Lighting fundamentals connect source output, light on the work plane, color quality, and visual comfort. Lumens describe source output, lux and footcandles describe light on a work plane, CRI describes color rendering, and color temperature describes appearance. Use 1 fc = 10.76 lux when moving between U.S. footcandle targets and metric lux references.
For a worked room layout, use the Lighting Design Calculator. For fixture count, coefficient of utilization, light loss factor, and lighting power density workflow, continue to the Lighting Calculations Guide.
Physics of Light
Electromagnetic Spectrum
Visible Light:
- Wavelength range: 380-780 nanometers
- Small portion of electromagnetic spectrum
- Human eye sensitivity varies with wavelength
- Color perception based on wavelength
Spectral Sensitivity:
- Peak sensitivity: 555 nm (green-yellow)
- Photopic vision: Daylight conditions
- Scotopic vision: Low-light conditions
- Mesopic vision: Intermediate conditions
Color Temperature:
- Measured in Kelvin (K)
- Warm light: 2700-3000K
- Neutral light: 3500-4100K
- Cool light: 5000-6500K
- Daylight: 5500-6500K
Light Behavior
Reflection:
- Specular reflection: Mirror-like surfaces
- Diffuse reflection: Matte surfaces
- Mixed reflection: Most real surfaces
- Reflectance values: 0-100%
Transmission:
- Direct transmission: Clear materials
- Diffuse transmission: Translucent materials
- Selective transmission: Colored filters
- Transmittance values: 0-100%
Absorption:
- Energy conversion to heat
- Complementary to reflection and transmission
- Absorptance + Reflectance + Transmittance = 100%
Photometric Units and Measurements
Luminous Flux (Lumens)
Definition: Total amount of visible light emitted by a source in all directions.
Symbol: Φ (phi) Unit: Lumen (lm)
Typical Values:
- 100W incandescent: 1600 lm
- 23W CFL: 1600 lm
- 16W LED: 1600 lm
- Candle: 12 lm
Luminous Intensity (Candela)
Definition: Luminous flux per unit solid angle in a specific direction.
Formula: I = Φ / Ω Symbol: I Unit: Candela (cd)
Applications:
- Directional lighting design
- Beam angle calculations
- Glare analysis
- Photometric data
Illuminance (Lux/Footcandles)
Definition: Luminous flux incident on a surface per unit area.
Formula: E = Φ / A Symbol: E Units:
- Lux (lm/m²)
- Footcandle (lm/ft²)
- Conversion: 1 fc = 10.76 lux
Typical Illuminance Levels:
- Sunlight: 100,000 lux
- Office work: 300-500 lux
- Reading: 200-300 lux
- Moonlight: 0.1 lux
Luminance (Nits)
Definition: Luminous intensity per unit projected area of a surface.
Formula: L = I / (A × cos θ) Symbol: L Units:
- Candela per square meter (cd/m²)
- Nit (cd/m²)
- Footlambert (cd/ft²)
Applications:
- Display brightness
- Glare evaluation
- Contrast calculations
- Visual comfort assessment
Light Sources
Incandescent Lamps
Operating Principle:
- Electric current heats tungsten filament
- Thermal radiation produces light
- Continuous spectrum
- Warm color temperature (2700K)
Characteristics:
- Efficacy: 10-20 lm/W
- Life: 750-2000 hours
- Excellent color rendering
- Instant on/off
- Dimmable
Applications:
- Residential lighting
- Decorative applications
- Specialty lighting
- Heat applications
Fluorescent Lamps
Operating Principle:
- Electric discharge in mercury vapor
- UV radiation excites phosphor coating
- Phosphor converts UV to visible light
- Requires ballast for operation
Types:
- T12: 1.5" diameter (being phased out)
- T8: 1" diameter (common)
- T5: 5/8" diameter (high performance)
- Compact fluorescent (CFL)
Characteristics:
- Efficacy: 50-100 lm/W
- Life: 10,000-20,000 hours
- Various color temperatures
- Good color rendering (CRI 70-85)
- Requires warm-up time
High-Intensity Discharge (HID)
Types:
- Mercury Vapor: Older technology, poor efficacy
- Metal Halide: Good color rendering, high efficacy
- High-Pressure Sodium: Very high efficacy, poor color
- Low-Pressure Sodium: Highest efficacy, monochromatic
Characteristics:
