What is blue light?

When discussing the benefits of our circadian lighting, we often mention "blue" or "blue-free" light. But what does that mean? We delve into the concept and explore the various aspects of white light.

In most places we frequent, the electric lighting remains the same colour and intensity throughout the day. Whether in a supermarket, waiting room, or at work, the light is rarely anything other than bright and white. This stark contrast with the sunlight outside, where the white light changes with the passing hours, is quite noticeable.

However, white light is not simply white. It is much broader than one might initially think. White light contains all the colours of the rainbow. Therefore, white light can vary from being cool and blue to more warm and golden, or even reddish in appearance. This colour variation depends on two key factors, colour temperature and wavelengths. To better understand white light, it is necessary to understand what colour temperature and wavelengths are.

Colour Temperature - the Warmth and Coolness of Light

Colour temperature and the wavelengths of light are two distinct aspects of light, yet they are closely intertwined and influence each other. The wavelength of light is crucial for determining colour temperature. When looking at white light, colour temperature describes our visual perception of the light, whether it appears warm or cool. Colour temperature is measured in the unit kelvin (K). A lower kelvin measurement (1000K-3000K) results in a warm, golden, or reddish light, while a higher kelvin (5000K or higher) produces a clear, cool, and blue light.

As the colour temperature increases, the blue tone of the light becomes more noticeable. Amidst the warm and cool hues, there lies the neutral white light. However, it's essential to distinguish between what we perceive and the physical properties that determine whether the light feels warm or cool – all of which depend on the wavelengths of the light.

Wavelength – the Colors and Physical Properties of Light

White light consists of a broad spectrum of colours in different wavelengths. This means that each wavelength emitted by the light represents a specific colour. At one end of the spectrum is blue light, characterized by a short wavelength, while at the other is red light, with a longer wavelength. In between these extremes, a plethora of other colours can be found, reminiscent of a rainbow.

When discussing white light and its colours, it is essential to differentiate between electric LED light and sunlight. While electric white LED light contains all the visible colours from blue to red, sunlight emits not only all visible colours but also some "colours" or wavelengths that humans cannot see. The sun also emits ultraviolet and infrared radiation, invisible to the naked eye. We can, however, observe the effects of ultraviolet waves on our skin, turning it brown or red, just as we can feel the warmth from infrared radiation when basking in the sun.

Understanding Blue Light

In essence, white light with a higher colour temperature contains a larger proportion of the short-wavelength blue and green colours, giving the light an overall cool white – or bluish – appearance. Conversely, white light with a lower colour temperature includes the long-wavelength warm, yellow, and reddish colours. Colour temperature and wavelengths are two ways to describe the same variation, with colour temperature more tied to our subjective perception of colour, while wavelengths are an objective measure of the physical properties of light. When discussing blue light, it refers to short-wavelength white light (5000K or higher) that appears cool and blue to the human eye. In contrast, blue-free light refers to long-wavelength white light (1000K-3000K) that looks warm, golden, or amber-like, akin to a flame. These are commonly referred to as cool white and warm white, respectively, in everyday conversation.