Light uniformity is often discussed as an optical problem, but in a real OEM product, the LED module is only one part of the optical system.
The final appearance can be affected by:
- LED quantity and spacing,
- LED viewing angle,
- distance between the LEDs and the cover or diffuser,
- diffuser characteristics,
- fixture geometry,
- LED-to-edge spacing,
- and even the space occupied by connectors and other PCB components.
For many OEM projects, some of these conditions have already been fixed before the LED module is designed. The customer may already have the fixture dimensions, diffuser position, mounting points, and available PCB space.
In these cases, improving uniformity does not mean redesigning the complete optical system. The practical task is often to develop the LED layout around the mechanical and optical conditions that already exist.
The goal is to achieve the required light output while reducing visible hotspots and dark areas within the constraints of the customer's actual product.
1. What Causes Hotspots and Dark Areas in an LED Module?
A visible hotspot occurs when light from an individual LED remains noticeably brighter than the surrounding illuminated area.
Dark areas occur when the light distribution from adjacent LEDs does not overlap sufficiently or when the LED layout does not adequately cover part of the illuminated surface.
These effects are not determined by LED spacing alone.
A typical design needs to consider several interacting factors:
- LED-to-LED spacing,
- LED-to-edge spacing,
- LED viewing angle,
- LED-to-cover distance,
- diffuser or cover characteristics,
- fixture geometry,
- and required brightness.
For example, two modules using the same LED and the same spacing can look different when installed behind different covers or at different distances from the illuminated surface.
This is why light uniformity should be evaluated as part of the final product configuration rather than from the PCB layout alone.
2. LED Spacing Is Usually the First Module-Level Design Consideration
For many NKT fixture projects, the height and position of the diffuser or PC cover are already determined by the customer's mechanical design.
That means the LED module cannot simply increase the optical distance whenever more uniform light is required.
Instead, one of the main variables available during module development is the LED layout.
When the LED quantity is known, NKT normally considers the spacing before the PCB design is finalized and distributes the LEDs as evenly as practical across the effective illuminated area.
The objective is not simply to create equal distances between neighboring LEDs.
Both of these need to be considered:
LED-to-LED spacing + LED-to-edge spacing
If the LEDs are evenly spaced in the center but leave excessive space at the edges, the finished product can still appear less uniform near the perimeter.
A good LED layout therefore needs to consider the complete illuminated area, not only the distance between adjacent LEDs.
3. Why Perfect LED Spacing Is Not Always Possible on a Real PCB
An ideal optical layout might place every LED at perfectly regular intervals.
A real OEM LED module usually cannot.
The PCB also needs space for:
- connectors,
- wiring points,
- mounting holes,
- and other required components.
This becomes particularly noticeable when compact SMD LEDs are used.
For example, a 2835 LED occupies relatively little PCB area, while a wire connector can occupy considerably more space. Depending on the board dimensions and connector position, the connector may force one or more LEDs away from their theoretically ideal locations.
Directly soldered wires can sometimes require less board area and provide more layout flexibility.
However, many OEM customers prefer connectors because they make final assembly easier: the module can be installed and connected using a plug rather than requiring wires to be soldered during fixture assembly.
The result is a practical OEM trade-off:
Optical spacing ↔ PCB space ↔ connector position ↔ assembly convenience
For this reason, a good LED module layout is not necessarily the one with mathematically perfect spacing. It is the layout that provides suitable light distribution while still meeting the customer's mechanical and assembly requirements.
This type of constraint is considered during custom LED module development.
4. LED Viewing Angle Affects How Adjacent LEDs Overlap
LED viewing angle also influences uniformity.
A wider viewing angle spreads the emitted light across a larger area, allowing the light patterns from neighboring LEDs to overlap.
In many fixture applications NKT works with, 2835 LEDs with an approximately 120° viewing angle are commonly used.
This does not mean that 120° is the correct angle for every product.
Rather, wide-angle LEDs can be useful where broad illumination is required and the fixture relies on the combined output of multiple LEDs.
The final result still depends on:
- LED spacing,
- distance to the cover,
- fixture geometry,
- and the optical properties of the cover.
Some OEM projects also use commercially available secondary lenses with different beam angles when the application requires more specific light distribution.
NKT can integrate suitable standard lenses into an LED module when required, but the optical requirement still needs to be evaluated in the context of the customer's final product.
5. Why LED-to-Diffuser Distance Matters
The distance between the LED and the illuminated cover is another important factor.
When a wide-angle LED emits light, its illuminated area expands with distance.
If adjacent LEDs are very close to the diffuser, their individual light patterns may not have enough distance to overlap before reaching the visible surface. This can make individual LED points more noticeable.
With greater LED-to-cover distance, adjacent light patterns generally have more opportunity to overlap before reaching the cover, which can reduce visible hotspots.
