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Bottom Camera Exposure for OpenPnP: A Calibration Guide That Actually Explains Why

· 5 min read
PikkoBot Team
PikkoBot Team
Precision Robotics

The bottom camera is arguably the most important vision system on your desktop pick-and-place. It's what verifies that the component is correctly oriented on the nozzle before placement. Get the exposure wrong, and you're either blind to misaligned parts or drowning in false detections.

Most OpenPnP guides tell you to "adjust exposure until it looks right." That's not helpful. Here's what's actually happening and how to calibrate it properly.

Why the Bottom Camera Matters​

After the nozzle picks up a component, the bottom camera looks up at it from below. This serves three critical functions:

  1. Component verification: Confirms the right part is on the nozzle. If you picked up a capacitor instead of a resistor, the vision system catches it.
  2. Lead/pad alignment: Measures the component's position and orientation on the nozzle so OpenPnP can compensate during placement. A 0.5mm offset on the nozzle becomes a 0.5mm offset on the board.
  3. Fault detection: Catches bent leads, missing pads, or tombstoned components before they go down.

If your bottom camera exposure is wrong, all three functions degrade. Too dark and you can't see fine leads on QFN or TSSOP packages. Too bright and the vision pipeline gets confused by blown-out reflections on solder pads.

The Physics of Bottom Camera Exposure​

Bottom camera exposure is different from top camera exposure because of the lighting geometry. The bottom camera typically uses ring lighting or structured illumination from below, which creates specific challenges:

  • Metallic reflections: Component leads and pads are highly reflective. Overexposure creates blooming that obscures edges.
  • Dark substrates: The component body (black IC packages, dark capacitors) absorbs light, requiring higher exposure to see details.
  • Mixed contrast: A QFN package has both dark mold compound and bright exposed pad. The exposure needs to balance both.

The goal is an image where:

  • Component leads/pads are clearly visible with distinct edges
  • The component body is visible but not crushed to black
  • No regions are blown out to pure white

Step-by-Step Calibration​

1. Start with a known component​

Place a component you use frequently on the nozzle. A QFN-32 or SOIC-16 is ideal because it has fine leads that reveal exposure problems. Avoid starting with large passives—they're too forgiving.

2. Open the OpenPnP camera view​

Navigate to Machine → Cameras → Bottom Camera in OpenPnP. You should see the live feed.

3. Set exposure manually​

Don't start with auto-exposure. Set it to a middle value (typically 50-70% of range) and adjust from there.

Too dark indicators:

  • Component leads appear as faint gray lines
  • Vision pipeline reports low confidence scores
  • Edge detection fails on fine-pitch leads

Too bright indicators:

  • Leads and pads merge into bright blobs
  • The component outline is indistinct
  • Vision pipeline detects phantom features

4. Adjust in small increments​

Change exposure by 5-10% at a time. After each change:

  • Check that leads are visible as distinct lines against the background
  • Verify that the exposed pad (on QFN packages) is visible but not blown out
  • Run the vision pipeline's test mode if available

5. Lock down the settings​

Once the image looks correct, disable auto-exposure. You want consistent, repeatable results. Auto-exposure will cause problems when:

  • You switch between dark IC bodies and bright metallic shields
  • Ambient lighting changes (even slightly)
  • The component size changes significantly

Common Mistakes​

Using auto-exposure: The most common mistake. Auto-exposure adjusts per-frame based on average brightness, which means your calibration is never stable. Always use manual exposure for the bottom camera.

Ignoring lighting angle: If your ring light is uneven or partially blocked, exposure adjustments won't fix the problem. Check that all LEDs are firing and the light is centered.

Calibrating with the wrong component: A 1206 resistor is easy to see at almost any exposure. Calibrate with your most challenging component—the one with the finest pitch or lowest contrast.

Forgetting to recalibrate after hardware changes: If you change the bottom camera position, ring light, or nozzle type, recalibrate. Even small mechanical changes affect the image.

PikkoBot's Dual-Camera Advantage​

PikkoBot ships with both top and bottom cameras pre-calibrated and coordinated. The bottom camera uses a structured ring light system designed specifically for component verification:

  • Uniform illumination: 360-degree LED ring eliminates shadow artifacts
  • Adjustable intensity: Software-controlled brightness without physical access to the machine
  • Fixed geometry: Camera and light positions are mechanically constrained, so calibration is stable across sessions

The pre-calibrated setup means you spend less time wrestling with exposure and more time placing components. When you do need to recalibrate (typically when switching nozzle types), the process is straightforward because the mechanical foundation is solid.

The Bottom Line​

Bottom camera exposure calibration is one of those tasks that takes 10 minutes to do right and costs hours of debugging when done wrong. Take the time to calibrate properly with a challenging component, disable auto-exposure, and your vision pipeline will reward you with reliable, repeatable component verification.

Next steps: Once your bottom camera is calibrated, move on to top camera fiducial calibration for complete vision system setup.

For the full calibration workflow including top camera setup, nozzle concentric calibration, and test job execution, see the OpenPnP v4 Calibration Guide.

Looking for feeder setup? Check the Feeder Overview.