Luminous Flux Converter
Convert lumens (lm), candela steradian (cd·sr), and spherical candlepower (scp).
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Relative Value
*Diagram shows values relative to the selected base unit (Lumen).
Unit Information
What are Lumens (lm)?
The lumen (symbol: lm) is the SI derived unit of luminous flux. It quantifies the total amount of visible light emitted by a source, weighted according to the human eye's sensitivity to different wavelengths (the luminosity function). A higher lumen rating means a brighter light source. Light bulb packaging, for instance, prominently displays lumen values.
What is Candela Steradian (cd·sr)?
Candela steradian (cd·sr) is dimensionally equivalent to the lumen. The candela (cd) is the SI unit of luminous intensity (light emitted in a particular direction), and the steradian (sr) is the SI unit of solid angle. Luminous flux is defined as luminous intensity per unit solid angle, so 1 lumen = 1 candela × 1 steradian. This unit emphasizes the relationship between the total light output and its directional intensity.
What is Spherical Candlepower (scp)?
Spherical Candlepower is an older unit of luminous intensity. It represents the intensity of a theoretical point source that radiates light uniformly in all directions (isotropically). The total luminous flux (in lumens) from such a source is 4π times its luminous intensity (in candelas). Therefore, 1 scp corresponds to approximately 12.57 lumens.
Formulas
1 Lumen (lm) = 1 Candela Steradian (cd·sr)
This direct equivalence arises from the definition of luminous flux.
1 scp = 4π lm ≈ 12.57 lm
Spherical candlepower (scp) is an older unit, where 1 scp is the luminous intensity of a source that emits 1 candela uniformly in all directions; its total flux is 4π lumens.
Lumens = Lux × Area (m²)
The total luminous flux falling on a surface is the illuminance (in lux) multiplied by the area of the surface.
Key Reference Points
- A standard 60W incandescent bulb (older type): ~800 lumens.
- An equivalent LED bulb (using ~9-10W): ~800 lumens.
- A typical office projector: ~2000-4000 lumens.
- A candle flame: ~12-13 lumens (highly dependent on the candle).
- A high-powered flashlight: Can be 1000 lumens or more.
- A standard street light: 5,000 - 15,000 lumens.
- A stadium light fixture: 100,000+ lumens.
- The Sun (total luminous flux): Approximately 3.8 × 10²⁸ lumens.
- A 32-inch television at full brightness: ~200-400 lumens (total output).
- A typical car headlight (low beam): ~700-800 lumens per bulb.
Did You Know?
Historically, light bulb brightness was often associated with wattage (power consumption). However, with the advent of more efficient technologies like LEDs and CFLs, wattage is no longer a reliable indicator of brightness. Lumens directly measure light output, making it the standard for comparing brightness across different bulb types. A higher luminous efficacy (lumens per watt, lm/W) indicates a more energy-efficient light source.
The lumen is based on the photopic luminosity function, which models the human eye's response to light under well-lit conditions. There's also a scotopic luminosity function for low-light (night vision) conditions.
An integrating sphere is a device used in photometry to measure the total luminous flux of a light source. It has a highly reflective interior coating to ensure light is evenly diffused before measurement.
This is a phenomenon where the human eye's peak sensitivity shifts towards the blue end of the spectrum at low light levels. This means that red objects may appear darker relative to blue objects in dim light, even if they have similar brightness in full light.
Luminous flux (lumens) only tells you the quantity of light, not the quality. The Color Rendering Index (CRI) measures how accurately a light source reveals the colors of objects compared to a natural light source. A high CRI is important for tasks where color accuracy matters.
A standard wax candle emits approximately 12-13 lumens. This provides a tangible, if somewhat variable, reference for the lumen unit.
The American National Standards Institute (ANSI) has a standardized procedure for measuring projector brightness, resulting in 'ANSI lumens'. This provides a more consistent and reliable measure than a simple lumen rating, as it averages measurements taken at several points on the projected image.
A typical incandescent bulb has an efficacy of about 15 lumens/watt. A fluorescent lamp might be 60-100 lumens/watt, while modern white LEDs can exceed 150 lumens/watt, demonstrating their superior energy efficiency.
The talbot is a non-SI unit of luminous energy, equal to one lumen-second. It's used in contexts like measuring the total light energy from a camera flash.
While standard lumens are photopic (for day vision), scotopic lumens measure light based on the eye's night vision sensitivity (which peaks at a different, bluer wavelength). A light source can have different photopic and scotopic lumen values.
Specific wavelengths of light, particularly in the UV-C range, are not visible to the human eye (and thus have zero lumens) but are very effective at killing bacteria and viruses by damaging their DNA. This is a case where lumens are irrelevant.
For horticulture, lumens are a poor measure of a grow light's effectiveness. Instead, growers use metrics like PAR (Photosynthetically Active Radiation), which measures the light spectrum that plants actually use for photosynthesis.
The 'Sky Beam' at the Luxor Hotel in Las Vegas is one of the brightest man-made lights on Earth. It consists of 39 xenon lamps with a combined output of billions of candela and an immense luminous flux, reportedly visible to aircraft from hundreds of miles away.
A 'young lumen' is a humorous, informal term referring to the fact that manufacturers' lumen ratings for new bulbs are often based on initial output. The actual light output can decrease over the bulb's lifespan.
Inefficient outdoor lighting that directs lumens upward into the sky contributes to light pollution, which wastes energy and disrupts ecosystems and astronomical observations. 'Full-cutoff' fixtures are designed to direct all their lumens downward.
The brightness (luminous flux) of a flame depends on its temperature and the presence of soot particles. A clean-burning blue flame from natural gas has very low luminosity, while a sooty yellow flame from a candle is much brighter.
CCT, measured in Kelvin, describes the color appearance of a white light source ('warm' vs. 'cool'). Two bulbs can have the same lumen output but different CCTs, creating very different ambiances.
Luminous flux (lumens) measures the perceived power of light to the human eye. Radiant flux (watts) measures the total power of all electromagnetic radiation emitted, including invisible infrared and ultraviolet. Luminous efficacy (lm/W) is the ratio of these two quantities.