Introduction
Selecting an LED for a custom LED module may appear straightforward. Search for the required CCT, CRI, lumen output, and efficacy, then choose a suitable component.
In practice, LED selection is more complex.
The LED becomes part of an electrical, thermal, optical, and mechanical system. A component that performs well under datasheet test conditions may behave differently when operated at another current, installed on a compact PCB, placed behind a diffuser, or used inside an enclosed product.
For OEM applications, component availability and long-term supply can also be as important as optical performance.
The goal is therefore not to identify the LED with the highest specification on paper.
The goal is to select an LED—or combination of LEDs—that provides the required performance within the actual product architecture.
1. Start With the Finished Product Requirements
LED selection should begin with the requirements of the finished product rather than with a particular LED part number.
Useful starting specifications include:
- Target lumen output
- CCT
- CRI
- Color consistency
- Available PCB dimensions
- Input voltage
- Driver architecture
- Target power
- Optical requirements
- Operating temperature
- Expected lifetime or reliability requirements
- Estimated production volume
Mechanical constraints also matter.
A small circular PCB, long linear board, compact high-output module, and large-area panel may require very different LED packages and layouts even if their total lumen requirements are similar.
If the overall module specification is still being defined, see How to Choose the Right LED Module for Your OEM Product.
2. Determine the Required Light Output
The required lumen output is one of the most obvious LED selection criteria, but it should be considered at module or system level.
A product requiring a particular lumen output does not necessarily need LEDs whose datasheet values add up exactly to that number.
Actual output can be affected by:
- LED operating current
- Junction temperature
- LED bin
- Optical losses
- Diffuser transmission
- Lens efficiency
- Driver performance
For example, if the finished product needs 2,000 lumens, the LED module may need to generate more than 2,000 lumens before optical losses are considered.
This is why system-level requirements should be defined before determining LED quantity.
3. Select the Appropriate CCT and CRI
For white-light modules, CCT and CRI are key optical specifications.
CCT determines the general appearance of white light, while CRI describes aspects of color rendering.
Common CCT requirements may include:
- 2700K
- 3000K
- 3500K
- 4000K
- 5000K
- 5700K
- 6500K
CRI requirements may include:
- CRI 70+
- CRI 80+
- CRI 90+
- CRI 95+
The correct combination depends on the application.
Higher CRI may be important in retail, architectural, display, inspection, hospitality, or other color-sensitive environments, while other products may prioritize efficacy or cost.
For a detailed explanation, see Understanding CCT, CRI and Luminous Efficacy in LED Module Design.
4. Do Not Compare LEDs by Maximum Efficacy Alone
Luminous efficacy is important, but headline lm/W values can be misleading if the test conditions are different from the intended operating conditions.
LED efficacy can vary with:
- Operating current
- Junction temperature
- CCT
- CRI
- LED bin
- Package design
An LED may achieve an excellent efficacy value at a relatively low test current but provide a different result when operated at the current required by the actual product.
For OEM design, it is therefore more useful to compare LED performance at realistic operating conditions.
A slightly lower datasheet maximum does not necessarily mean lower system performance.
5. Choose the LED Package Size
LEDs are available in many package sizes and architectures.
Common SMD LED package families may include dimensions such as:
- 2835
- 3030
- 3535
- 5050
- Other application-specific packages
Package selection can influence:
- PCB layout
- LED spacing
- Power density
- Thermal path
- Optical design
- Assembly process
- Cost
A smaller package can allow higher component density, but placing many LEDs in a compact area can increase local thermal concentration.
A larger or higher-power package may reduce LED quantity but can create different optical and thermal requirements.
Package size should therefore be selected according to the complete module architecture.
6. Consider LED Quantity Versus Operating Current
There are often multiple ways to achieve the same target lumen output.
One design may use fewer LEDs operated at relatively high current.
Another may use more LEDs operated at lower current.
