LED lighting system design can be performed in six steps: determining lighting requirements, defining design objectives, estimating the efficiency of optical, thermal, and electrical systems, calculating the required number of LEDs, selecting the optimal design scheme, and completing the final step.

I. Determining Lighting Requirements

LED lighting must meet or exceed the lighting requirements of the target application. These requirements must be determined before establishing design objectives. For some applications, existing lighting standards exist, allowing for direct determination of requirements. For others, identifying the characteristics of existing lighting is a good approach.

II. Determining Design Objectives

Once the lighting requirements are determined, the design objectives for the LED lighting can be established. As with defining lighting requirements, key design objectives relate to light output and power consumption. Ensure that other design objectives that may also be important to the target application are included, including the operating environment, bill of materials (BOM) cost, and lifespan.

III. Estimating the Efficiency of Optical, Thermal, and Electrical Systems

One of the most important parameters in the design process is the number of LEDs required to meet the design objectives. Other design decisions revolve around the number of LEDs, as this directly affects light output, power consumption, and lighting costs. It is tempting to consult the typical luminous flux listed in the LED datasheet and divide that number by the design target lumens. However, this method is too idealistic, and designs based on it will not meet the lighting requirements of the application. The luminous flux of an LED depends on several factors, including drive current and junction temperature. To accurately calculate the required quantity, the efficiency of the optical, thermal, and electrical systems must first be estimated.

1. Optical System Efficiency The effectiveness of the optical system is estimated by examining light losses. There are two main sources of light loss to estimate:

First, secondary optics, which are not part of the LED's optical system, such as lenses or diffusers on the LED. Losses associated with secondary optics vary depending on the specific components used. Typical optical efficiencies for each secondary optical element are between 85% and 90%.

Second, light losses within the luminaire. Luminaire light losses occur when light strikes the luminaire cover before reaching the target object. Some light is absorbed by the luminaire cover, and some is reflected back to the luminaire. The efficiency of the fixture is determined by the lighting layout, the shape of the luminaire housing, and the material of the luminaire cover.

2. Thermal Losses The relative flux output of an LED decreases as the junction temperature increases. Most LED datasheets list typical luminous flux values ​​at 25°C, but most LED applications use higher junction temperatures. When the junction temperature Tj > 25°C, the luminous flux will definitely be worse than the value given in the LED datasheet. LED datasheets include a curve showing the relationship between relative light output and junction temperature, such as for the XLamp XR-E white LED. This curve provides other characteristic values ​​by selecting a specific relative light output or junction temperature. 85% relative luminous flux is an estimate of the lighting thermal efficiency in this example.

3. Electrical Losses LED driver electronics convert available power sources (such as AC power from a wall socket or battery) into a stable current source. Like all power supplies, this process will not be 100% efficient. Electrical losses in the driver reduce overall lighting efficiency because the input power is wasted on heat generation instead of light emission. Electrical losses should be considered when designing an LED system. Typical LED drivers have an efficiency between 80% and 90%. Drivers with efficiencies higher than 90% are significantly more expensive. Note that driver efficiency can vary with output load. For indoor applications, an estimated driver efficiency of 87% is appropriate. For outdoor applications or drivers with very long lifespans, the efficiency may need to be lower.

IV. Calculating the Required Number of LEDs

1. Actual Luminous Flux Required: Actual luminous flux required = Target luminous flux / (Optical efficiency × Thermal efficiency)

2. Operating Current: Operating current is crucial in determining the efficiency and lifespan of LED lighting. Increasing the operating current increases the light output of each LED, thus reducing the number of LEDs required. However, increasing the operating current also introduces several disadvantages, such as reduced efficiency and shortened lifespan.

3. Number of LEDs: Once the operating current is determined, the luminous flux of each LED can be calculated. Since the heat loss of LEDs has already been considered in the calculation of the actual required luminous flux, the quantities provided in the LED supplier's documentation can be used directly. The formula for calculating the number of LEDs is as follows:

Number of LEDs = Actual required luminous flux / Luminous flux per LED

V. Considering All Design Possibilities and Selecting the Optimal Design

After calculating the required number of LEDs, the next step is to consider all design possibilities that meet the design goals. Designers can fully utilize the directionality of LED light and the large number of available secondary optics to construct the initial design. Secondary optics refer to optical components added to the primary optics of an LED, used to shape the light output of the LED. Common types of secondary optics are reflection (light reflected back from a surface) or refraction (light bent through a refractive material, typically glass or plastic). Secondary optics can be designed and customized by purchasing standard parts, off-the-shelf components, or through ray tracing simulation using lighting models.

VI. Final Steps

After selecting the optimal design, the final steps include: circuit board layout, prototype construction, prototype testing against design goals, finalizing the design scheme, and writing observation reports and improvement suggestions.