Three colour spectra
The human eye is a sophisticated organ that adapts to the available light. This ability to adapt allows us to see surprisingly well at night with only the light of the moon and stars. Using the right colour of light in street lighting can ensure better visibility.
For the first time, it is possible to produce light that makes use of the way the human eye sees at night. The Dutch manufacturer Innolumis Public Lighting offers a unique RGB (R – red, G – green, B – blue) LED technology without phosphor or other substances that would burden the environment. The concept is based on the fact that the human eye perceives different colour spectra at different light intensities. The light is perfectly adapted to the needs and sensitivity of the human eye in evening and night-time vision. A considerable benefit for businesses and nature alike is the reduction in CO2 emissions.
The light sources in the LED luminaires are red, green and blue LEDs. With this RGB concept, the optimum light spectrum is created to guarantee excellent conditions for the human eye, all with low electricity consumption.
But how can we take into account a biological effect that cannot be measured with conventional light-measuring instruments? By looking at the S/P ratio (the ratio between the scotopic and photopic curves), which shows the extent to which light sources emit the wavelengths of light to which our eyes are particularly sensitive in the dark.
How our eyes work
Humans have excellent daytime vision, but our eyes are also surprisingly effective in the dark. This is because we have two types of light-sensitive cells in our eyes: rods and cones. Cones are located mainly in the centre of the retina, while the rest of it consists mainly of rods. Cones are used in bright light (photopic light levels), during the day and in almost all lit rooms. Rods work only in very dark situations (scotopic light levels), such as at night. Rods are therefore much more sensitive to light than cones, and thanks to them we can still distinguish shapes and contrast even in near-total darkness.
In the transition between light and darkness, both cones and rods transmit information. These light levels are called the mesopic range (twilight). During the day, when we use our cones, our vision is best in yellow light (the photopic curve), but at night the sensitivity of our vision shifts towards green and blue light (the scotopic curve), when our rods are most active.
Visibility at dusk
The light intensity provided by most street lighting is in the mesopic range (twilight). People walking along streets lit by street lighting therefore use both cones and rods. Most conventional street lighting is orange, which stimulates the cones but not the rods. The rods are active, but not effectively stimulated. Street lighting that also contains green and blue light allows us to distinguish contrast better, as our rods are most sensitive to these colours.
The cones in our eyes let us see during the day, the rods let us see at night. At dusk we use both cones and rods.
How well we see at dusk therefore depends not only on light intensity but also on the light spectrum. This understanding of mesopic vision is by no means new. In 1819, Jan Evangelista Purkyně discovered that the sensitivity of our rods peaks at a different frequency from that of the cones. The use of LEDs in street lighting puts this knowledge to practical use. It is possible to produce light of a specific wavelength without wasting energy.
What is the S/P ratio?
It is useful to know how effectively a light source stimulates the rods. There is a special ratio that expresses this information: the S/P ratio. The S/P ratio is defined as the ratio between blue-green light (scotopic), i.e. night vision, and green-yellow light (photopic), i.e. day vision. The greater the proportion of blue-green light, the higher this number. Light sources with a high S/P ratio allow us to see well even when the light intensity is low. A few figures to illustrate the concept: daylight has a high S/P ratio = 2.47, while a sodium lamp has a very low S/P ratio = 0.23.
The effect of the S/P ratio depends on the light intensity. When light intensity decreases, we use our rods more and our cones less. The reverse also applies: the higher the light intensity, the smaller the effect of the S/P ratio, as cones are more active when they have more light. Above a certain light intensity, rods contribute hardly anything to the quality of our vision. There is as yet no scientific consensus on the exact threshold above which the S/P ratio no longer has an effect. Practical findings show, however, that human mesopic vision is active with almost all types of street lighting.
Measuring lighting
Commercially available photometric instruments determine lux values by measuring all light frequencies and weighting them along the photopic curve. The scotopic curve is therefore not included in the measurement. Luminaires with a high S/P ratio provide better visibility at the same lux values. This creates a problem for street lighting calculations, since the calculations are based on lux values alone. This can result in more light being emitted than is needed in practice when we use light sources with a high S/P ratio.
A better solution is to include the spectral distribution of light sources in lighting calculations. The S/P ratio is an excellent tool that can help us here. There are scientific publications that put forward proposals for using the S/P ratio to factor the light spectrum into lighting calculations.
Examples include the Unified System of Photometry (Rea et al., 2004) and the ASSIST report (LRC, 2009). These publications present correction factors based on the S/P ratio. This calculation method is called unified luminance.
Unified luminance in practice
Unified luminance is a system that allows the S/P ratio of a light source to be included in measured photopic light values. The process consists of three steps:
- Determine the level of visibility required for a given situation and task: for example, street lighting class S5 with Em 3 lux, Uh 0.2.
- Incorporate visibility into the illuminance using the unified luminance method. Find the S/P ratios of the various light sources you are considering in the manufacturer’s documentation. Calculating unified luminance gives the following Em results:
SOX (S/P ratio 0.23) SON T (S/P ratio 0.62) Lumis-LED (S/P ratio 3.2) 0.88 lux 2 lux 5 lux The results of the calculations show that we need less capacity when the light source has a high S/P ratio. This, of course, saves energy. The poor spectral distribution of sodium light sources (SOX = low-pressure sodium, SON T = high-pressure sodium), which cannot stimulate our rods, means lower corrected lux values. The corrected lux value of Nicole Mesopic (Lumis-LED) is very high, as the spectrum has been optimised for twilight (mesopic light levels).
- Choose a suitable light source for the area to be lit.
This result gives better information about the actual visibility you can achieve in practice at a given location. This means that the required visibility can be achieved with lower electricity consumption.
This LED lighting can be used to light residential areas, parks, car parks or roads. It is equally suitable for areas where light pollution must be kept to a minimum, for example in the countryside and in nature. What is more, the colour spectrum is tuned for the human eye and even for security cameras. The camera image gets suitable contrast and shows colours far more realistically than under conventional sodium lamp lighting.
Car park lit by SON T luminaires vs. car park lit by Lumis-LED luminaires.
New LED lighting for public spaces allows precise selection of the colours needed to create optimal visibility while saving energy. Colour combinations make the spectral distribution of light sources a relevant factor in lighting calculations.
Would you like your calculations to include the spectral distribution of light? Please do not hesitate to contact us.