Both approaches can work, but they may produce different results in terms of:
- Efficacy
- PCB size
- LED cost
- Thermal performance
- Optical uniformity
- Driver requirements
- Reliability
Operating LEDs at a moderate current can sometimes improve efficacy and reduce thermal stress, but it requires additional components and PCB area.
There is no universal optimum.
The correct balance depends on the product requirements.
7. Evaluate the LED's Thermal Characteristics
LED selection and thermal design cannot be separated.
Important thermal-related characteristics can include:
- Package thermal resistance
- Recommended operating conditions
- Maximum junction temperature
- Power dissipation
- Temperature-dependent performance
However, even a thermally capable LED package requires an effective PCB and mechanical thermal path.
Heat still needs to move through:
LED → solder joint → PCB → thermal interface → housing/heatsink → ambient environment
For a detailed discussion, see LED Module Thermal Management: Design Considerations for Long-Term Reliability.
8. Match the LED to the PCB Architecture
The LED package needs to work with the selected PCB construction.
Depending on power density and circuit complexity, an LED module may use:
- FR4 PCB
- Aluminum PCB
- Other application-specific PCB constructions
The LED footprint, thermal pad structure, soldering requirements, copper layout, and PCB stack-up should all be considered.
A high-power LED package does not automatically require an aluminum PCB, and using aluminum PCB does not automatically guarantee adequate thermal performance.
For more information, see FR4 vs Aluminum PCB for LED Modules: How to Choose.
9. Consider Forward Voltage and Electrical Configuration
LED forward voltage affects how LEDs can be arranged within the circuit.
Depending on the product, LEDs may be configured in:
- Series
- Parallel
- Series-parallel combinations
The configuration influences:
- Module voltage
- Operating current
- Driver selection
- Power
- Failure behavior
- PCB routing
LED selection therefore needs to be coordinated with the electrical architecture.
For example, a constant-current driver has an output voltage range that needs to accommodate the forward voltage of the LED string.
A constant-voltage module requires appropriate current regulation within the system.
See Constant Current vs Constant Voltage LED Modules: Which Should You Choose?
10. Evaluate Color Consistency and Binning
For many OEM products, nominal CCT alone is not enough.
LEDs manufactured within the same nominal CCT category can have chromaticity variation.
This becomes particularly noticeable when:
- Multiple LEDs are visible through one diffuser
- Several fixtures are installed side by side
- Modules need to match between production batches
- Replacement modules must visually match existing products
Depending on the application, the LED specification may therefore include a defined chromaticity tolerance or SDCM requirement.
Tighter binning can improve visual consistency, but it may also affect availability and cost.
The specification should reflect what the finished product actually requires.
11. Consider Optical Compatibility
The LED package needs to work with the optical system.
The final product may use:
- Diffusers
- Reflectors
- Secondary lenses
- Light guides
- Mixing chambers
- Other optical components
LED spacing, emitting surface size, viewing angle, and package geometry can influence the final light distribution.
For example, an LED layout that appears uniform without a diffuser may produce visible hotspots once integrated into another optical structure—or the opposite.
For this reason, optical performance should ideally be evaluated using the intended mechanical and optical components during prototype development.
12. Consider Dimming and Control Requirements
LED selection may also be influenced by the control architecture.
A simple fixed-white module may have relatively straightforward requirements.
Other products may require:
- PWM dimming
- Analog dimming
- Tunable white
- RGB
- RGBW
- Multi-channel control
Multi-channel modules require appropriate LEDs, electrical architecture, driver selection, and optical mixing.
For specialized systems, see NKT's Specialty OEM LED Module Applications.
13. Component Availability Matters for OEM Production
For OEM products, technical performance is only part of LED selection.
The component also needs to be commercially practical.
Important supply considerations can include:
- Manufacturer availability
- Lead time
- Product lifecycle
- Minimum order quantity
- Binning availability
- Alternative components
- Production volume
- Long-term sourcing stability
Selecting an unusual LED that is difficult to source consistently can create problems later even if its technical specifications are excellent.
