LED street lamp datasheet for dummies

Real consumption of a LED lamp

The consumption of an LED lamp is obtained with this calculation:

C = n x Vf x i / μ

Where:

n is the total number of LEDs;

Vf is the Forward Voltage of the lamp, a function of drive current and of junction temperature, typically from 3V to 3.5 V;

i is the drive current of the LEDs, typically from 0.35 A to 0.7 A;

μ is the efficiency of the power supply, typically from 0.8 to 0.92.

Thus the performance of an LED lamp with 100 XP-E LEDs driven at 350mA with a power supply efficiency of 85% is:

C = 100 x 3,2 x 0,35 / 0,85 = 131,8 W

Then, saying “1-Watt LED” is incorrect, usually an LED driven at 350mA current consumes 1.12 W and 1.3 W when we consider power supply.

Nominal efficiency of LEDs

The flux shown in the LEDs datasheets does not display the efficiency in lumens per watt but the flow delivered at 350 mA with 25°C of junction temperature. The efficiency in lumens per watt is calculated therefore as follows:

F1 = f / (Vf x 0,35)

Where:

f is the nominal flux at 350 mA;

Vf è is the Forward Voltage of the lamp at 350mA, typically from 3V to 3.3 V;

Going back to the first example, an XP-E LED, 100 lumens, has an efficiency of 100 lumens per watt but:

F1 = 100 / (3,2 x 0,35) = 91,4 lumen/watt

Real flux of LEDs

As mentioned above, the flux of the LEDs are rated for a junction temperature of 25 ° C at a drive current of 350 mA.

Maintaining a junction temperature of 25 ° C in a streetlight is impossible and there is therefore a minimum decay of 10-15% (but it can easily become higher) on the nominal flux of the LEDs, to be assessed in the laboratory on real streetlight lamps.

In addition, while increasing drive current, the luminous flux increases not in direct proportion to the current. It should therefore be assessed according to the efficiency of the LED current.
Therefore, the real flux of the LEDs is:

F2 = f x μ1 x μ2

Where:

f is the nominal flux at 350 mA;

μ1 is the change in luminous flux because of the actual driving current;

μ2 is the change in luminous flux because of the actual junction temperature.

If we consider again the example of XP-E LEDs, based on diagrams, if LEDs are driven at 500 mA with a junction temperature of 100 °C, we have the following flux:

F2 = 100 x 1,356 x 0,822 = 111,5 lumen

with a power consumption C = 3.3 x 0.5 = 1.65 W and therefore with an efficiency, without considering the power supply, of 67.6 lumens / Watt.

Luminous efficiency of the LED module

UNI regulation 11356:2010 defines this efficiency as the “quotient of the luminous flux emitted by the LED module divided by the power input source including mechanical components such as, for example, any heatsinks excluding the power dissipated by the unit of power, at a temperature test environment specified. It is measured in lumens per watt. ”

Optical efficiency of a LED lamp

The real flux of the LEDs is the “starting” flux of the lamp.

The fixture, however, in order to achieve the lighting requirements, introduces some efficiency losses.

On average there is about a 10% loss of the glass plus a loss of 10% to 40% for the usage of secondary optics or for internal losses.

This efficiency is also known as LOR, resulting in:

LOR =  Fl / (n x F2)

Where:

Fl is the actual lamp flux measured in a photometric lab;

n is the total number of LEDs;

F2 is the real flux of the LEDs previously calculated;

The best lamps have an overall efficiency of around 85%.

Luminous efficiency of the LED lamp

UNI regulation 11356:2010 defines this efficiency as “the ratio between the luminous flux emitted by the lamp and the electrical power consumption of the appliance. It is measured in lumens per watt ”

Efficacy of the LED lamp

This is probably the most important parameter to be evaluated but also the most difficult to be measured.
This parameter expresses how much of the luminous flux leaving the lamp illuminates the target, and how it intercepts it.

In order to have a good street optics, the light exiting the lamp should be well distributed on the street, therefore without any peaks under the pole, with an opening angle of 120/150 degrees, without upward emissions and with small backward emissions behind the pole.

The shape of the emission profile helps to understand the effectiveness of a streetlight.

Usually the emission profile is made by two curves: one shows the intensity of the luminous flux from 0 ° to 180 °, another one from 90 ° to 270 °.

The 0-180 ° curve shows the profile of the luminous flux on the left and on the right of the lamp, the curve 90-270 ° shows the profile of the luminous flux in front and behind the lamp post.

The curve 0-180° must have a peak emission at about 60 ° and a flux that decreases from 60 ° to 0 °, the 90-270° curve must be very unbalanced on one side (with a few emissions behind the lamp) and the peak around 30 °, to cover the second lane.

Another factor to consider is the lack of jagged curves (showing shadows) or bumps (showing dips and light peaks).

For example, if we look at the profile below, we note that:

  1. The peak emission of the curve 0-180° is at 0° with a consequent exaggerated peak under the light pole and therefore with low lighting uniformity along the street.
  2. 0-180° curve has many “bumps” which are shadows and lowering of light along the street.
  3. The values at angles greater than 50 ° are too low.
  4. The opening angle is correct.
  5. Regarding the curve 90-270°, it is properly piqued in the two centerlines, but with lowering of light on the second lane and lack of uniformity within the lanes.
  6. Backward emissions (right part of  the curve  90-270°) are correct.

Let’s analyze another profile:

In this case:

  1. The curve 0-180 ° has a peak emission at around 60 ° resulting in correct angular aperture and uniformity.
  2. The curve 0-180° has an optimal opening angle (around 140°).
  3. The curve 90-270° has its highest emission on the second lane, thereby ensuring a good uniformity of light on both carriageways.
  4. Behind the lamp there is an optimal light with only a small peak of light at -60 °, probably due to the geometry of the lamp.

Alberto G. Gerli

http://www.ariannaled.com

Informazioni su ariannaled

The right reflection leads you forward.
Questa voce è stata pubblicata in English. Contrassegna il permalink.

Lascia un commento

Inserisci i tuoi dati qui sotto o clicca su un'icona per effettuare l'accesso:

Logo WordPress.com

Stai commentando usando il tuo account WordPress.com. Chiudi sessione / Modifica )

Foto Twitter

Stai commentando usando il tuo account Twitter. Chiudi sessione / Modifica )

Foto di Facebook

Stai commentando usando il tuo account Facebook. Chiudi sessione / Modifica )

Google+ photo

Stai commentando usando il tuo account Google+. Chiudi sessione / Modifica )

Connessione a %s...