Lighting terms pop up on packaging all the time, but honestly, they can get confusing fast. Lumens, lux, and candelas all deal with light, but each one means something a little different. Lumens tell you the total amount of visible light, lux shows how much of that light actually lands on a surface, and candela measures the intensity of light in one direction.
If you get a handle on these units, it’s much easier to compare lighting options and pick what works for your space. A bulb with tons of lumens might not feel bright in a big room if the light spreads everywhere, while a narrow beam with fewer lumens can look much stronger thanks to a higher candela value. Lux ties both ideas together by showing how much light actually hits your walls, floors, or workbench.
Let’s break down these units step by step. Looking at luminous flux, illuminance, and intensity gives you a solid way to compare products, design better spaces, and just understand how light works in real life.
Fundamentals of Photometric Units
Photometric units describe how we measure light in ways that match how people actually see. They connect physical radiation with the eye’s sensitivity, so they’re essential for lighting design, engineering, and setting measurement standards.
What Are Photometric Quantities?
Photometric quantities measure light as humans see it, not just as raw energy. That’s what sets them apart from radiometric quantities, which track electromagnetic radiation without caring how it looks to us.
The main photometric quantities are:
- Luminous flux (lumens, lm): total visible light coming out of a source.
- Luminous intensity (candelas, cd): light output in a specific direction.
- Illuminance (lux, lx): light falling on a surface.
- Luminance (cd/m²): brightness of a surface as seen from an angle.
Each of these values uses weighting functions based on the human eye’s sensitivity curve, the luminosity function. That way, green light, which our eyes pick up best, counts more than red or violet light with the same energy.
Visible Light and the Electromagnetic Spectrum
Visible light covers just a tiny chunk of the electromagnetic spectrum, from about 380 nanometers (violet) to 780 nanometers (red). Anything outside that range is invisible to us.
Photometry only cares about this visible slice. Unlike radiometry, which measures every wavelength, photometry uses the CIE standard observer function to weight light by human vision. This makes the measurements line up with how we actually experience brightness.
Our eyes are most sensitive around 555 nm, right in the green-yellow zone. At that wavelength, a certain amount of radiant power looks brightest to us. This idea is at the heart of photometric units and gets locked in by groups like CIE and ISO.
SI Units in Lighting Technology
We use the International System of Units (SI) for photometric units, so everyone stays on the same page in science and industry. The base unit is the candela (cd), which measures luminous intensity in a certain direction.
From the candela, we get other units:
- Lumen (lm): 1 lm = 1 cd·sr (luminous flux).
- Lux (lx): 1 lx = 1 lm/m² (illuminance).
- Candela per square meter (cd/m²): luminance, or a nit.
These units form the backbone of modern lighting tech. Engineers use them to specify lamp brightness, compare displays, and design spaces with just the right amount of light. Standardization keeps measurements consistent no matter where or how you use them.
Lumens: Measuring Luminous Flux
Lumens tell you the total visible light a source puts out. It’s the main way to compare how much useful light different bulbs or fixtures deliver. The lumen rating on a bulb helps you figure out its brightness, and luminous efficacy ties that brightness to energy use.
Definition and Role of Lumens
A lumen (lm) is the SI unit for luminous flux, measuring the total visible light a source emits. Unlike radiant flux, which includes all electromagnetic radiation, luminous flux only counts wavelengths humans can see.
One lumen is the light that comes from a source of one candela within a solid angle of one steradian. So, lumens connect directly to how bright things look to us, not just the energy involved.
Lumens matter because they let people and engineers compare light sources on equal footing. A lamp with a higher lumen value kicks out more visible light, no matter how much power it uses.
For example, a candle gives off about 12 lumens, while a 100‑watt incandescent bulb puts out around 1,300–1,750 lumens. That’s a huge range, and it really shows how lumen values jump between old and new tech.
Lumen Ratings and Light Bulbs
Most bulbs come with a lumen rating that tells you their total visible light output. This number is usually more helpful than wattage, since wattage just measures energy use—not brightness.
Typical numbers look like this:
| Light Source | Approx. Lumens |
|---|---|
| Candle | ~12 lm |
| 40 W incandescent bulb | ~325 lm |
| 7 W LED bulb | ~450 lm |
| 18 W fluorescent tube | ~1,250 lm |
| 100 W incandescent bulb | ~1,750 lm |
| 100 W fluorescent lamp | ~8,000 lm |
Modern bulbs like LEDs or fluorescents put out way more lumens per watt than the old-school incandescents.
People usually pick bulbs based on lumen ratings to get the brightness they want for a room or task. Higher lumens mean more light, but efficiency depends on what kind of tech you’re using.
