Candela vs Lumens: Light Measurement Units
Light is measured in many ways: candela, lumens, lux, and foot-candles all describe brightness but in different contexts. Understanding these units (and how they interrelate) helps you pick the right light bulbs, flashlights, or headlamps. This comprehensive guide defines each term, compares them, provides real examples, includes conversion formulas and a comparison table, and gives tips on when to use which metric.
What Are Candela and Lumens?
Candela (cd) is the SI base unit of luminous intensity. It describes the brightness of a light source in a specific direction per unit solid angle. One candela is roughly equal to the light from a single common candle. In practical terms, a higher candela rating means a more concentrated beam: useful for spotlights, flashlights, and signaling lights. Candela remains the same even if part of the light is blocked, since it measures intensity in the unobscured direction.
Lumen (lm) is the SI derived unit of luminous flux, meaning the total amount of visible light emitted by a source. Lumens tell you how much light comes from a bulb in all directions. For example, an LED bulb might be rated for 800 lumens – indicating it outputs 800 units of visible light overall. Lumens are commonly used on light bulb and flashlight packaging to convey brightness. By definition, 1 lumen = 1 candela × 1 steradian. In other words, a 1-candela light emitting uniformly into a solid angle of 1 steradian produces 1 lumen of flux. Roughly speaking, one candle (at about 1 cd) emits around 12 lumens.
Key difference: Lumens measure the total light output of a source, regardless of direction. Candela measures directional intensity, answering “how bright is the beam if I look at it from a particular angle?”. For example, a floodlight and spotlight might both output 1000 lumens, but the spotlight could have far more candela in its narrow beam, making it shine brighter at distance, whereas the floodlight spreads its light widely with lower candela.
What Are Lux and Foot-Candles?
Lux (lx) is a unit of illuminance, measuring luminous flux per unit area. It tells you how bright a surface is lit. One lux equals one lumen spread over one square meter. For instance, if a 100-lumen lamp illuminates a 1 m² area, that surface has 100 lux. Move the lamp farther so it covers 4 m², and the illuminance drops to 25 lux (100 lm/4 m²). Lux is important for setting proper lighting levels: office lighting is typically around 400–500 lux, moonlight is ~1 lux, and bright sunlight can exceed 100,000 lux.
Foot-Candle (fc) is the imperial equivalent of lux (used mainly in the US). It is defined as one lumen per square foot. One foot-candle ≈ 10.764 lux (so for rough estimates one fc ≈ 10 lux). In lighting design, foot-candles are often cited by architects and photographers. For example, typical indoor general lighting might be 5–40 fc (50–430 lux), whereas a task area (like a workshop bench) might require 70–90 fc. Since nearly all of the world uses lux, remember: 1 lux = 1 lumen/m² and 1 fc ≈ 10.76 lux.
Key Differences Between Candela and Lumens
Measurement Type: Lumens measure luminous flux (total light) while candela measure luminous intensity (flux per solid angle). This means lumens describe overall brightness, whereas candela describes beam strength. A light source with high lumens but broad distribution may have lower candela; conversely, a narrow-beam flashlight might have modest lumens but very high candela.
Lighting Purpose: Use lumens to choose bulbs for lighting up a room or area. Higher lumens mean more light everywhere. Use candela to choose lights where distance or focus matters. A flashlight or headlamp with high candela will project light farther, creating a bright hotspot in the distance.
Blocking/Beam Spread: If you block part of a light, the lumen output drops (because less total light comes out) but the candela (in the remaining beam direction) stays the same. Lumens count all light, so any obstruction reduces lumens. Candela only looks in one direction, so hiding half the light source doesn’t change its directional intensity.
Conversion Relationship: Direct conversion depends on beam angle. For a uniformly emitting point source: 1 cd spread into all directions (4π steradians) equals 4π lumens (~12.57 lm). In practice:
Formula: Φv (lm)=Iv (cd)×2π[1−cos(A2)]\Phi_{v}\ (\text{lm}) = I_{v}\ (\text{cd}) \times 2\pi [1-\cos(\tfrac{A}{2})] where AA is beam angle.
Thus, knowing a light’s candela and beam angle, you can compute lumens. But since most lights don’t distribute light uniformly, precise conversion requires detailed beam patterns.
Comparison Example: Consider a 1000 lm LED bulb. If it emits equally in all directions, its intensity is only ~80 cd (1000 lm / 12.57). If the same light is focused into a tight 10° beam, its candela jumps much higher. Conversely, a laser pointer has a tiny lumen output but an enormous candela in its narrow beam (its light is intense in one direction).