This is why the same LED spacing can produce different visual results in two fixtures with different internal heights.
However, increasing the distance is not always an available solution.
In an OEM project, the fixture height is often already fixed by the customer's mechanical design.
In that situation, the LED module has to be designed around the available optical distance rather than assuming the fixture can be changed to suit the PCB.
6. Frosted vs. Clear PC Covers: Uniformity and Light Transmission
The cover itself introduces another design trade-off.
A frosted or diffusing PC cover scatters light and can help make individual LED points less visible.
However, diffusion also introduces optical loss.
A clear PC cover generally preserves more transmitted light, but individual LED points can be more noticeable depending on LED spacing, viewing angle, optical distance, and fixture geometry.
The practical trade-off is therefore:
Light transmission / apparent brightness ↔ visual uniformity
This does not mean that a clear cover always produces poor uniformity or that a frosted cover always eliminates hotspots.
For example, when there is sufficient distance between wide-angle LEDs and the cover, the light from neighboring LEDs has more opportunity to overlap before reaching the visible surface.
The final result still depends on the complete fixture geometry.
For this reason, cover type should not be evaluated independently from the LED module layout.
7. Diffuser Loss Should Be Considered When Setting the Module Lumen Target
Another important distinction is between:
LED module output
and
finished-fixture output
They are not necessarily the same.
If the customer's finished fixture needs a specified lumen output, the LED module may need to produce more light before the cover in order to compensate for optical loss through a frosted or diffusing PC.
In many projects, customers do not provide the exact transmission data for their cover during the early module-design stage.
In that situation, NKT may use an estimated transmission value for initial lumen planning based on common market conditions.
For example, approximately 85% transmission may be used as an initial engineering assumption for a typical frosted PC application when the actual material data is unavailable.
This is an estimate—not a fixed transmission value for all frosted PC materials.
Actual transmission can vary with:
- material,
- thickness,
- haze,
- surface treatment,
- and diffuser design.
If the customer has actual transmission data for the selected cover, providing it during module development allows the required module-level output to be estimated more accurately.
The final fixture output should still be confirmed using the customer's actual optical components.
8. If Dark Spots Remain, Repositioning the Same LEDs Is Not Always the Best Solution
LED spacing is normally considered before the PCB layout is finalized.
Therefore, if the customer installs a prototype in the actual fixture and still sees dark areas, simply moving the same number of LEDs may not provide enough improvement.
In some projects, a more practical solution is to increase the LED quantity.
More LEDs can reduce the distance between adjacent light sources and increase optical overlap across the illuminated area.
For example:
fewer LEDs + wider spacing
↓
greater possibility of visible dark areas
Compared with:
more LEDs + closer spacing
↓
more overlapping light distribution
But adding LEDs does not automatically mean that the finished product needs to become brighter.
The operating current can also be adjusted.
This creates an important design option:
More LEDs + lower operating current per LED can improve uniformity while maintaining a similar overall output target.
Whether this approach is appropriate depends on the electrical architecture and the customer's performance requirements.
9. Adding More LEDs Does Not Automatically Mean More Lumens
Suppose a customer approves the overall brightness of a fixture but identifies dark areas during prototype testing.
If more LEDs are added to improve uniformity, there are two possible objectives.
Option A — Improve Uniformity While Keeping Similar Light Output
The LED quantity is increased, but the operating current is reduced so that the total output remains close to the original target.
This can improve light coverage without unnecessarily increasing fixture brightness.
Option B — Improve Uniformity and Increase Light Output
The customer may decide that the fixture should also become brighter.
In that case, the new LED quantity and operating current need to be evaluated against the revised lumen and power targets.
Therefore:
Adding LEDs is an optical-layout decision. The resulting lumen output is also an electrical-design decision.
The two should be considered together.
10. Constant-Current and Constant-Voltage Modules Need Different Adjustments
How the current is adjusted after increasing the LED quantity depends on the module architecture.
For a more detailed explanation of these two electrical approaches, see Constant Current vs. Constant Voltage LED Modules.
Constant-Current LED Modules
With a constant-current module, the customer controls the total operating current within the approved electrical limits of the module.
If additional LEDs are used primarily to improve uniformity and the customer wants to maintain approximately the same overall light output, the operating current can be reduced accordingly.
If higher output is also required, the appropriate operating current needs to remain within the validated range of the new module configuration.
Constant-Voltage LED Modules
For a constant-voltage module, increasing the LED quantity can require the electrical design to be recalculated.
If the customer does not want additional lumen output, the LED operating current can be reduced in proportion to the new design target.
If the customer also wants higher output, the required current needs to be determined based on:
- the new LED quantity,
- revised lumen target,
- LED performance,
- circuit configuration,
- and power requirements.