- High efficacy: 50-150 lm/W
- Long life: 10,000-24,000 hours
- High lumen output
- Requires ballast and warm-up time
- Restrike delay after power interruption
Applications:
- Street lighting
- Industrial facilities
- Sports facilities
- Large area lighting
Light-Emitting Diodes (LEDs)
Operating Principle:
- Electroluminescence in semiconductor
- Direct conversion of electricity to light
- Monochromatic emission
- White light through phosphor conversion
Characteristics:
- High efficacy: 80-200+ lm/W
- Long life: 25,000-100,000 hours
- Instant on/off
- Excellent dimming capability
- Directional light output
Advantages:
- Energy efficiency
- Long life
- Durability
- Environmental benefits
- Design flexibility
Visual Perception and Comfort
Human Visual System
Eye Anatomy:
- Cornea: Light focusing
- Pupil: Light control
- Lens: Fine focusing
- Retina: Light detection
- Optic nerve: Signal transmission
Photoreceptors:
- Rods: Low-light vision, no color
- Cones: Color vision, high acuity
- Distribution: Cones concentrated in fovea
Visual Adaptation:
- Light adaptation: Bright to dim
- Dark adaptation: Dim to bright
- Chromatic adaptation: Color temperature changes
Visual Comfort Factors
Illuminance Levels:
- Task requirements
- Age considerations
- Contrast needs
- Duration of task
Uniformity:
- Even light distribution
- Avoid extreme variations
- Typical ratios: 3:1 maximum
- Gradual transitions preferred
Glare Control:
- Direct Glare: From light sources
- Reflected Glare: From surfaces
- Disability Glare: Reduces visibility
- Discomfort Glare: Causes discomfort
Color Rendering
Color Rendering Index (CRI):
- Measures color accuracy
- Scale: 0-100
- CRI 80+: Good color rendering
- CRI 90+: Excellent color rendering
Color Temperature Effects:
- Warm light: Relaxing, intimate
- Cool light: Alerting, productive
- Circadian rhythm impacts
- Psychological effects
Lighting Quality Metrics
Illuminance Uniformity
Average to Minimum Ratio: Uniformity = E_average / E_minimum
Typical Requirements:
- Offices: 0.7 minimum
- Industrial: 0.5 minimum
- Sports: 0.5-0.7 minimum
Glare Evaluation
Unified Glare Rating (UGR): International standard for discomfort glare evaluation.
Visual Comfort Probability (VCP): Percentage of people finding lighting comfortable.
Typical Limits:
- Offices: UGR < 19
- Classrooms: UGR < 16
- Precision tasks: UGR < 13
Contrast and Visibility
Luminance Contrast: C = (L_object - L_background) / L_background
Visibility Level: Combines illuminance, contrast, and size effects.
Design Guidelines:
- Positive contrast preferred
- Minimum contrast ratios
- Task-specific requirements
Basic Lighting Calculations
Point Source Illumination
Inverse Square Law: E = I / d²
Where:
- E = Illuminance (lux or fc)
- I = Luminous intensity (cd)
- d = Distance (m or ft)
Example Calculation: 1000 cd source at 2m distance: E = 1000 / 2² = 250 lux
Cosine Law
Angular Illumination: E = (I × cos θ) / d²
Where θ is angle from normal to surface.
Example: Same source at 30° angle: E = (1000 × cos 30°) / 2² = 217 lux
Lumen Method
Average Illuminance: E_avg = (Φ × CU × LLF) / A
Where:
- Φ = Total lamp lumens
- CU = Coefficient of utilization
- LLF = Light loss factor
- A = Area
Lighting Design Principles
Layered Lighting
Ambient Lighting:
- General illumination
- Overall visibility
- Uniform distribution
- Base lighting level
Task Lighting:
- Specific work areas
- Higher illuminance
- Reduced shadows
- Adjustable positioning
Accent Lighting:
- Highlighting features
- Creating interest
- Directional lighting
- Decorative purposes
Design Process
Requirements Analysis:
- Space function
- User needs
- Visual tasks
- Aesthetic goals
Preliminary Design:
- Lighting concept
- System selection
- Layout planning
- Control strategy
Detailed Design:
- Calculations
- Specification
- Documentation
- Cost analysis
Energy Considerations
Lighting Power Density
Definition: Installed lighting power per unit area.
