What Makes a Good Illumination Design?
A well-designed illumination system is defined not by image quality, but by how light is distributed, transferred, and perceived at the target. Unlike imaging optics, illumination design focuses on energy flow, uniformity, efficiency, and color control.
This article explains the core performance goals of illumination systems by introducing the key units of measure and evaluation targets commonly used in illumination design.
Units of Measure in Illumination Systems
Illumination performance is typically described using radiometric or photometric (photopic) quantities, depending on whether the system is optimized for physical energy or human visual perception.
| Quantity | Symbol | Radiometric Name | Units | Photometric Name | Units |
|---|---|---|---|---|---|
| Flux | Φ | Radiant Power | Watts (W) | Luminous Flux | Lumens (lm) |
| Flux / Area | E | Irradiance | W/m² | Illuminance | lux (lm/m²) |
| Flux / Solid Angle | I | Radiant Intensity | W/sr | Luminous Intensity | candela (cd) |
| Flux / Area·Solid Angle | L | Radiance | W/m²·sr | Luminance | cd/m² (nit) |
- Radiometric units describe absolute optical power
- Photometric units weight power by the human eye’s sensitivity
A schematic visualization of these quantities often helps clarify their physical meaning and relationships (e.g., flux vs intensity vs radiance). The figure is shown as below:
Source: PencilofRays.com
Key Performance Parameter 1: Uniformity
Uniformity describes how evenly light is distributed and is one of the most critical metrics in illumination design.
Types of Uniformity
Angular Uniformity
Uniformity over a solid angle
Common in projector optics, beam shaping, and LED collimation
Surface Uniformity
Uniformity over a target area
Critical in machine vision, flat-field illumination, microscopy, and display backlighting
Uniformity is often expressed as:
- Min / Max ratio
- RMS deviation
- Percent non-uniformity
Key Performance Parameter 2: Throughput and Efficiency
Throughput determines how much of the source light actually reaches the target.
Factors Affecting Throughput
- Étendue (Optical Invariant): Limits how much light can be transferred through an optical system
- Material Absorption: Losses in lenses, light guides, and diffusers
- Surface Reflectivity: Especially important for reflective integrators and mirrors
- Fresnel Reflections: Losses at each refractive interface due to index mismatch
High-performance illumination systems aim to preserve étendue while minimizing optical losses.
Key Performance Parameter 3: Color and Chromaticity
The perceived color of illumination is quantified using chromaticity, defined by the CIE (Commission Internationale de l’Éclairage).
CIE Color Representation
CIE 1931 Chromaticity Coordinates (x, y)
Based on standardized color matching functions
Used to define:
LED color bins
White point accuracy
Color uniformity across the field
Color performance is critical in:
- Display backlighting
- Biomedical illumination
- Inspection and metrology
- Architectural and automotive lighting
