Chromatic quality of the LED

The renewal of all the streetlights in Terrassa can already be seen in almost all the streets, parks and squares in our city. However, there is one aspect that stands out as the most visible of the whole TEI project: the change in the type of light and its color, which went from a yellowish hue to a whiter one. The real question is: beyond the aesthetic aspect, what does imply this change?

We can explain that with an indicator known as CRI, or Color Rending Index. CRI is a numeric value obtained by comparing the appearance of one color palette under a specific type of light, which in our case would be LED lamps, and the appearance it would have under the average light of the sun or another standardized reference light. The CRI allows us to evaluate with a number the quality of a light source and to compare with others. So, the higher the value, the more similar will it be to the sun light, and so the visible color hue will be wider.

Figure 1. Color tone perception for an IRC of 60, 80 and 90. Source: My Led Lighting Guide (https://www.myledlightingguide.com/blog-color-rendering-index-cri).

So let's apply this to our city. Until the TEI project was implemented, streetlights in Terrassa featured old-fashioned mercury vapor or high pressure sodium-vapor lamps. Both technologies produced a great light performance, as they emitted a really broad spectrum of light but in a very specific hue: the one which belonged to yellowish colours, the main characteristic of this type of lamps.

Figure 2. Crossing of Igualtat Street and Prior Tàpia Street. The orange color belongs to sodium lamps while the white or clear yellow color light is belongs to LED lamps.

However, this kind of lamps presented many inconveniences, as these:

1. Despite a high visual range, the CRI value was very low.
2. Orange or yellowish color hues made it hard to distinguish real colors.
3. The economic cost was really high, as it was the power consumption.
4. They generated a lot of diffuse lighting and, in consequence, light pollution.

The current trend between cities is to opt for LED lamps. They have many advantages and very diverse ones, but here's some of the main ones:

• The lamps have a low energy consumption: they are more ecological and they bring economic savings as well.
• The illuminated areas are more concise and light pollution is reduced.
• The CRI value is high, allowing to perceive shapes and colors at night.
• The lamp lifespan is increased: a LED lamp is expected to last between 30.000 and 100.000 hours on, while sodium lamps had to be changed after between 12.000 and 20.000 hours of use. This allows to reduce the maintenance costs.