This is why a uniformity change can sometimes lead to an electrical-design change as well.
11. What Information Helps When Uniformity Matters?
When light uniformity is important to the finished product, the LED module should ideally be developed with information about the actual fixture rather than from a lumen requirement alone.
Useful project information includes:
- available PCB dimensions,
- illuminated area,
- mounting-hole positions,
- connector requirements,
- LED-to-cover distance,
- cover or diffuser type,
- cover transmission data, if available,
- target finished-fixture lumens,
- power requirements,
- supply or driver type,
- CCT and CRI requirements,
- and any restrictions on visible LED spots.
If the customer already uses a standard lens, information about the lens and beam angle is also useful.
The more accurately the final fixture conditions are defined, the less the LED module design needs to rely on assumptions.
12. Prototype Testing in the Actual Fixture Is Important
A PCB can look well balanced in the layout drawing and still behave differently once installed behind the customer's actual cover.
That is because the final result depends on components outside the LED module itself.
For this reason, prototype validation is an important part of a custom project.
NKT can test relevant module-level electrical and optical parameters during prototype development.
For white-light modules, these may include:
- voltage,
- current,
- luminous flux,
- luminous efficacy,
- CCT,
- and CRI.
However, visual uniformity should ultimately be evaluated with the module installed in the customer's actual fixture, using the intended cover, diffuser, or lens.
The customer can then determine whether:
- individual LED points are visible,
- dark areas remain,
- the edges are sufficiently illuminated,
- and the finished-fixture brightness meets the product requirement.
If a problem remains, the feedback can be used to evaluate changes such as LED quantity, layout, or operating conditions before production.
Learn more about this development stage under Prototype Validation.
13. Is There a Universal LED Spacing Rule for Good Uniformity?
No.
There are useful optical relationships between LED spacing, viewing angle, and LED-to-diffuser distance, but a single spacing value cannot guarantee uniformity across different fixtures.
For example, the appropriate spacing for a 120° LED installed relatively far from a frosted cover may not work equally well when:
- the cover is much closer,
- the cover is clear,
- the illuminated area has a different shape,
- a narrower-angle lens is used,
- or PCB components force changes to the LED positions.
For an OEM product, LED spacing should therefore be evaluated in the context of the actual fixture.
The correct question is not simply “How far apart should the LEDs be?”
It is:
“How should the LEDs be distributed within the available PCB and optical space to achieve the required output and acceptable uniformity?”
How NKT Approaches Light Uniformity in a Custom LED Module
NKT normally develops the LED module around the mechanical and performance conditions already defined by the customer's product.
When the fixture dimensions and cover height are fixed, the focus is placed on the module-level variables that can still be optimized, including:
- LED quantity,
- LED-to-LED spacing,
- LED-to-edge spacing,
- layout,
- LED viewing angle,
- standard lens integration when required,
- and operating current.
The design also needs to accommodate practical PCB requirements such as connectors, mounting holes, and customer assembly preferences.
If a frosted cover is used and exact transmission data is unavailable, an estimated optical loss may be considered during initial lumen planning rather than assuming that module output and finished-fixture output are identical.
After the prototype is installed in the customer's actual fixture, visual feedback can be used to determine whether further changes are needed.
If dark areas remain, increasing LED quantity and adjusting the operating current can sometimes provide a better solution than simply repositioning the original LEDs.
The objective is not perfect spacing on a PCB drawing. It is a practical LED module configuration that works inside the customer's actual product.
Conclusion
Improving LED light uniformity is rarely the result of changing one parameter.
LED spacing matters, but so do LED quantity, viewing angle, distance to the cover, diffuser characteristics, fixture geometry, PCB component placement, and required brightness.
In many OEM projects, the fixture and diffuser position are already fixed.
The LED module therefore needs to work within those constraints.
A well-planned layout distributes the LEDs as evenly as practical across the illuminated area while also accounting for edge spacing, connectors, mounting requirements, and electrical design.
If prototype testing reveals dark areas, increasing the LED quantity can improve optical overlap, while the operating current can be adjusted according to whether the customer wants to maintain or increase the overall light output.
And when a frosted or diffusing cover is used, its optical loss should be considered when translating a finished-fixture lumen requirement into an LED module output target.
Light uniformity is therefore not only an LED-spacing problem. It is a balance between optical performance, electrical design, PCB constraints, and the mechanical structure of the finished OEM product.
Request a Quote
If you are developing an OEM product and need a custom LED module with specific brightness or uniformity requirements, send NKT the available fixture information, PCB dimensions, LED-to-cover distance, target output, power requirements, and optical-component details.
If the final LED quantity or layout has not yet been defined, these requirements can be reviewed as part of the initial module development.