Real-World Examples of Light Units
Flashlights and Headlamps: Manufacturers list both lumens and candela. For a tactical flashlight, lumens indicate total brightness (how much overall light), and candela indicates beam throw (how far/bright the center spot is). For example, a searchlight might output 2000 lumens with 100,000 cd (long beam) versus a lantern with 2000 lumens but only 1000 cd (wide flood).
Room and Task Lighting: Home or office light bulbs are rated in lumens. A 60-watt incandescent (~800 lm) vs a 9W LED (~800 lm) give similar brightness. When designing a room, one might calculate lux: e.g. a 100 lm light spread over 1 m² yields 100 lux on the desk. Lighting guidelines often specify required lux (or foot-candles) for tasks, so you choose bulbs accordingly.
Automotive Lights: Headlight legal limits are specified in candela (intensity of beam) while bulb specs are often in lumens. For instance, U.S. regulations cap high-beam intensity to ~20,000–75,000 cd per lamp, but auto bulb ads typically tout lumen output (e.g., “6000 lm set”). This mismatch means you can’t directly tell legality from lumens alone. In practice, car designers engineer beam patterns (candelas) to meet safety standards while providing enough total light.
Photography and Filmmaking: Lux is critical. A photo studio might measure 300 lx on a subject for correct exposure. Flash meter readings (often in foot-candles) help set lighting. A bare lamp rated 1500 lm could produce vastly different lux on a subject depending on distance and reflectors.
Everyday Analogies: Think of lumens like gallons of water and candela like water pressure. 1000 lm (gallons) can pour into a large pool (flood beam) or a narrow tube (spot beam) – the total amount is the same, but the pressure (cd) in the narrow tube is much higher. Lux/foot-candle then tell you how quickly the pool fills (illumination of a surface).
When to Use Candela vs Lumens
General Lighting (Rooms, Outlets): Use lumens as your guide. If you’re lighting a living room or hallway, you want a certain number of lumens (light output) to achieve brightness. For example, a bedroom might need ~2000–3000 lumens total. Bulb packaging usually highlights lumens for this purpose.
Focused Lighting (Spotlights, Flashlights, Automotive): Use candela for tasks requiring distance or pinpoint focus. A spotlight used on stage or a flashlight for signaling should be evaluated by its candela (throw) because it tells you how bright the beam is far away. A higher candela means a more intense, far-reaching beam.
Task vs Ambient: For desk work or reading, consider lux/foot-candle – you care about how much light falls on the paper or screen. A table lamp might not need ultra-high candela if it’s close by; instead you ensure a certain lux level on your workspace (e.g. ~300–500 lux). Online calculators or apps can use lumens and distance to estimate lux.
Product Selection Tips:
LED Bulbs: Look at lumens and lumens-per-watt to compare brightness and efficiency. If lumens aren’t given, convert from watts (e.g. LED: ~100 lm/W). You rarely see candela for general bulbs.
Flashlights and Torches: Check both lumens (total output) and candela (beam intensity). A flashlight with 1000 lm and 50000 cd is very different from one with 1000 lm and 5000 cd – the first has a much tighter, long-distance beam.
Automotive Lights: Look for beam charts or candela graphs if available. As a rule, higher claimed lumens might mean more glare without useful throw unless optics focus it (candela). High candela is crucial for high beams, while wide-beam fog lights rely on lumens spread.
Choosing by Need: Start with application. If you need to light a large area (garage, plant shelf), prioritize lumens (or lux) so enough light covers the space. If you need to see far (trail, road, sign), prioritize candela to ensure a strong beam. Often the best lights strike a balance (e.g. floodlights with moderate candela + high lumens).
Conversion Insights and Relationships
Candela–Lumen Relationship: One candela corresponds to 1 lumen per steradian. Since a sphere has 4π steradians, a 1 cd point source (uniformly radiating) emits approximately 12.57 lumens. In general, luminous flux (Φ, in lumens) = luminous intensity (I, in cd) × solid angle of emission (Ω, in steradians).
For a beam with apex angle A:
Φ = I × 2π × [1 - cos(A / 2)]Example: A lamp emitting 590 cd in a 40° cone yields about 224 lumens.
Candela–Lux Relationship: Illuminance (E, in lux) from a point source is calculated as:
E = I / r²Where I is intensity in candela and r is distance in meters. For example, a 1000 cd source at 10 m distance gives:
1000 / (10 × 10) = 10 luxLumen–Lux Relationship: Lux is lumens per square meter. If a light emits 1000 lm and evenly lights a 10 m² area, that area receives 100 lux. Spread over 20 m², the illuminance drops to 50 lux.
