Lighting metrics: lumens, foot-candles, CRI, CCT, and efficacy

One-line orientation

Five metrics — lumens, foot-candles, CRI, CCT, and efficacy — are the language of lighting specification; understanding which metric describes the source vs. the space vs. the energy cost is the exam’s central lighting vocabulary test.

Key points

• Lumens: total light output of a lamp; a source property. A 60 W incandescent = ~800 lm; a 10 W LED = ~800 lm. Same lumens, very different watts.
• Foot-candles (fc): illuminance at a surface — lumens arriving per square foot of work plane. Measured with a light meter. Governs whether a space meets occupancy lighting requirements.
  • 1 fc = 1 lumen/ft²
  • 1 lux = 1 lumen/m² (metric equivalent; 1 fc ≈ 10.76 lux)
• CRI (Color Rendering Index): 0–100 scale comparing how a source renders a standardized set of test colors vs. a reference illuminant.
  • CRI ~100: incandescent / halogen (reference-quality)
  • CRI 85+: LED (good), Metal Halide (~85)
  • CRI 75–85: Fluorescent
  • CRI ~20: High-Pressure Sodium (very poor — colors shift to yellow-orange)
  • Design rule: retail, medical, art galleries → CRI ≥ 90; general commercial → CRI ≥ 80.
• CCT (Correlated Color Temperature): Kelvin scale indicating the “color appearance” of the light.
  • 2700–3000 K: warm white (residential, hospitality, ambiance)
  • 3500–4100 K: neutral white (offices, general commercial)
  • 5000–6500 K: cool / daylight white (medical, warehouses, some offices)
  • CCT does not correlate directly with CRI — a lamp can be warm (low CCT) and still have poor color rendering.
• Efficacy (lm/W): energy efficiency of a source.
  • Incandescent: ~20 lm/W (most energy lost as heat)
  • Halogen: ~15–25 lm/W (standard); ~25–35 lm/W (IRC/infrared-reflecting halogen)
  • Fluorescent: ~60–90 lm/W
  • HID (Metal Halide): ~80–100 lm/W
  • LED: ~90–120 lm/W (leading current standard)
• Lamp life: rated hours to 70% of initial lumens (L70 standard for LEDs). Not a hard failure point — LEDs dim gradually rather than burn out suddenly.
• Daylight Factor (DF): ratio of indoor natural light to outdoor daylight under overcast sky, expressed as a percentage.
  • < 2%: low daylight — artificial lighting required most of the time.
  • 2–5%: moderate — supplemental artificial lighting often needed.
  • Above 5%: high daylight availability — also check glare and unwanted heat gain or loss.
• Visible spectrum: about 380–750 nm. UV (< 380 nm) and IR (> 750 nm) are outside visible range.
  • UV comes from arc sources — fluorescent and HID (metal halide, mercury vapor) generate UV in the arc, which is why HID lamps need a protective outer envelope or lens. UV is what fades fabrics and artwork, so it drives lamp choice in museums and retail displays.
  • IR comes from hot filaments — incandescent and halogen radiate most of their energy as IR (heat) rather than light, which is why their efficacy is so low.
  • LEDs emit essentially neither, so they add little radiant heat or fading risk.

Confusions / comparison

Metric	What it describes	A property of	Unit	Higher = ?
Lumens	total light output of a source	the lamp	lm	brighter source
Foot-candles	illuminance on a surface	the space / work plane	fc (lm/ft²)	more light on surface
CRI	color accuracy vs. reference	the lamp	0–100	better color rendering
CCT	color appearance (warm vs. cool)	the lamp	Kelvin (K)	cooler / bluer light
Efficacy	lumens per watt of electricity	the lamp	lm/W	more efficient

Space type	Typical target fc	Notes
Egress paths, corridors	1–10 fc	minimum for safe passage
Lobbies, hallways	10–20 fc	ambient orientation
General offices, retail	30–50 fc	comfortable general tasks
Typical classrooms / general study	about 30–50 fc	detailed tasks may need more
Labs, medical, manufacturing	100+ fc	precision / critical tasks

Lighting metrics: what leaves, what arrives, and how the source behaves

Location separates lumens from foot-candles; separate scales distinguish CCT from CRI.

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Scroll horizontally to explore →

CCT is not CRI: a warm or cool source can still render colors well or poorly.

Related

→ Lighting: Lamp types (which lamp achieves which CRI/CCT/efficacy) · Illuminance & daylight (using fc levels and DF in design) · Lighting controls (maintaining target fc automatically) · Systems: HVAC (lighting heat load → cooling).