Illumination and Photometry: Comprehensive Glossary

Luminous Flux (Φ)

Luminous flux (Φ) is the fundamental measure of the total visible light emitted by a source per unit time, weighted to match the sensitivity of the human eye. The unit is the lumen (lm). Unlike radiant flux, which considers all electromagnetic energy, luminous flux only counts light that contributes to human vision (using the photopic response curve V(λ)). For white light sources, it is calculated by integrating the spectral power distribution, weighted by the eye’s sensitivity:

Φ = 683 lm/W × ∫ P(λ) × V(λ) dλ

• Measurement: Integrating spheres are used to capture and integrate light from all directions, ensuring accurate total flux measurements.
• Importance: Luminous flux is reported on lighting products and is key for comparing lamp outputs—the higher the lumens, the more visible light is produced.

Illuminance (E)

Illuminance (E) measures the amount of luminous flux incident on a unit area, indicating how much light falls onto a surface. It is expressed in lux (lx), where 1 lx = 1 lm/m².

• Formula: E = Φ / A (where A is area in m²)
• For a point source: E = I / r² (I: luminous intensity, r: distance)
• Measurement: Luxmeters with cosine-corrected sensors ensure accurate readings regardless of light angle.
• Application: Illuminance is specified in standards for offices (300–500 lx), operating rooms (10,000+ lx), and more.

Luminance (L)

Luminance (L) expresses the brightness of a surface as perceived from a specific direction. Its unit is candelas per square meter (cd/m²) or “nits.” Luminance combines intensity and projected area, and is the photometric quantity most closely related to visual sensation.

• Formula: L = dI / (dA × cosθ)
• Law of conservation: Luminance remains constant along a line of sight in a lossless medium.
• Measurement: Luminance meters or imaging photometers evaluate emitted, reflected, or transmitted light from a surface.
• Use: Essential in evaluating display brightness, road visibility, glare, and ergonomic lighting.

Luminous Intensity (I)

Luminous intensity (I) quantifies how much luminous flux is emitted in a particular direction per unit solid angle. Its SI unit is the candela (cd), where 1 cd = 1 lm/sr (steradian).

• Formula: I = Φ / Ω
• Use: Directional sources—spotlights, headlights, and signaling devices—are specified by luminous intensity.
• Measurement: Goniophotometers map intensity as a function of angle, producing beam pattern diagrams.

Solid Angle (Steradian, sr)

A solid angle (steradian, sr) is the 3D equivalent of a planar angle, defining how large an object appears from a point. The total solid angle around a point is 4π sr. One steradian is the angle subtending an area equal to the square of the radius on a sphere.

• Formula: Ω = A / r²
• Role: Essential for defining luminous intensity and for photometric calculations.

Luminous Efficacy (η)

Luminous efficacy (η) measures how efficiently a light source converts energy into visible light, in lumens per watt (lm/W).

• Formula: η = Φ / P
• Maximum theoretical: 683 lm/W at 555 nm (most sensitive for the human eye)
• Typical values: Incandescent: 10–17 lm/W, CFL: 40–70 lm/W, LEDs: 100–200+ lm/W
• Significance: Higher efficacy means more light for less energy, a key metric for sustainability.

Illuminance vs. Luminance

Illuminance and luminance are often confused but serve distinct roles:

• Illuminance: Amount of light arriving on a surface (lux).
• Luminance: Apparent brightness of a surface (cd/m²), considering emission or reflection.

A white and black surface under the same illuminance will have different luminance due to different reflectances.

Luxmeter

A luxmeter measures illuminance (lux) at a point using a sensor matched to the human eye’s spectral response. Used by lighting professionals to verify compliance and optimize lighting in workplaces, schools, and public spaces.

• Best practice: Place the sensor at the working plane, ensure no shading, and take multiple readings for uniformity.
• Advanced features: Data logging, cosine correction, and color sensitivity adjustments.

Integrating Sphere

An integrating sphere is a hollow sphere with a diffuse, reflective interior, used to measure total luminous flux from a light source. The sphere scatters light uniformly, and a photodetector measures the integrated output.

• Features: Multiple ports, internal baffles, and calibration against standards.
• Use: Laboratory-grade measurement of lamps, LEDs, and luminaires.

Goniophotometer

A goniophotometer measures the angular distribution of luminous intensity. By rotating the light source and recording intensity at different angles, it produces a polar plot of the beam pattern.

• Application: Vital for certifying and designing spotlights, headlights, and luminaires.
• Feature: Automated measurements, spectral/color sensors, and compatibility with large fixtures.

Luminance Meter

A luminance meter measures surface luminance (cd/m²) from a given angle, using a lens and field stop to define the area. Imaging photometers provide luminance maps for entire scenes.

• Use cases: Display brightness, road lighting, glare assessment, and ergonomic compliance.

Photometric Standards and Regulations

Lighting measurement and design are governed by standards to ensure safe, comfortable, and efficient environments:

• EN 12464 (Europe): Workplace lighting—minimum illuminance, glare, uniformity.
• IESNA Lighting Handbook (USA): Guidelines for diverse lighting applications.
• ISO 8995: International standard for workplace lighting.

Compliance ensures proper lighting for safety, productivity, and comfort, and is enforced through regular measurement and verification.

Application Examples

Office Lighting Calculation

For a 30 m² office needing 500 lux:

Φ = E × A = 500 lx × 30 m² = 15,000 lm

With 3,000 lm LED panels, at least five are required (not accounting for fixture loss).

Roadway Lighting Assessment

Road lighting standards require not only measuring pavement illuminance but also luminance from the driver’s viewpoint to ensure visibility and prevent glare. Uniformity and maximum/minimum luminance ratios are specified to avoid shadows and bright spots.

Display Technology

Display luminance (in cd/m²) ensures readability and comfort. Standard laptops: ~400 cd/m²; HDR monitors: 1,000+ cd/m².

Energy Efficiency Comparison

LEDs provide much greater visible light per watt, enhancing energy savings.

Conclusion

Understanding illumination and photometry is essential for designing, measuring, and regulating lighting in all environments. These foundational quantities and instruments ensure visual comfort, safety, and energy efficiency, underpinning modern lighting technology and standards.