Luminous Efficacy and Efficiency
Luminous efficacy tells you how well a light source turns electrical power into visible light. You’ll see it listed as lumens per watt (lm/W). The higher the number, the more light you get for your energy.
A typical incandescent bulb only manages 10–17 lm/W, but modern LEDs often hit 80–120 lm/W or even higher. Fluorescents usually fall somewhere in the 60–90 lm/W range.
Luminous efficiency is related, but it compares luminous flux to the maximum possible flux for a certain radiant power. It shows how close a source gets to the best possible human visual sensitivity.
Knowing both efficacy and efficiency explains why two bulbs with the same lumen rating might use very different amounts of electricity. Efficient bulbs give you the same light with less power, which saves money over time.
Lux: Understanding Illuminance
Lux measures how much visible light hits a surface. It’s a good way to tell if a space is bright enough for what you want to do. Lux connects the raw output of a light source to how that light spreads across an area.
Definition and Calculation of Lux
Lux (lx) is the SI unit for illuminance. It describes how many lumens land on each square meter of surface. One lux equals one lumen per square meter (1 lx = 1 lm/m²).
This measurement cares about the light received, not just what’s emitted. For example, a lamp might put out 800 lumens, but the lux level depends on how far it is from the surface and how the light spreads out.
You can measure lux with a lux meter, which checks light intensity at a specific spot. Engineers and designers use these readings to see if lighting meets recommended standards.
Some places still use foot-candles instead, where 1 foot-candle is about 10.76 lux. That’s handy for industries sticking with imperial units.
Lux Versus Lumens
Lux and lumens are connected, but they’re not the same. Lumens measure the total light a source gives off, no matter where the light goes. Lux measures how much of that light actually lands on a certain area.
A 1000-lumen bulb can create very different lux levels depending on distance and beam angle. If the light spreads out, the lux drops. If it’s focused tightly, the lux goes up.
That’s why lighting specs often mention both lumens and lux. Lumens help you guess a lamp’s brightness, but lux shows how effective that brightness really is in a space.
If you mix up the two, you might end up with a high-lumen bulb that still doesn’t light up your work area enough.
Applications in Lighting Design
Lighting designers use lux values to make sure spaces meet visual needs. Offices usually aim for 300–500 lux for desks, warehouses need less, and surgical rooms might need over 1000 lux.
Designers follow recommended lux levels from professional standards to plan lighting. These guidelines help balance energy savings with comfort and safety.
Lux values also guide where you put fixtures, how far apart they go, and what beam angles to use. By modeling how light spreads across a room, designers can predict whether surfaces will be evenly lit or if shadows will pop up.
In outdoor spaces, lux helps set street lighting so roads stay safe without wasting energy. Sports stadiums, galleries, and classrooms all use lux-based rules to get lighting just right.
Candela: Luminous Intensity Explained
Candela measures how bright a light source looks in a specific direction. It connects the light’s power with how sensitive our eyes are and shows how the light is distributed, not just how much there is.
What Is Candela?
The candela (cd) is the SI base unit for luminous intensity. It tells you how much visible light a source emits in a certain direction, weighted for human brightness perception.
Lumens measure total light, but candelas focus on directional brightness. For reference, a candle flame has a luminous intensity of about 1 candela.
Candela is crucial because it lets you compare lights that don’t send light everywhere equally. A lamp might make thousands of lumens, but its candela value tells you how strong the light is in a specific direction.
You’ll see candela values given in millicandelas (mcd) for tiny LEDs or kilocandelas (kcd) for big lamps and projectors.
Solid Angle and Steradian
To define luminous intensity, we use a geometric thing called a solid angle. A solid angle shows how big something looks from a point, and its unit is the steradian (sr).
One steradian is the solid angle that covers an area on a sphere equal to the square of the sphere’s radius. A whole sphere has 4π steradians.
Candela comes out as lumens per steradian.
- 1 cd = 1 lumen per steradian (lm/sr)
That’s why two light sources with the same total lumens can have very different candela values, depending on how the light is aimed.
Candela in Focused Lighting
Candela really shines when you talk about focused lighting like spotlights, flashlights, or car headlights. These devices take a bunch of luminous flux and squeeze it into a narrow beam angle.
Say you have a bulb making 1700 lumens and it spreads light evenly in all directions. That works out to about 135 cd. But if you focus that same bulb into a 20° beam, you can hit tens of thousands of candelas.
That’s why candela ratings matter when direction is more important than total light. A high-candela spotlight can throw a bright, tight beam a long way, while a lamp with the same lumens but lower candela just spreads light everywhere.
