Equipment · Controlled Procedure
VC-SOP-0124 — Spectral Canopy Array Calibration & Imaging
The controlled procedure for the periodic calibration of the multispectral and hyperspectral Spectral Canopy Array imaging rigs, and for running an imaging pass whose radiometric and spectral output can be trusted by the PhytoSense Mesh and the plant-health analytics chain.
No imaging pass is published from an uncalibrated rig. Every Spectral Canopy Array run must be bracketed by a valid dark frame, a white-reference frame, and a wavelength check against the onboard reference target. If any reference frame fails tolerance (Section 6), the run is flagged provisional, withheld from the Mesh, and the rig is recalibrated before the data is trusted. A spectral index computed from an out-of-tolerance frame can mask early stress or a containment-relevant change.
1. Purpose
To define the controlled procedure for calibrating the Spectral Canopy Array rigs — radiometric, spectral (wavelength), and geometric — and for executing imaging passes over the greenhouse canopy. The objective is repeatable, traceable spectral data so that derived indices (vegetation, water, pigment, and stress indices) reflect true canopy state rather than instrument drift.
2. Scope
Applies to all overhead Spectral Canopy Array rigs (multispectral CA-M units and hyperspectral CA-H units) covering greenhouse grid zones G-01 through G-24 and the Phyto-Synthesis canopy bay. Covers calibration, imaging-run execution, and first-line data QC. Ingestion of imaging products into the PhytoSense Mesh and downstream analytics is governed by VC-SOP-0223. Excludes the Pollinator Drone Swarm's onboard cameras (governed by VC-SOP-0301) and any forensic imaging of containment specimens.
3. Definitions
| Term | Definition |
|---|---|
| Dark Frame | A capture with the shutter closed; characterizes sensor dark current and bias for subtraction. |
| White Reference | A capture of a calibrated near-Lambertian panel of known reflectance, used to normalize to reflectance. |
| Wavelength Check | Verification of band-center wavelengths against the onboard spectral reference target's known emission lines. |
| SNR | Signal-to-noise ratio of a band; below threshold the band is unusable for index computation. |
| Co-Registration | Geometric alignment of bands so a pixel maps to the same canopy point across the spectrum. |
| Provisional Run | An imaging pass withheld from the Mesh because a reference frame failed tolerance. |
4. Responsibilities
| Role | Responsibility |
|---|---|
| Imaging Engineer (Owner) | Owns the calibration schedule; signs off calibration validity; releases runs to the Mesh; owns this SOP. |
| Imaging Technician | Executes calibration frames and imaging passes; performs first-line QC against tolerances. |
| Mesh Integration Engineer | Confirms ingest compatibility and timebase alignment per VC-SOP-0223. |
| Plant Health Analyst | Consumes released indices; flags index anomalies back for re-imaging. |
| CHLORA | Schedules calibration windows; archives reference frames; blocks Mesh publication of any provisional run; trends band SNR over time. |
5. Equipment & Pre-Checks
- CA-M / CA-H rig powered and thermally stabilized (sensor at setpoint ±0.5 °C for ≥ 15 min)
- Calibrated white-reference panel, in-cert, clean and unmarked
- Onboard spectral reference target deployed and unobstructed
- Canopy illumination at the imaging setpoint from the climate plant per VC-SOP-0212
- Lens / window clean; no condensation; integration time set for the band plan
- Storage and timebase synced to the MDF before the run begins
6. Calibration Reference Targets & Spectral Bands
Each imaging pass is validated against the reference frames and band tolerances below. A band failing any tolerance is excluded from index computation and the run is flagged provisional.
