Light perception and chromaticity
How we perceive artificial light is determined by several factors, such as colour temperature, colour rendering, and colour quality. The light that the human eye perceives as white is a mixture of all colours. LEDs are technically different from incandescent bulbs, and their characteristics are therefore not directly comparable.
Black Body Locus (BBL)
Light colour and colour temperature are part of the colour triangle. Another part of the colour triangle is the Planckian Locus. Also known as the Black Body Locus (BBL), it indicates colour temperatures (in Kelvin) and corresponds to the actual colour of a heated black body. The higher the colour temperature, the more incandescent the body is. In incandescent light bulbs, the colour temperature is the same as the actual temperature (°K) of the filament. Since modern light sources do not have filaments, a correlated colour temperature (CCT) is stated instead. This is measured by the chromaticity of the light source where one of the isotherms* crosses the Planckian Locus. *Isotherm: a line connecting points with the same temperature.
Fagerhult’s range includes luminaires with colour temperatures between 1800 and 6500 K, with the majority being 3000 K or 4000 K. We also offer luminaires with Tunable White where colour temperature in the range 2700K-6500K can be selected.
The illustration shows how to calculate correlated colour temperature. The plotted curve shows the colour temperature in degrees Kelvin. To obtain a correlated colour temperature, the colour temperature of the light source is measured. This value lies on one of the isothermal lines, and the correlated colour temperature is set at the point at which the line crosses the curve.
BBBL
As you can read in the section above on colour temperature, the Planckian Locus (Black Body Locus) describes the white light in the colour triangle. Normally, our products provide light with a colour coordinate at the intersection of the curve and the isothermal lines of colour temperature. In some applications, mainly in shop lighting, we have chosen to develop a specific colour coordinate that is placed below the Planckian Locus (BBBL - Below Black Body Locus). This colour coordinate was chosen to still give us a warm white light (normally 3000 K) but also give us a “whiter” white. Our BBBL products also provide very good colour rendering (CRI > 90) and enhance contrasts.
White light in different colour temperatures
White LEDs are available with colour temperatures, from warm white to very cool (2700–8000 K). White light is usually created by coating a blue LED with a layer of phosphor.
An LED with a warmer colour temperature is less efficient than one with a cooler colour temperature because more of the light from the blue chip must be converted to warmer colour.
Amber – light in the orange-red spectrum
Light has an evident biological impact on humans. Research shows that high levels of cool light stimulate cortisol production, which makes us alert, while low levels of warm light calm us down.
Amber is light in the orange-red spectrum, and a relatively new concept in the field of lighting. It reduces the production of the stress hormone cortisol and can be used in healthcare to calm patients or stimulate sleep.
Daylight, or its equivalent, is mainly in the blue and green part of the spectrum. It triggers cortisol production. Low levels of night light, or light with a lot of red, instead promote the production of the sleep hormone melatonin and make us want to sleep.
Melanopic Ratio (Melanopic Daylight Efficacy Ratio - MDER) is a factor used in lighting design to calculate the melanopic lux of the lighting installation. Since daylight controls the human circadian rhythm, daylight is also the reference for the Melanopic Ratio. A low Melanopic Ratio value minimally affects the production of melatonin, providing a light that “calms”.
Choose an LED with a low Melanopic Ratio for evening and nighttime lighting and an LED with a higher value for daytime lighting. LED with Amber has a very low Melanopic Ratio.
Tunable White
Tunable white means that the colour temperature can be adjusted between warm and cool light, usually between 2700-6500K. A Tunable White LED circuit has two sets of LEDs, half 2700 K and half 6500 K. A two-channel driver determines how much each will shine and thus the colour temperature produced by the luminaire. A COB can also be Tunable White where the phosphor over the chips at the bottom is striped.
RGBW
An RGBW luminaire can provide all colours by mixing light from red, green, blue and white LEDs. The LEDs are mounted together on an LED module. The whole system is controlled with a four-channel driver. RGBW is mainly used in surgery in operating theatres to create conditions for better light, mainly in keyhole surgery.