USABLE LIGHT - Talking about real life lumen values

Page Contents:

[ 7 Relationships | Calculating Your Real Life Lumen Value | Comparing High Power LEDs ]

In its basic nature, a lumen is a measure of photometric flux emitted by a light source in all directions.

In the design of any luminaire, designers are concerned with how much light (lumens) the light source emits. If the designer knows this, he/she can predict how much light they are able to project on the intended target. Today, when designing a luminaire with an incandescent, fluorescent or any other conventional bulb, designers usually specify a particular wattage of bulb. (These bulbs typically have an associated datasheet lumen value). However, unlike conventional technology, high power LED datasheet values should be viewed more critically. The reality is that high power LEDs have adverse reactions to heat and designers need to know how to calculate the amount of light that they will see in their applications. This leads to the discussion of USABLE LIGHT.

When characterized for flux (lumens), high power LEDs are given a short pulse of current and the temperature of the die is controlled at 25°C. In a real world application, the junction temperature (Tj), otherwise known as the die temperature, will rarely remain static at 25°C and will be a lot higher than that. In order to understand why this is important, we need to analyze the characteristics of high power LEDs as they are introduced to elevated temperatures.

 

 

LED Thermal Characteristics

A. Junction Temperature (Tj): This refers to the temperature of the die. In Figure A., you can see the relationship between light output and junction temperature. As the Tjincreases, light output decreases.

B. Thermal Resistance: Measure of the ability of heat to escape the device. Unit of measurement is °C/W. LED thermal resistance is measured from junction to slug. Another way to look at it is that if an LED had a thermal resistance of 10°C/W as you drive the LED with 1W of power, you would have a 10°C rise in die temperature.

 

 

7 Relationships:
In addition to the LED thermal characteristics listed above, there are 7 relationships at work which accurately model the LED and these relationships must be taken into account when determining usable light.

 

1. The more current that you supply to the LED, the brighter the LED will be
2. As forward current increases so too does the forward voltage
3. Power will increase as voltage increases
4. An increased amount of heat will be generated from the LED as the power increases
5. Light output will decrease as the LED heats up
6. The LED Vf will drop as the LED heats up
7. Power will increase as current increases

 

 

Calculating Your Real Life Lumen Value:
It is important to determine how the LED will operate in a real life application and thus you must derate the part to a designed lumen value.

Here are the steps in determining how much light you can expect from a high power LED in your application.

 

1. Figure out your junction temperature.
   The formula to determine Tj is:
   
   Tj = Ta + ((Rth)*Pd)
   
   Tj = Junction temperature
   Ta = Ambient temperature
   Rth = Thermal resistance from junction to ambient =(Rth of LED + Rth of board + Rth of heat sink)
   Pd = Power dissipated by the LED
2. Use the light output versus temperature graph to determine how much light you will see from your LED.
3. This new, derated lumen value is the number that you should use when designing.

 

 

 

Comparing High Power LEDs
The usable light concept should be used when comparing high power LEDs. Company A may say that they get 30 lumens from their LED versus Company B which says that they get 40 lumens at the same current. Does this mean that we should think that Company B's LED is the brighter one? In the real world, you must derate both parts as a result of heat and do true 'apples' to 'apples' comparison. In doing this comparison and using the Tj formula, it becomes apparent that there are many other factors which need to be accounted for to determine which LED will be brighter in a real life application. For example, if Company B's LED has a significantly higher Rth, then their LED, Tj will be hotter, resulting in a higher loss of light. This concept should NOT be overlooked as it is the only way to compare the light output characteristics of a high powered LED. In this case, if Company A has a lower Rth, or lower Vf then it is entirely possible that you will get more usable light from Company A rather than Company B's LED.

 

 

Future Lighting Solutions has developed an online tool which will allow customers determine how much light they can expect to see from their LUXEON® LED in their application, i.e. usable light. The output (see Fig.1) showcases both a usable flux (lumen) conversion as well as usable efficacy.

Usable efficacy is just the amount of usable light divided by the power dissipated by the LED. This online calculator can be found at www.futureelectronics.com/promos/Lumileds/UsableLight/.

 

 

As shown in the input screen, (see Fig. 2) the user must enter the product, color, ambient temperature and the heat sink Rth for the two products in a question. This online tool is modeled around the 7 relationships as well as the polynomial equations which model the LUXEON® LED output over temperature. This online calculator can be found at www.futureelectronics.com/promos/Lumileds/UsableLight/