LED LIGHTING KNOWLEDGE BASE


WHAT IS A LED?

It can be said that a LED is an inverse of a photovoltaic cell. Like a cell, it consists of P- and N-type semiconductors. The difference is that the supply of electricity to the LED is followed by the emission of photons and heat. By contrast, a photovoltaic cell works by generating a flow of electricity after photons are supplied. The LED abbreviation derives from an English phrase “light-emitting diode”.

WHAT ARE THE ADVANTAGES OF THE LED TECHNOLOGY IN LIGHTING?

In the last few years, there has been a real revolution in the production of LEDs for the lighting industry. Firstly, LEDs themselves became much cheaper. Secondly, their service life has increased up to 150,000 hours. Finally, today’s LEDs emit more light and less warmth (as a loss) than those produced just one or two years ago. This means that if we consume the same amount of electricity (kWh) that was consumed 2 or 3 years ago to power a LED, we will get much more light for the same amount of energy and we will not need as much cooling as we did previously.

WHAT IS THE LUMINOUS EFFICACY OF LEDs?

One of the most commonly tested properties of technical equipment is its efficiency, or in the case of LEDs and LED luminaires and bulbs – their luminous efficacy. The efficacy of light sources is determined by the ratio of lumens per 1 W (lm/W), i.e. the amount of light that we are able to obtain from 1 W of electricity. The bigger this index is, or the more lumens from 1 W there are, the better. The luminous efficacy of LEDs did not exceed 80-90 lm/W before 2015. Today, the most efficient LED luminaires provide more than 120 lm/W and this index is constantly growing. The number of lumens from 1 W means that, for example, in order to get a LED luminaire with a net flow of 5000 lumens, you need to use as much as 50 W of electricity at the luminous efficacy of 100 lm/W. On the other hand, if you have a LED luminaire with the efficacy of 120 lm/W, you need only 41.6 W to get 5000 lm. It can be said that every single lumen less from 1 W means that the LED luminaire takes approximately 1% more energy from the electricity network, i.e. it is 1% less efficient. Therefore, when choosing a LED luminaire, we should always base our decisions on the number of lumens per 1 W and it should preferably be at least 120 lm/W.

DOES THE LUMENS/WATT INDEX FOR A LED MEAN THE SAME AS THE LUMENS/WATT INDEX FOR AN ENTIRE LED LUMINAIRE?

Definitely NOT. Please note that each LED luminaire has its efficiency, which is usually between 60 and 80%. The losses are consumed by the power supply (approx. 10-15%), lens (8-12%) and bell, i.e. diffuser, (7-10%). So, the same LED can have a luminous efficacy of as much as 160-180 lm/W but the entire LED luminaire will get about approx. 30% less, i.e. only 112 lm/W. Therefore, you should always keep that in mind and check if the luminous efficacy provided by the manufacturer applies to the LEDs or the entire LED luminaire.

WHAT IS LED LIFE?

LED life is the time that the LED is able to maintain its technical characteristics. Unlike typical light sources, a LED does not stop operating in an abrupt manner but loses its initial brightness over time. The life of the LED is measured by setting a certain threshold level below which it ceases to meet the pre-set technical specifications and should be replaced. The best LED manufacturers declare that their LED life can exceed even 100,000 hours, i.e. approx. 25 years in operation (a street luminaire works for approx. 4,000 hours per year).
An extremely important element affecting the LED life is the cooling system. In order for the LED and the entire luminaire to work for a long time, natural and well-designed cooling must be provided. One of such solutions is the application of aluminum fins, which are the most effective. LED luminaires with poor cooling, i.e. a small number of fins, will last only approx. 50,000 hours while those luminaires that have a much better designed cooling system will work for 100,000 hours. Except for LEDs, no other light sources have a service life of 20-25 years. For example, sodium-vapor light bulbs need to be replaced once per 1-3 years.

WHAT IS COLOR TEMPERATURE?

The light color is a very important property of light. Color temperature is expressed in Kelvins (K). The higher the color temperature is, the bluer (colder) the light gets. For example, the candle light has a color temperature of about 2000 K and the of daylight is about 6,500 K. It is assumed that warm light has a temperature below 3,500 K (from light yellow to orange) and cold light is above 5,300 K (light blue). Thanks to the application of LED lighting with a higher color temperature, you can achieve the parameters that are more similar to daylight than with the illumination from sodium-vapor lamps. However, a cold light color can make the users of roads or squares feel discomfort.
Manufacturers of street LED luminaires have chosen a golden mean in the form of the color temperature of 4,000 K, which is a warm yellow/white color that is neutral to the consumer and does not cause any negative impressions.

WHAT IS CRI (OR COLOR RENDERING INDEX)?

CRI (abbreviation for Color Rendering Index) is a ratio used to determine color rendering. This parameter takes values from 0 to 100. The higher CRI index is, the more natural objects look, as their colors are better reflected. Sodium-vapor lamps generally have a low color rendering index. Significantly higher CRI values are attained by LED lighting. The CRI value of 100 means that an item looks as if it were lit by sunlight. In practice, LED luminaires are typically characterized by CRI>70, sometimes above 80 or 90.

WHAT IS LUMINAIRE PHOTOMETRY?

Photometric measurements are made to determine the luminous flux and the light distribution characteristics of the tested light sources and luminaires. Lenses that are part of a luminaire are used to distribute, filter or transform the light emitted by the diodes. Lenses combined with diodes determine the key lighting characteristics, such as efficiency. In addition, optical systems allow for precise illumination of a particular area, thus reducing energy consumption.