Fundamental concepts about lighting

In order to choose a good lighting solution it is necessary to understand some basic concepts.

Colour temperature

In lighting, color temperature is based on Planck's Law, also known as Planckian Locus. This theory argues that the color of a black body - a hypothetical object that neither reflects nor is passed through by any light - varies depending on its temperature. The color temperature tells us the tone of the light from that light source, it is represented in Kelvin and is divided into warm, neutral and cold color temperatures.

Warm lights are more yellow and have values below 3500K, with 2700K and 3000K being the most common values.

Neutral lights are whiter, their values vary between 3500K and 4500K.

Cold lights have a bluish appearance and start at 5000K, with the most common values being 5000K, 5700K and 6500K.

The higher the Kelvin number, the cooler the color temperature.

Planckian Locus - black line - on the CIE 1931 chromaticity diagram.

CRI

CRI means Colour Rendering Index and represents the ability of that lighting source to represent colors reliably. The higher the CRI value, the better the colors will be presented.

We recommend a minimum CRI of 80 for general lighting, in residential, work and commercial contexts.

CRI >90 is best suited for specific uses, such as:

• At home, in the bathroom mirror and on the food preparation counter;

• In spaces where detailed work is carried out or where excellent color visualization is important;

• In a commercial context, where good product presentation is essential to boost sales.

Mainly intended for commercial contexts, there are already some LEDs developed specifically to illuminate certain types of products. At Nexled, we have the Thrive 3000K LED, with CRI >93, to illuminate fruits and vegetables and the NW403 HE R9 LED that enhances reds, being recommended for butchery lighting.

Flux

Flux or luminous flux is basically the amount of light emitted by a light source. It is measured in lumen (lm).

Power

Power is represented in W (watt) and, unlike what happened with incandescent lamps, it does not demonstrate the amount of light emitted. The power only represents the consumption of that lamp.

Luminous efficacy

Efficacy is the relationship between flux and power, it indicates how many lumens that lamp emits for each watt of energy consumed. Lumen is represented by Watt – lm/W.

Luminous efficiency

Efficiency is expressed as a percentage (%) and refers to the relationship between the light emitted by the LEDs and the light that is actually emitted by the luminaire.

Illuminance and Luminance

The illuminance of a light source is the amount of light that falls on a surface. It is calculated by dividing the luminous flux by the square meters of the illuminated area and represented in lux.

The rays of light that fall on the surface are not seen unless they are reflected by the surface itself. This reflection is called luminance.

Luminance is the amount of light that is visible to the human eye as it is reflected by the surface where the light falls and is represented in candelas per square meter (cd/m2).

Illuminance is the light that falls on the surface. Luminance is the light that is reflected.