfaces in the room. Therefore, in addition to lighting, the colours of the walls and furniture must be taken into account. Dark elements
    absorb a lot of light and will affect large contrasts.
Example of an LED luminaire with a microprismatic diffuser
  • The right level of illumination measured in lux [lux] horizontally on the desk top in the task area (on the desk) and in the immediate surroundings and vertically on the board and its surroundings. A modern classroom is often a multimedia classroom, so it is
    important to take into account the principles of designing classrooms with monitors and adjusting the lighting intensity parameters for screens and projectors.

We should also remember about the overall illuminance level in the room, not only on the table or on the board. We must not only light up work places, but also provide a general level of visibility in the room to comfortably see other people and objects in the environment.

where:
δ – uniformity of electric lighting
E av – mean value of illuminance

 

  • Reduction of glare by using low UGR luminaires or indirect luminaires that partially illuminate the ceiling, thus reducing the contrast between luminance of the luminaire and its surroundings. This is very common in German-speaking countries. Unfortunately,
    increasing the comfort of work in this case is associated with the deterioration of light efficiency parameters, i.e. the amount of light energy in relation to the consumed electricity [lm/W].

  • Appropriate correlated colour temperature (CCT) and colour rendering index (CRI) – both of these parameters depend on the spectral distribution of the light (spectrum). This is due to the optical radiation that is produced by the light source; part of the light source reflected from the objects falls into our eye. This allows us to see objects, recognise details and distinguish colours. The right white light is a mixture of colours, i.e. different wavelengths. The better the mixture, the fuller it is, the better we perceive objects.
  • Limitation of the flicker and stroboscopic effect is possible by means of an appropriate power supply system. The better the power supply used in the luminaire, the better the optical parameters of the lighting installation. Light flicker is related to the type of power supply, and flicker, i.e. short-term changes in intensity, is caused by interference. Both phenomena may not only cause visual discomfort at work, but may also cause distraction and cause various physiological effects (headaches, migraines), especially in young and vulnerable people.

At the moment there are no unified recommendations for permissible levels of light flicker, but it seems that this may change soon. Currently, in Europe, work is underway to adopt new, revised minimum functional ecodesign requirements to include stroboscopic visibility measure (SVM) – more on this subject later in this article.

Specialist measurements

A very important step is to check the lighting system after implementation and then during operation. Traditional practices for measurements at the acceptance stage of indoor lighting installations are only focused on verifying the appropriate illuminance levels measured in lux.
Measurements after the period of use are made very rarely or are not made at all. In a way we assume that the designer's assumptions concerning maintenance factors have been correctly adopted and actually implemented. The appearance of LED technology has, however, introduced additional important parameters, which are worthy of attention when measuring lighting.

Basic measures of flicker describe the shape, depth of modulation and frequency of flicker

Flicker percent (FP)- is the simplest measure of flicker based on the measurement of maximum and minimum amplitude of flicker.

FP =      max – min  x 100%
max + min
where max – is the maximum amplitude of the signal, and min – is the minimum amplitude of the signal
Flicker index (FI)- is a measure of flicker based on the determination of the area under the curve. The advantage of this measure is its sensitivity to amplitude, shape and filling.
FI = A
A + B
where A is the area under the curve to the level of the mean value, and B is the area under the curve from the level of the mean value to zero.

These are quality parameters, which according to the so-called minimum functional requirements are to be met by the manufacturer of lighting equipment, such as: correlated colour temperature (CCT) closest to T b measured in Kelvin degrees and colour rendering index
(CRI). It is worth measuring these parameters after the useful life of the lighting to check whether the values adopted by the designer in the design of lighting are actually maintained (periodic measurements). This applies to general lighting, as well as to emergency and evacuation lighting, which contributes to safety in the event of an emergency.

Example of a diffuse LED luminaire with low luminance and low luminous efficiency [lm/w].
Investor awareness and increasing knowledge of LED lighting have led to additional criteria for luminaires which are not required by the standard, but which affect the comfort of use, being taken into account in the new tender conditions.

Most of these parameters can also be verified at the place of installation. For such measurements, however, it is not enough to use a simple luxmeter. A spectroradiometer is necessary that will measure the spectral power distribution of the light and on this basis it will be possible to calculate additional quality parameters of lighting, including the impact on the round-the-clock cycle – the biological clock of man, or impact in the sphere of the so-called human centric lighting.

In the absence of standards specifying how to adjust lighting to the daily cycle of people, we can benefit from the experience and recommendations of WELL Building Standard – an organization that has developed detailed procedures and criteria for the evaluation of modern office spaces, taking into account environmental protection, energy efficiency, but primarily focused on ensuring comfort at work and access to light and air. It is worth noting that on the market there are lighting companies that offer products in a higher standard, meeting not only the minimum functional requirements and related only to vision, but also to the well-being of people, their ability to remember, the ability to focus and the speed of response.

Knowledge on this subject is becoming more and more available, and companies offering this type of solutions definitely stand out in the competitive lighting market.

It is also important that the current measurement technology enables the testing of all these additional parameters, not resulting from the current standards. Portable measuring devices are also available which combine the functions of a spectroradiometer with a photometer, allowing photometric measurements (e.g., for example, lux measurement) with colorimetric measurements (e.g. correlated colour temperature and flicker parameters).

Example of a school classroom lighting system

The new flicker measure, which takes into account the stroboscopic effect level, is called the SVM (stroboscopic visibility measure). It is planned to adopt changes to the so-called minimum functional ecodesign requirements for LED lighting through the introduction of a minimum SVM level.

This parameter determines the level of stroboscopic effect visibility and is related to the flicker frequency and the way the power supply is controlled. This is particularly important in LED lighting because, unlike incandescent light sources, the light emitting diode, which is a
semiconductor, reacts immediately to changing power supply conditions. For this reason, the power supply parameters have a direct influence on the light parameters of LED luminaires.

The SVM parameter will be introduced as a change to the ecodesign probably in autumn 2019. It will be extended to include lighting manufacturers. It will be the first point of reference for the flicker.

Flicker measures have been known for years, but there is no standard that defines an acceptable flicker level.

If the change comes into effect as a functional requirement for lamps and luminaires, it will consequently be possible to use this determinant when verifying lighting also on site. It may also turn out that some LED products available on the market will be withdrawn from the market.

It seems that the SVM parameter, i.e. the stroboscopic effect visibility measure that takes into account both the frequency and the way the lighting is controlled and the shape of the modulation will be just as important a functional requirement as, for example, a colour rendering index or the energy efficiency of the product.

Until now, power supply parameters have not been studied in such detail as regards their influence on the optical properties of lighting products. Attention has been focused only on disturbances in the power grid.

Literature:
1. European Standard EN 12464-1:2012 – Light and lighting – Lighting of work places – Part 1:
Indoor work places.
2. http://www.lighting.philips.pl/edukacja

 

Posts created 21

Leave a Reply

Your email address will not be published. Required fields are marked *

ten − 8 =

Related Posts

Begin typing your search term above and press enter to search. Press ESC to cancel.

Back To Top