Units: W/m² or W/ft²
Energy Codes:
- ASHRAE 90.1
- IECC
- Local energy codes
- Green building standards
Efficacy Comparisons
System Efficacy: Total system lumens / Total system watts
Includes:
- Lamp efficacy
- Ballast/driver losses
- Fixture efficiency
- Control losses
Example Comparison:
- Incandescent system: 15 lm/W
- Fluorescent system: 75 lm/W
- LED system: 100 lm/W
Controls and Automation
Manual Controls
Switches:
- On/off control
- Multi-level switching
- Dimming controls
- Scene controls
Applications:
- Residential lighting
- Small commercial spaces
- Simple requirements
- Cost-sensitive projects
Automatic Controls
Occupancy Sensors:
- PIR (passive infrared)
- Ultrasonic
- Dual technology
- Daylight integration
Daylight Harvesting:
- Photosensors
- Continuous dimming
- Stepped dimming
- Energy savings
Time Scheduling:
- Astronomical time clocks
- Programmable schedules
- Holiday schedules
- Override capabilities
Maintenance and Life Cycle
Lamp Life
Rated Life:
- Average life expectancy
- Laboratory test conditions
- 50% survival point
- Varies by lamp type
Factors Affecting Life:
- Operating hours
- Switching frequency
- Ambient temperature
- Voltage variations
Light Loss Factors
Lamp Lumen Depreciation (LLD):
- Gradual lumen output reduction
- Varies by lamp type
- Typical values: 0.85-0.95
Luminaire Dirt Depreciation (LDD):
- Dust accumulation effects
- Depends on environment
- Maintenance schedule
- Typical values: 0.80-0.95
Total Light Loss Factor: LLF = LLD × LDD × other factors
Future Lighting Technologies
Advanced LED Systems
Tunable White:
- Variable color temperature
- Circadian lighting
- Dynamic control
- Health benefits
Color-Changing LEDs:
- RGB systems
- RGBW systems
- Entertainment applications
- Architectural lighting
Smart Lighting
IoT Integration:
- Wireless connectivity
- Cloud-based control
- Data analytics
- Predictive maintenance
Human-Centric Lighting:
- Circadian rhythm support
- Biometric feedback
- Personalized lighting
- Health optimization
Summary
Lighting fundamentals provide the foundation for effective lighting design:
- Light Physics: Understanding electromagnetic spectrum and light behavior
- Photometric Units: Lumens, lux, candela, and luminance measurements
- Light Sources: Characteristics and applications of different lamp types
- Visual Comfort: Factors affecting human visual perception and comfort
- Quality Metrics: Uniformity, glare, and color rendering evaluation
- Basic Calculations: Illuminance calculations and design methods
- Future Technologies: Smart lighting and human-centric applications
Understanding lighting fundamentals enables effective lighting system design and energy-efficient solutions.
Next Steps
Continue your lighting design education with these related topics:
- LED Technology and Applications: Learn advanced LED systems and design
- Lighting Calculations: Master detailed illumination calculations and methods with the Lighting Calculations Guide
- Lighting Design Calculator: Estimate fixture count and average illuminance with the Lighting Design Calculator
- Lighting Controls and Automation: Understand advanced control systems
- Energy Efficient Lighting Design: Learn energy optimization strategies
Lighting Fundamentals FAQ
What is the difference between lumens and lux?
Lumens measure how much visible light a source emits. Lux measures how much of that light reaches a surface, so it changes with fixture output, distance, beam spread, room reflectance, and mounting height.
How do footcandles convert to lux?
Use 1 fc = 10.76 lux. A 30 fc office target is about 323 lux, while a 50 fc task target is about 538 lux.
Where should lighting fundamentals hand off to calculations?
Use this guide to define terms such as lumens, lux, footcandles, CRI, and color temperature. Then use the Lighting Calculations Guide or Lighting Design Calculator when you need fixture count, maintained illuminance, CU, LLF, and lighting power density.
Mastering lighting fundamentals is essential for all lighting design and energy management applications.