| Band / Reference | Center / Target | Tolerance | Purpose |
|---|---|---|---|
| Blue | 470 nm | ±3 nm center · SNR ≥ 200:1 | Pigment / chlorophyll-b context. |
| Green | 550 nm | ±3 nm center · SNR ≥ 250:1 | Canopy greenness, pigment indices. |
| Red | 670 nm | ±3 nm center · SNR ≥ 250:1 | Chlorophyll absorption, NDVI red term. |
| Red-Edge | 720 nm | ±2 nm center · SNR ≥ 200:1 | Early stress; the most drift-sensitive band. |
| NIR | 840 nm | ±4 nm center · SNR ≥ 300:1 | Biomass, NDVI NIR term, water context. |
| Dark Frame | Shutter closed | Mean bias drift ≤ 1.5% | Dark-current / bias subtraction. |
| White Reference | 99% panel | Reflectance error ≤ 2% | Reflectance normalization. |
| Co-Registration | All bands | ≤ 0.5 px RMS | Cross-band pixel alignment. |
Red-Edge is the canary. The 720 nm band carries the tightest tolerance (±2 nm) because early canopy stress and several toxin-pathway shifts express there first. A red-edge band out of tolerance fails the whole run regardless of the other bands.
7. Procedure
7.1 Thermal & Geometric Setup
- Power the rig and allow the sensor to reach its setpoint ±0.5 °C and hold for ≥ 15 min before any frame.
- Deploy and inspect the white-reference panel and the onboard spectral reference target.
- Confirm canopy illumination is at the imaging setpoint and stable; defer the run if illumination is drifting.
7.2 Radiometric Calibration
- Capture a dark-frame series (shutter closed); verify mean bias drift ≤ 1.5% against the stored reference.
- Capture the white-reference frame; verify reflectance error ≤ 2% across all bands.
- Store both frames; CHLORA timestamps and links them to the upcoming run.
7.3 Spectral & Geometric Check
- Image the spectral reference target; confirm each band center is within its Section 6 tolerance.
- Verify per-band SNR meets threshold; exclude any band that fails from index computation.
- Verify cross-band co-registration ≤ 0.5 px RMS; re-run alignment if exceeded.
7.4 Imaging Pass Execution
- Run the canopy pass over the assigned zones at the planned integration time and overlap.
- Bracket the pass with a closing dark frame; compare to the opening frame to bound drift over the run.
- Apply dark subtraction and white normalization; compute the planned indices (NDVI, red-edge stress, pigment).
7.5 First-Line QC & Release
- Imaging Technician checks every Section 6 tolerance and the opening/closing dark-frame consistency.
- A clean run is signed by the Imaging Engineer and released to the Mesh per VC-SOP-0223; consult the live feeds via Monitoring.
- Any failed tolerance flags the run provisional; CHLORA blocks Mesh publication and a recalibration is scheduled.
7.6 Quality Checkpoints
- Sensor thermally stabilized before any frame
- Dark and white reference frames within tolerance and archived
- All band centers and SNR within Section 6 limits; red-edge specifically verified
- Co-registration ≤ 0.5 px RMS
- Run signed and released, or flagged provisional and withheld
8. Records
- Calibration frame set (dark, white, spectral target) per run (MDF, retained 3 years)
- Per-band tolerance and SNR QC sheet
- Imaging-pass metadata: zones, integration time, illumination, timebase
- Release sign-off or provisional flag with reason
- Band SNR trend exported to Monitoring
9. References
- VC-SOP-0223 — PhytoSense Mesh Calibration & Sync
- VC-SOP-0212 — Greenhouse Climate Control
- VC-SOP-0301 — Pollinator Drone Swarm Operations
VC-SOP-0125— Reference Panel Care & Recertification- Monitoring Systems · Operational Metrics
10. Revision History
| Rev | Date | Author | Summary |
|---|---|---|---|
| A | 2087-02-15 | Imaging Engineer | Initial issue; dark/white reference framing and five-band multispectral plan. |
| B | 2089-06-21 | Imaging Engineer | Hyperspectral CA-H units added; per-band SNR thresholds and co-registration limit set. |
| C | 2090-09-30 | Imaging Engineer | Red-edge tightened to ±2 nm; provisional-run withhold rule and opening/closing dark-frame bracket formalized. CBO approval. |