Calculate Number of LEDs Needed

Actual Lumens Needed

With all the system efficiencies estimated, the actual number of LED lumens required to achieve the design goals can be calculated. For this calculation, only the light efficiencies (optical and thermal) are used. The electrical efficiency affects only the total power consumed and fixture efficacy, not the amount of light coming out of the luminaire. The calculation of “actual lumens needed” for the example luminaire is shown below:

Actual Lumens Needed = Target Lumens / (Optical Efficiency x Thermal Efficiency)

Actual Lumens Needed = 810 / (91% x 85%)

Actual Lumens Needed = 1,050 lm


Operating Current

Another decision to be made is what operating current to use for the LEDs. Operating current plays an important role in determining the efficacy and lifetime of the LED luminaire. Increasing the operating current will result in more light output from each LED, thus reducing the number of LEDs needed. However, increasing operating current also has several drawbacks, shown in Table 6 (below). Depending on the application, these drawbacks may be acceptable trade-offs for the higher per-LED lumen output.

For the example luminaire, lifetime and efficacy are top priority design goals. The luminaire will run at the minimum operating current listed on the XLamp XR-E datasheet (350 mA) to maximize LED efficacy and lifetime.



Reduced efficacy Higher operating current reduces the efficacy of current generation power LEDs. In general, the size of the power supply will increase as operating current increases, since it takes more power to generate the same number of lumens.

Reduced maximum ambient temperature


Decreased lifetime

Higher current will increase the temperature difference between the LED junction and the LED’s thermal path. In practical terms, since the maximum junction temperature is already decided, this reduces the maximum ambient temperature for the luminaire.

If instead of lowered maximum ambient temperature, the maximum junction temperature is raised, the LED will degrade in light output faster over its operational life.

Table 6 - Drawbacks of High Operating Current in LED Luminaires

Number of LEDs

After deciding on operating current, the lumen output of each LED can be calculated. Since the thermal loss of the LED has already been taken into account through the actual-lumens-needed calculation, the numbers specified in LED-supplier documentation can be used directly without further interpretation.

For this calculation, it is important to use the minimum flux listed for your LED order code and not the typical number on the data sheet. Most LED companies sell to minimum flux ranges. By designing against this minimum number, you are ensuring that all luminaires made with that LED order code meet the target requirements.

The example luminaire will use XLamp XR-E LEDs at 4000K CCT with minimum luminous flux of 67.2 (P2 flux bin) @ 350 mA. The number of LEDs is calculated below.

Number of LEDs = Actual Lumens Needed / Lumens per LED

Number of LEDs = 1,050 lm / 67.2 lm

Number of LEDs = 16 LEDs

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