- Technical Note / Measurement Methods
- Title: Irradiance Non-Uniformity: Measurement Setup Tips
- 1-line summary: Common pitfalls and a simple checklist for spatial uniformity measurements.
- Date: 2026-02-22
- Spectrum: General
- Level: Standard
- Category: Measurement Methods
- Related: Temporal Instability: What to Log and Why / (Example) Non-Uniformity Report Template
1. Purpose
Irradiance non-uniformity is easy to “measure,” but also easy to measure incorrectly due to setup errors.
This note lists practical setup tips so your map reflects the simulator field, not artifacts from:
- sensor geometry,
- positioning errors,
- wrong working distance,
- warm-up/temporal instability during scanning.
2. Definitions (quick)
2.1 Irradiance non-uniformity
Spatial variation of irradiance across the specified test plane / target area.
2.2 “Map” vs “single-point”
- Single-point reading is not enough; non-uniformity needs a defined grid (or scan path).
- A map is only valid if the plane, distance, and aperture/sensor condition are controlled.
Key point: Non-uniformity depends on (a) target area, (b) working distance, (c) sensor aperture and cosine response, (d) stabilization condition.
3. Common pitfalls (and the fix)
3.1 Typical pitfalls
- Measuring at an undefined distance (or mixing distances across runs).
- Sensor not normal to the plane → cosine error across positions.
- Using a sensor head that is too large relative to the spatial gradient → “smoothed” map.
- Scanning too slowly while the source is drifting → spatial map becomes a time-history artifact.
- Not controlling cable tension / operator hand pressure → sensor height changes point-to-point.
- Not specifying whether the map is for irradiance or effective irradiance (band-limited).
3.2 Fix: state these 5 setup items
Whenever you report temporal instability/drift, state:
- Target area / aperture: ______ (e.g., 150×150 mm, 200 mm dia)
- Working distance (reference plane): ______ mm
- Grid definition: ______ (e.g., 3×3 / 5×5 / step size)
- Sensor condition: model, active area, cosine diffuser (yes/no): ______
- Temporal control: warm-up and drift handling (time per point / total scan time): ______
4. What to say (recommended wording examples)
A) If you report a standard non-uniformity map:
- “Irradiance non-uniformity was evaluated on a × mm plane at ___ mm working distance using a × grid (step ___ mm). The sensor active area was ___ mm² with cosine diffuser ___. Total scan time was ___ min after warm-up of ___ min.”
B) If you used a fast method (to avoid drift):
- “A fast grid measurement (total ___ min) was used to minimize temporal drift influence; drift during scan was monitored separately.”
5. Quick checklist table
| Check | Why | Pass condition (example) |
| Fix working distance and reference plane | Non-uniformity changes with distance | Distance recorded and repeatable |
| Keep sensor normal to plane | Avoid cosine error | Tilt < ___° (or use jig) |
| Define grid and step size | Makes results comparable | Grid stated; symmetric around center |
| Control scan time vs drift | Spatial map should not become time trend | Total scan time << drift time scale |
| Confirm sensor aperture suitability | Avoid smoothing or aliasing | Active area appropriate for gradient |
| Mark center and orientation | Prevent map rotation errors | Center defined; X/Y axes defined |
6. Notes (SAN-EI recommendation)
- If temporal instability is non-negligible, treat non-uniformity measurement as a two-problem system:
(1) spatial map, (2) time drift during scan.
Log drift at center (or a fixed point) while scanning. - If the reader may compare across systems, always include the target area and working distance in the summary line