A good LED

Besides a good appearance? When is a LED lamp or LED module a good one? We don’t try here to make a science out of it, but explain with simple words. A good LED has five main characteristics:

1) Good cooling, thermal design

A lot of people think that LEDs don’t create a lot of heat. This is not correct. The efficacy is of course much better compared to an incandancent light bulb, but still there are losses.  Therefore a good thermal design with a well sized heat sink is a prerequisite for a good quality and long lasting LED lamp.

2) Good optical system

You can have a lot of luminous flux, but still illumination can be lousy.   The luminous flux is the condition for a good illumination, but a good and well design optics brings the light where it is needed.  In addition the optics should be designed in a way to reduce glare.

3) Excellent color rendering 

Excellent color rendering is defined by a (so called) color rendering index CRI. The reference is the sun light. It’s CRI is 100. The incandescent light bulb comes pretty close to 100. A good LED for inside applications should have a CRI > 90. For outside applications a CRI >80 is sufficient in most cases.  What does the value mean? With easy words. Things appear as  they really are.  There is a trade off between CRI and energy efficiency.

Most of our LEDs for inside applications have a CRI of typical 95. Despite that we in general still reach in the majority an energy efficacy of A+. But we also offer for inside applications solutions with CRI of 85 for those for whom energy efficiency is the most important factor.

4) Consistent light colors

If you choose a light color you will not want to see deviations from LED to LED on one lamp, or from lamp to lamp.

We reach a consistent color by bining. This means that only pre-selected LEDs are used with such a narrow BIN that the deviations are not visible for the human eye.

Even though the LED is good when it is new this might change over lifetime. The change will be more significant the worse the thermal design is.  We will explain this in greater detail later.

5) Driver type and driver design

Driver separated from heatsink?

Are the driver and heatsink separated from each other?  Why is this important?
Again heatsink and electronics create heat. In this case the heat from the heatsink accelerates (mainly) the aging of the driver. Such a design reduces lifetime of the driver significantly. This is the problem of probably almost any LED retrofit solution. If the driver should fail, since it is separated you can exchange it without having to replace the entire module. Sometimes it might be unavoidable to put both in the same housing. Appropriate measures should be taken to reduce the described effect.

What kind of driver is used?

Constant current or constant voltage?

This might sound a little harsh, but people who try to run an LED with constant voltage driver have not understood that an LED is semi-conductor. It’s always a specialized solution for one type of LED. Such a design is very sensitive to changes of the voltage. We will also explain this further down.

That’s about it.

Good to know

Lifetime definition

When does the service life of an LED end? At the point in time it is not emitting light anymore? No.

Today there is no uniform common definition which has to be used, but what you should find is something which looks like this:

Lifetime: 50000h /L70B50

What does it mean?

L – luminous flux rate:
in this case: L70- The LEDs used reach after 50000h 70% of the initial luminous value

B – Mortality rate:

in this case: B50 – 50% of the population of LEDs fail  to reach the L70 criteria

Other example: 50000h/L70B10

Is it better? In theory yes, because only 10% of the LEDs don’t fulfill the criteria anymore.

You should know that these lifetime values are estimations based on measurements from so called LM-80 reports. These test cycles are normally 6000h-10000h. These measurements have to be performed at 55°C, 85°C and a third one which can be freely choosen by the LED manufacturer. Beyond the real test cycle extrapolations are used to determine the lifetime according to the above definition. Which extrapolation has to be used is not defined for the time being and individual extrapolations are used by the manufacturers.  Which makes a real comparison eventually very difficult, if not impossible, unless you know the methods which were used for extrapolation.

So now you will understand the statement “In theory yes”. Because you have two values which might or might not be comparable to each other.

The other decisive point in then the temperature under which the LED is operating which brings us back to obvious and hidden thermal  design of the LED spot or luminaire.

 

We will continue soon here….

History of the LED - a brief overview

A lot of people and companies speak about LED light as “light of the future”.  This sounds like past to us. LED is the light of the presence.

But the history of the LED starts probably sooner than you think. The physical effect of electroluminescence, the operating principle of the LED based on the, was discovered over 100 years ago.

Here the major milestones:

1907
Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat’s-whisker detector.

1927
The Soviet inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades.

1935
The French physicist Georges Destriau discovered a light emission of zinc sulfide. He referred to the effect in honor of the Russian physicist  as “Losev light”. Today Georges Destriau regarded as the inventor of the electroluminescence.

1951
Kurt Lehovec, Carl Accardo and Edward Jamgochian, explained these first light-emitting diodes in 1951 using an apparatus employing SiC crystals with a current source of battery or pulse generator.

1957
Rubin Braunstein eported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Braunstein observed infrared emission generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77 kelvins.

1961
James R. Biard and Gary Pittman discovered infrared light emission from a tunnel diode they had constructed on a GaAs substrate. Later that year, they were able to demonstrate efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector.

1962
The first visible-spectrum (red) LED was developed in 1962 by Nick Holonyak, Jr.. Holonyak first reported his LED in the journal Applied Physics Letters on the December 1, 1962.

1976
T. P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.

1993
Shuji Nakamura developed the first blue LED and a very efficient LED in the green spectral (InGaN diode). A little later, he also designed a white LED.

1995
The first LED with white light created by luminescence conversion is presented and launched on the market two years later.

2006
The first LEDs with 100 lumens per watt are produced. This efficiency can now only  be trumped only by gas discharge lamps.

2010
LEDs  of a certain color with a gigantic luminous efficiency of 250 lumens per watt are already developed under laboratory conditions.

to be continued….

source: Wikipedia

LED in General lighting applications

The diversity of the LED are at the same time its advantage and its problem. Why?  It’s advantage, because it can be used to solve a lot of different lighting problems, its disadvantage because people don’t understand the LED. Suddenly parameters become important for the purchase of a lamp we never thought of in the past. Or did you think about parameters like color temperature, Color rendering index ?

Here follows the try to explain the different parameters in a common language:

 

…. here we still need to do  some homework

 

Contact

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