Tallinn University of Technology

Regardless of street lighting complying with the requirements, a person on the street might feel that visibility is lacking. Toivo Varjas, researcher at TalTech, proposed a solution for assessing the level of light on the street perceived by humans at dusk and at night more precisely, easily, and quickly than was previously possible. 


In his doctoral thesis, Varjas introduces an innovative measuring device and the methodology for using it, which allows to take more factors into consideration when planning street lighting. For a road user, it means more user-friendly street lighting in the future, but also increased safety on Estonian roads.

What is it exactly? According to Toivo Varjas, several aspects affect how we sense light. If street lighting is not ‘eye-friendly’, it could mean that the road surface material, the reflective properties of the road surface and road markings, the angle of incidence of light, other light sources next to the road, including large flashing and bright LED advertisements or the environment in general, have not been taken into consideration when constructing the lighting system. For example, the area being an urban or rural area, whether the road is long and straight or winding, the climate being cold or warm, a nearby pavement or a pedestrian crossing, and other such aspects.

In addition, humans sense light and colours differently during the day than in the evening or at night – which is also not considered. After all, it is not a secret that even our blue, black, and white flag does not have the right colours in every artificial light. So what should we do to see colours correctly in an intense beam of light or in the dark? Toivo Varjas recently advised the leaders of this country on precisely these issues.

The subject of street lighting can also be linked to psychology because there are issues that humans are not capable of explaining or understanding. Varjas explains, for example, that a colder light temperature at intersections with heavier traffic helps increase the safety of traffic because it helps to be more alert and sense the surroundings better. In a domestic situation, colder light can be preferred in the morning and warmer light in the evening. ‘This, of course, also depends on the person,’ Varjas admits.

Normative documents are outdated

Why is this novel solution important? The main reason is that, at dusk and in the dark, the quality of street lighting plays an important role in the level of road safety and the security of the living environment.

The measuring device developed as an invention of Toivo Varjas can be used for the spectrographical measurement and assessment of various values characterising the reflection of light from surfaces, such as the luminance of the road surface, the luminance coefficient, the reduced luminance coefficient, and the colour temperature. Until now, these aspects could not be promptly measured with a single device and be used in practice straight away. Our streets are already gradually fitted with street lights which operate on different colour temperatures of light. ‘This provides an opportunity to use lighting for a better assessment of perceived light for humans and the environment,’ Varjas explains.

The properties of novel LED light sources used for lighting streets and the characteristics of road surfaces have created a controversy where the specifications in normative documents for assessing lighting have become outdated. ‘Therefore, a need was experienced in the scientific research to develop a new measurement method, which would increase the universality of measurement, decrease substantially the measurement operations and ensure a significantly higher accuracy of measurement by excluding weather and environmental conditions as well as the disturbing effect of sidelight,’ Varjas states in the abstract of his thesis.

Using the innovative invention

A faster and more precise assessment is conducted as follows:

  1. as a result of implementing the measurement method and the relevant measuring equipment, the characteristic values of illuminance emitted by the luminaires are imitated and these values are taken as base values;
  2. the base values allow for the estimation of the values of the light-reflecting characteristics of many different road surface materials (including asphalt and concrete pavements) and the various additives used in them;
  3. by using the device, the effect of the spectral composition of visible light is taken into account when estimating the light reflected from a surface, helping to propose safer and more efficient solutions for the traffic environment.

The relative importance of the side effects of measurement is now smaller

Varjas emphasised that existing measurement methods were subjected to the analysis and classification of components of measurement uncertainty. The new method enables to minimise the relative importance of side effects of measuring results, and therefore, significantly reduce the overall uncertainty of measurement results (reflecting values) when measuring road surface lighting.

The innovative solution described and analysed in the doctoral thesis also renders possible a considerably higher measurement certainty – as a result, measurement uncertainty decreases from the current 40–50 per cent to 10 per cent!

Due to the simplicity of use and a better measurement accuracy, the practical value of the new method and device is using them first and foremost for light measurements in accredited measurement laboratories, which must comply with stricter requirements for measurement procedures (including measurement uncertainty).

Read the doctoral thesis of Toivo Varjas ‘Research and Development of Measurement Solution and Methodology for Assessment of Light Reflection from Surfaces’.

The article was published in the science portal Novaator.