For long-term OEM programs, engineering and procurement considerations should therefore be evaluated together.
14. Should You Specify an LED Brand?
Some OEM customers specify a particular LED manufacturer or exact part number.
Others specify performance requirements and allow the module manufacturer to recommend an appropriate component.
Both approaches can be valid.
Specifying an exact LED may be appropriate when:
- The product is already qualified around that component
- A customer specification requires it
- Optical characteristics need to remain consistent
- Regulatory or certification documentation references it
A performance-based specification may provide more flexibility when:
- The product is still under development
- Multiple suitable LEDs are available
- Cost optimization is important
- Supply continuity is a priority
The best approach depends on the project.
15. Prototype With the Actual LED Configuration
Datasheet analysis is important, but the final decision should ideally be supported by prototype evaluation.
A prototype can help verify:
- Light output
- CCT
- CRI
- Color consistency
- Optical uniformity
- Power consumption
- Operating temperature
- Driver compatibility
- Mechanical fit
The module should be evaluated as closely as practical to the conditions of the final product.
This is especially important when the design uses a diffuser, lens, enclosed housing, or thermally demanding configuration.
Learn more about Custom LED Module Design: From Concept to Production.
A Practical LED Selection Process
For many OEM projects, the LED selection process can be simplified into the following sequence:
Define product requirements
→ Determine target optical performance
→ Select candidate LED packages
→ Evaluate electrical operating point
→ Evaluate PCB and thermal requirements
→ Check optical compatibility
→ Review availability and cost
→ Build prototype
→ Test and validate
→ Finalize for production
The process is iterative.
Changing one parameter can influence several others.
For example, changing from CRI 80 to CRI 90 may influence efficacy and LED quantity. Increasing LED quantity may affect PCB dimensions. Changing PCB dimensions may influence thermal behavior and optics.
This is why LED selection should be part of the complete engineering process rather than a separate purchasing decision.
Common LED Selection Mistakes
Choosing the LED With the Highest Datasheet lm/W
Performance needs to be evaluated at the intended current, temperature, CCT, and CRI.
Selecting LED Quantity Before Considering Thermal Design
The number of LEDs and their operating current influence power density and PCB temperature.
Ignoring Color Binning
Nominal CCT alone may not provide the visual consistency required by the product.
Choosing an LED Without Checking Driver Compatibility
Forward voltage, operating current, and circuit configuration need to match the power architecture.
Ignoring Long-Term Component Availability
An excellent technical solution can still create production problems if the selected component becomes difficult to source.
Frequently Asked Questions
What is the most important specification when choosing an LED?
There is no single most important specification. Light output, CCT, CRI, efficacy, forward voltage, operating current, thermal characteristics, package size, availability, and cost should be evaluated according to the application.
Should I use fewer high-power LEDs or more lower-power LEDs?
Either approach can work. The decision depends on optical uniformity, PCB area, operating current, efficacy, thermal performance, driver requirements, component cost, and reliability.
Does a higher lm/W LED always make a more efficient LED module?
Not necessarily. Datasheet efficacy may be specified under conditions different from the final application. Module and system performance also depend on current, temperature, driver efficiency, and optical losses.
Can I change the LED brand without redesigning the PCB?
Sometimes, but not always. Even LEDs with similar package dimensions may have different footprints, electrical characteristics, optical performance, thermal behavior, or binning systems. Compatibility should be evaluated before substitution.
Do I need to specify the exact LED part number for a custom module?
Not necessarily. OEM customers can specify either a particular component or the required optical, electrical, thermal, and commercial performance. The appropriate LED can then be selected during engineering development.
Need Help Selecting LEDs for Your Custom Module?
NKT develops custom LED modules for OEM applications with support for LED selection, PCB design, electrical architecture, thermal design, optical requirements, prototyping, testing, and production.
If you already have an LED specification, mechanical drawing, existing PCB, product sample, or target performance requirements, our engineering team can evaluate the project and recommend an appropriate module architecture.