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History of Light Sources - Color Rendering Index (CRI)

12 March 2024

Outdoor Lighting and CRI

The first post discussed general lighting and the second one addressed specialty lighting applications requiring a higher CRI. This post will explore outdoor lighting. While it is widely understood why CRI is important for indoor lighting, is it important for outdoor lighting as well? Many people have had the experience of driving into a tunnel and not being able to see clearly, not being able to distinguish car colors directly in front or not even being able to tell the color of fellow passenger’s clothing! This is due to the extremely low CRI of sodium lamps commonly used in tunnel lighting, until recently. As shown by the above example, CRI is also an important factor in outdoor lighting.

Despite the widespread adoption of LED lighting, high-pressure sodium lamps (“HPS”) remain the most widely used light source for outdoor lighting such as streetlights, road lights, and landscape lighting. Satellite images of the earth at night show cities around the globe glowing with a beautiful orange color. However, while HPS may look beautiful from the sky, the CRI of HPS is extremely low much like the lighting in tunnels. It is quite difficult to see objects clearly when walking under the illumination of HPS in older towns or container yards, again, sometimes even impossible to distinguish the colors of clothing.

One of the main reasons HPS is still the most widely used light source for outdoor lighting despite containing mercury and having extremely low CRI is that HPS is exempted from the RoHS Directive regarding "mercury in lamps" because there was no alternative technology. Another major reason is that people love the calming color of HPS, which closely resembles the color of gas lamps and flames. In fact, there have been extreme cases where streetlights that were once replaced by white LEDs have been converted back to HPS due to opposition by residents. People find relaxation in the colors they have grown familiar with from the past, and they tend to resist changing the environments where they have lived. Such human psychology often prevails over rational considerations such as efficiency, energy saving, and mercury free, thereby influencing society and everyday life.

In order to change this situation, Nichia has developed “HPS color LEDs”, which are low CCT LEDs that genuinely reproduce the emission color of HPS and serves as an alternative technology to HPS. Of course, LEDs do not contain mercury. HPS color LEDs are more efficient than traditional HPS and can lead to energy savings even when directly replaced. Furthermore, unlike HPS, which take more than 10 minutes to reach full brightness, HPS color LEDs can be turned on and off instantly. This capability enables sophisticated controls and smart lighting. In addition, the lifetime of the LEDs is more than five times longer than that of HPS, greatly reducing the frequency of lamp replacement. Nichia is hopeful that the market will evaluate and adopt these "HPS color LEDs,” benefit from all the advantages LED brings vs. HPS, remove the barrier of human psychology, and greatly contribute to the realization of a mercury-free and carbon-neutral society.

Nichia’s 3rd blog post answers the beginning question – yes, color rendering is important for outdoor lighting as well and understanding color rendering properties is critical. In fact, when developing these HPS color LEDs, Nichia’s goal was to maintain the nostalgic look, but also bring additional benefits and features that only LED can accomplish. As mentioned above, HPS color LEDs have the same emission color as HPS with a higher CRI of Ra≥70, making it easier to distinguish colors while maintaining the nostalgic atmosphere of a city illuminated by HPS. This can create a colorful and vibrant lighting space, as well as a safe and secure lighting space compared to HPS which have a lower level of color visibility.

Thus, by combining Nichia's expertise and technology in phosphors and LEDs, Nichia strives to create the optimal light for each specific application in accordance with its intended use, aiming to enrich people's lives through light.

For more information about the products, refer to Nichia's website.
HPS color LEDs