Foot-Candle Conversions: 1 foot-candle (fc) = 1 lumen per square foot ≈ 10.76 lux. For quick estimates, 5 fc is roughly 50 lux. Building codes and photography guides often use foot-candles for specifying lighting levels.
Practical Conversions:
- Uniform assumption: 1 cd ≈ 12.57 lm (spherical) or ≈ 6.37 lm (hemispherical).
- Flashlight example: A flashlight rated 1000 lm and 20000 cd likely has a narrow beam. This implies a small effective beam angle, concentrating light for distance.
Summary: Candela and lumens measure different light properties. Lumens = total output; candela = directional intensity. Use appropriate conversions and formulas to understand how light will behave in a real-world setting.
| Unit | Symbol | Measures | Definition/Relation |
|---|---|---|---|
| Candela | cd | Luminous intensity (directional brightness) | 1 cd ≈ luminous intensity of one common candle. 1 cd = 1 lm/sr. |
| Lumen | lm | Luminous flux (total light output) | 1 lm = 1 cd × 1 sr. Indicates the total visible light emitted. |
| Lux | lx | Illuminance (lumens per m²) | 1 lx = 1 lm/m². Measures how much light falls on a surface. |
| Foot-Candle | fc | Illuminance (lumens per ft²) | 1 fc = 1 lm/ft² ≈ 10.764 lx. U.S. customary illuminance unit. |
This table summarizes how the units relate. Candela and lumen both derive from human eye response: candela is weighted per direction, lumen is weighted total. Lux and foot-candle translate that light to surface illumination (lux = metric; foot-candle = imperial)
Recommendations for Choosing Lighting Products
Read the Specs: For general lighting fixtures, prioritize lumens and lumens-per-watt. A higher lumen rating means brighter light. Compare lumens instead of watts (since LED vs. incandescent differ drastically in lumens per watt). For example, replacing a 60W incandescent (≈800 lm) with an LED also rated 800 lm ensures similar brightness.
Flashlights: Check both. If you need distance (camping, security), choose a flashlight with a high candela (throw rating). If you need area illumination (flood light for work), look for high lumens with a wide beam (lower candela, higher beam spread). The ANSI FL1 standard ensures lumens and candela are tested consistently.
Automotive Lights: Focus on beam intensity (candela) charts if available. For headlights, higher candela in the legal beam angles will light the road further ahead. Aftermarket bulbs often advertise lumens, but that alone can be misleading (a 6000 lm LED bulb could have a very short throw if not focused). Always check beam pattern or measured candela if possible.
Work and Task Lights: Consider lux requirements. For a workshop or kitchen, you might aim for ~300–500 lux on work surfaces. You can calculate needed lumens by area (lux = lumens/area). For instance, to achieve 500 lux over a 4 m² bench, you’d need ~2000 lumens reaching the surface (allowing for losses and inefficiency).
Garden and Security Lighting: Use candela for outdoor spotlights that must reach far (paths, signage). Use lumens for floodlights illuminating wide areas (yard, doorway). Also note beam angle: a narrow 15° beam concentrates lumens into higher candela, while a 60° beam spreads it thinly.
Comparing Products: Use the same unit for apples-to-apples. Don’t compare lumens of one light to candela of another. For example, two flashlights: one is rated at 1200 lumens/10000 cd and another at 800 lumens/20000 cd. The second has a stronger beam despite lower lumens. Compare lumens to lumens, candela to candela.
Promotional Tip: If shopping on a website or catalog, use filters/keywords: “lumens” to find brightness levels (e.g. “800 lm LED bulb”), and “candela” or “throw” for directional lights (e.g. “5000 cd flashlight”). Our Product Range offers filters by lumen output and beam intensity, so you can match lights to your needs.
Energy Efficiency: A related metric is lumens per watt (lm/W). A product with high lm/W gives more light for less power. This matters for energy savings but also for performance, especially in battery-powered lights.
Conclusion
Candela and lumens (along with lux and foot-candles) are essential terms in lighting. Lumens tell you how much total light a source emits – great for general brightness. Candela tells you how intense that light is in a certain direction – crucial for focused beams and throw. Lux and foot-candles translate lumens into brightness on surfaces. By understanding these units and their relationships (as summarized in the table and examples), you can make informed choices: select higher lumens for bright room lighting, higher candela for long-distance illumination, and appropriate lux/foot-candle levels for tasks. Use the conversion tips to compare products, and always match the unit to your application. With these insights, you’ll shine a light on exactly what you need.
Key Takeaways: Candela measures a light’s intensity (in one direction); lumens measure total light output. Lux and foot-candles measure illumination on a surface. We explain each unit, show how they relate, give real-world examples (flashlights, room lighting, car headlights), and offer tips for choosing lights based on these metrics.
