Per-Key vs Underglow: Lighting Styles

When it comes to keyboard lighting, there are two primary aesthetics to consider: per-key backlighting and underglow. While both use LEDs, they produce very different visual effects. Many high-end custom keyboards even incorporate both for the ultimate light show.

Understanding these two approaches will help you choose the right style for your build and ensure you select compatible components.

Per-Key Backlighting: Lighting Up the Keys

Per-key backlighting is the most common form of keyboard illumination. As the name suggests, it involves placing an individual LED under each key switch. The light shines up through the switch housing and illuminates the keycap legend from below.

The Effect: This creates a functional and visually appealing effect, making the letters, numbers, and symbols on your keycaps visible in low-light conditions. With RGB LEDs, you can create stunning effects like reactive typing (where a key lights up when pressed), rainbow waves, or color-coding your keys by function (e.g., making all your modifier keys red).
What's Required:
- A PCB with support for per-key LEDs (either through-hole or SMD).
- Switches with transparent or translucent housings that allow light to pass through. Opaque switch housings will block the light.
- Shine-through keycaps: These are keycaps where the legend is made of translucent plastic, allowing the light to shine through it. Standard opaque keycaps will block the light, resulting in more of a "backwash" effect around the base of the keycap rather than an illuminated legend.

Underglow: The Atmospheric Effect

Underglow takes a different approach. Instead of lighting up the keys themselves, underglow involves placing a strip of LEDs on the underside of the PCB. These LEDs cast a vibrant, diffuse glow onto the desk surface beneath the keyboard.

The Effect: This creates a beautiful "floating" or "hovering" effect for the keyboard. It's a more atmospheric and ambient form of lighting that highlights the shape and design of the keyboard case itself. It can make a keyboard a stunning desk centerpiece.
What's Required:
- A PCB with support for an underglow LED strip.
- A keyboard case that is designed to show off underglow. This usually means the case has a translucent or frosted acrylic bottom or mid-layer that allows the light to escape and diffuse nicely. A solid metal or plastic case will completely block the underglow effect.

Practical Considerations: Maintenance, Visibility, and Power

Beyond looks, think about day-to-day use and constraints:

Cleaning: Per-key lighting sits under caps and stays fairly clean; underglow can highlight dust under the case or on the desk surface. Frosted bottoms help hide particles.
Glare and eye comfort: Very bright per-key LEDs can cause haloing on glossy keycaps. Underglow can create desk glare on shiny surfaces—use lower brightness or a mousepad edge to diffuse.
Power budget: Driving per-key RGB on 60–100+ LEDs taxes the USB current more than a modest underglow strip. Many boards cap global brightness to stay under 500 mA.
Latency and effects: On older MCUs (like ATmega32U4), heavy RGB effects can slightly reduce scan rate if not tuned. Newer ARM/RP2040 controllers handle complex animations more gracefully.

Which One Suits Your Use Case?

Consider these quick heuristics:

Gaming in the dark: Per-key with reactive effects helps highlight important keys without flooding your field of view.
Minimalist desk aesthetic: Underglow at low brightness provides ambience without busy legends.
Office/Shared spaces: Static white per-key at low brightness is functional and subtle; avoid flashy waves.
Photography/showpiece builds: Combine both with a frosted bottom and “pudding” caps for dramatic looks.

Build Complexity and Compatibility

Per-key through-hole: Most work to install, great for retro boards. Watch LED clearance under tight keycaps.
Per-key SMD (pre-soldered): Easiest; just pick compatible switches and caps.
Underglow: Usually pre-soldered and plug-and-play, but only shines with the right case materials and stand-offs that don’t block light.

Troubleshooting at a Glance

Per-key LED not lighting: Check polarity for through-hole, reflow joints on SMD pads, and test with a known-good LED.
Color mismatch on one key: A single channel (R/G/B) may be open; inspect that LED’s pad and the series resistor/driver pin where applicable.
Underglow segments dead: On addressable strips, check the first non-working LED—data flows one direction. Bad LED can break downstream data.
Flicker when typing: Lower brightness cap in firmware; heavy current draw can cause voltage droop on some USB ports/cables.

The Best of Both Worlds

You don't have to choose! Many modern PCBs support both per-key backlighting and underglow, and cases are often designed to take advantage of both. This allows you to have, for example, a functional white per-key backlight for typing, combined with a vibrant purple underglow for a pop of color.

Now that you know the styles, let's look at the practical steps for How to Add Backlighting to your build.

Quick Comparison Summary

Visual focus: Per-key highlights legends; underglow highlights case/desk.
Hardware needs: Per-key depends on switch/cap transparency; underglow depends on case diffusion.
Power impact: Per-key uses more current overall; underglow typically fewer LEDs.
Effort: Through-hole per-key is highest effort; pre-soldered SMD and underglow are low effort.