By checking candela values, you can figure out how “intense” a light will look when aimed. It’s a key unit for lighting design and picking the right product.
Comparing Lumens, Lux, and Candelas
Each photometric unit tells you something different about light. Lumens measure total output, lux measures the light that actually lands on a surface, and candela measures intensity in a single direction. Knowing how they fit together makes it way easier to pick the right lighting for any job, whether it’s general room lighting or something more specific.
Key Differences and Relationships
Lumens (lm) show how much visible light a source gives off in every direction. If a lamp has more lumens, it simply puts out more light overall.
Lux (lx) tells you how much light hits a surface. One lux equals one lumen per square meter. This unit really matters when you want to know how bright a space feels—like for reading, working, or taking photos.
Candela (cd) is about light intensity aimed in a specific direction. Take a spotlight, for example. It might have fewer lumens than a floodlight, but because it focuses its light, it can have a higher candela.
These units are tied together. Lumens cover total light output, candela shows how focused that light is, and lux tells you how much of it actually lands on a spot. Designers mix and match all three when they plan lighting for homes, offices, or even outdoor spaces.
Conversion Formulas and Practical Examples
Distance and beam spread play a big part in how lumens, lux, and candela relate.
-
Lux formula:
[ \text{Lux} = \frac{\text{Lumens}}{\text{Area in m}^2} ] -
Candela to lumens:
[ \text{Lumens} = \text{Candela} \times \text{Steradians} ]
Picture a lamp that puts out 1000 lumens and spreads it over 10 m². That setup gives you 100 lux.
Now, say you have a spotlight with 500 candela in a 10° beam. It’ll look way brighter in that direction than a regular bulb with the same lumens spread out wide. That’s why candela matters for things like flashlights or stage lights, while lux is what you want to check for workspace lighting.
Choosing the Right Unit for Your Application
Different lighting goals call for different units.
- General lighting in your living room or office? You’ll want to look at lumens, since total output decides how well you can see.
- Task lighting—think desk lamps or surgical lights—leans on lux, because the brightness on your work surface is what matters.
- Directional lighting like spotlights, car headlights, or projectors? Candela is key, because you care about intensity in a tight beam.
When you compare lights, lumens tell you about efficiency, lux tells you how usable the light is in a real space, and candela shows how strong the beam feels. Picking the right one helps you avoid lighting that’s too weak or way too harsh.
Additional Photometric Concepts
There’s more to measuring light than just lumens, lux, and candela. Some concepts explain how light looks to our eyes, how well it shows colors, and how we measure it consistently.
Luminance and Nits
Luminance is about how bright a surface looks when it emits or reflects light. People measure it in candelas per square meter (cd/m²), but you’ll often hear the term nits. Luminance considers both the light output and the size of the light source.
A tiny but intense source, like a filament, can have crazy high luminance. On the other hand, a big panel with the same total light might look way dimmer. That’s why luminance matters for things like glare or how bright a screen seems.
For example:
| Quantity | Unit | Meaning |
|---|---|---|
| Luminance | cd/m² (nit) | Brightness impression from a surface |
High luminance usually means sharper, more dazzling light. Lower values give you softer, more gentle lighting. Designers use luminance to keep things comfortable and visible, whether it’s a lamp or a screen.
Color Rendering Index and Color Temperature
The Color Rendering Index (CRI) tells you how well a light source shows colors compared to sunlight. People rate it as Ra on a scale from 0 to 100. For most indoor spaces, a CRI over 80 works fine, but if you need really accurate colors, look for 90 or above.
Color temperature—measured in Kelvin (K)—describes how the white light looks. Lower values like 2700K feel warm and yellowish, while anything above 5000K looks cool and blue. You might also hear the term Correlated Color Temperature (CCT).
- Warm white: < 3300K
- Neutral white: 3300K – 5000K
- Cool white: > 5000K
Both CRI and CCT shape how we see a space. High CRI means colors look natural, and the right color temperature sets the mood and helps a room feel just right.
Photometry and Measurement Standards
Photometry deals with visible light. It’s not quite the same as radiometry, which covers the entire electromagnetic spectrum.
Photometry weights radiant flux using the luminosity function, which tries to capture how the human eye responds to different wavelengths.
Standards lay out how we should measure luminous flux, intensity, and illuminance. That way, everyone gets consistent results.
Labs set up controlled environments to test luminaires and lamps. They pay attention to geometry, reflections, and spectral distribution.
You’ll see a few common photometric units pop up:
- Lumen (lm): total visible light output
- Candela (cd): luminous intensity in a specific direction
- Lux (lx): illuminance on a surface
When manufacturers and researchers stick to the same measurement practices, you can actually compare light sources across different applications.