LED performance, the gap between advertised and real luminous flux (1)

LED technology has made the decision to buy lighting products very complicated. We are promised important savings in energy and maintenance costs and asked in return to pay a higher price, at times dozens of times higher than other lighting technologies: incandescent, halogen or fluorescent.

But are the savings always real?  
The answer is: it depends...

In this series of articles we will discuss the elements that can define the answer to this question.

Brightness

When we want to replace our old lights with LED or have a new project that needs light, the first thing we take into consideration is brightness. In tech talk this is measured in lumens and is called luminous flux. Unlike a few years ago, today we can see the luminous flux on most lamp packing at the supermarket or find it on the internet at almost all online shops that sell lamps.

 It got like this because brightness has became a very important criteria in the buying decision, so important that some producers/distributors or retailers are willing to give misleading information just to score points in the race for the customers wallet.

Therefore, in the case of some products there is a gap between the advertised brightness and the real, so large that can turn the hole savings argument to buy LEDs on its head.

How temperature affects LED brightness

The performance of LEDs is greatly influenced by ambient and junction temperature (the one inside the LED) as every degree beyond 25 ºC decreases its brightness (lumen output). The amount of lumens lost is what makes buying LED lamps complicated.

A good LED, a Nichia SMD LED for example, will loose less than 10% if the nearby ambient temperature reaches 60 ºC, a very common value inside most fixtures.

 

On the other hand, a not so good LED, an OEM 3258 SMD for example,  will loose at 60 ºC almost half of its brightness.

Most LED producers advertise their products with the performance at 25 ºC but only some mention the temperature at which it was measured and even fewer provide a graph to show the connection between the two values, as the ones above.

Therefore, are the savings promised by LEDs always real?
The savings are real for a high performance LED and false for a low performance one.

When buying LED technology we should always take in consideration this aspect, especially if the price of the product is low or the producer is not a known brand.

Some top brands of LEDs: Cree, Nichia, Osram, Philips, Bridgelux, Citizen...