Light is measured in two broad families of units: radiometric units that quantify raw electromagnetic energy, and photometric units that weight that energy by how sensitive the human eye is to each wavelength. For everyday purposes, most people encounter three photometric units: the candela (luminous intensity), the lumen (total light output), and the lux (light falling on a surface). But the full picture includes a growing list of specialized metrics for everything from plant growth to screen brightness to the biological effects of light on your sleep cycle.
Two Families of Measurement
The reason light has so many different units traces back to a basic split in how scientists think about optical radiation. Radiometry measures all electromagnetic radiation in the optical spectrum, regardless of whether anyone can see it. Photometry narrows the focus to what the human eye actually perceives, weighting each wavelength by the eye’s sensitivity curve. A deep red or violet light might carry plenty of energy in radiometric terms but register as dim to your eye because photopic vision peaks in the yellow-green range around 555 nanometers.1NIST. Radiometry and Photometry: Review for Vision Optics
This distinction matters in practice. When you buy a light bulb, you care about photometric units because you want to know how bright the bulb looks to you. When an engineer designs a solar panel, radiometric units matter more because the panel absorbs energy the eye cannot see, including infrared. When a biologist studies how light affects plant growth, neither standard set of units does the job perfectly, which is why that field developed its own metric entirely.
The Candela
The candela is the SI base unit of luminous intensity, and it holds a unique distinction: it is the only base unit in the International System of Units that quantifies a biological process, namely human vision.2Metrologia. Photometry, radiometry and ‘the candela’: evolution in the classical and quantum world Think of luminous intensity as how concentrated a light source’s output is in a particular direction. A flashlight with a tight beam might have a high candela rating in the center of the beam, even if it emits relatively few total lumens.
The name comes from “candle,” which is no coincidence. Early photometric standards were literally based on specific types of candles and oil lamps. Over the centuries those standards evolved from candle and flame references to incandescent lamp standards, and eventually to the modern radiometric definition that ties the candela to a precisely specified amount of radiant power at a specific wavelength.3Lighting Research & Technology. The standardisation of light and photometry – A historical review Today the definition is anchored in physics rather than any physical artifact, but the practical meaning has stayed roughly the same: one candela is about the luminous intensity of a common wax candle.
Lumens and What They Tell You
Where the candela describes intensity in one direction, the lumen describes the total amount of visible light a source emits in all directions. When you shop for light bulbs, the lumen rating on the box is the most useful number for predicting how bright a room will be. A typical 60-watt-equivalent LED puts out around 800 lumens, while a powerful floodlight might produce tens of thousands.
Lumens are defined using the photopic luminous efficiency function, a standardized curve that describes how the average human eye responds to each wavelength of light under well-lit conditions. This function is used internationally to define light output from electric light sources and to regulate minimum luminous efficacy requirements, expressed as lumens per watt.4Lighting Research & Technology. A new rationale for setting light source luminous efficacy requirements Luminous efficacy is, in plain terms, how good a light source is at turning electrical power into visible light. Incandescent bulbs are notoriously wasteful, converting most of their energy into heat; LEDs convert a far larger share into lumens.
One subtlety worth knowing: lumens tell you total light output, not how that light is distributed. A bare bulb and a narrow-beam spotlight could have identical lumen ratings and produce completely different experiences in a room. That is why lighting designers rely on additional metrics like beam angle and candela diagrams rather than lumens alone.
Lux, or Light on a Surface
Lux measures illuminance, which is the amount of luminous flux arriving on a surface per unit area. One lux equals one lumen spread over one square meter. If lumens describe how much light leaves a source, lux describes how much light reaches you or your desk or a camera sensor. A sunny day outdoors might deliver 50,000 to 100,000 lux, a well-lit office around 300 to 500 lux, and a moonlit night well under one lux.
Lux follows a familiar physics pattern: the farther you move from a point source, the more the light spreads out, and the fewer lux land on any given area. For a simple point source, illuminance drops with the square of the distance. Move twice as far away, and you get roughly one-quarter the light. But that rule has limits. Researchers have demonstrated that for different source shapes, the falloff behaves differently: extended linear or planar sources can produce illuminance that decreases linearly or even stays roughly constant over a range of distances.5Journal of Physics: Conference Series. Light illuminance does not always decay with the inverse of the square of the distance: an open-inquiry laboratory This matters in workplaces with large overhead lighting panels, where illuminance is more even than the inverse-square rule would predict.
Nits and Screen Brightness
If you have ever compared phone displays or shopping for a television, you have probably seen the term “nit.” A nit is a unit of luminance, which describes how bright a surface appears when viewed from a particular angle. Technically, one nit equals one candela per square meter. Luminance is what your eye actually perceives when you look at a glowing screen, a sheet of paper reflecting daylight, or a sunlit wall.
In medical imaging, precise luminance measurements are critical. A study measuring the gray-scale performance of radiological workstations found luminance values ranging from 0.17 to 76 nits on one display and from 0.17 to roughly 25 nits on another.6PubMed. Evaluation of video gray-scale display Those numbers seem low by today’s standards because modern consumer displays routinely hit 500 to 1,000 nits, with HDR-capable screens pushing well beyond that. But the principle holds: luminance expressed in nits (or, equivalently, candelas per square meter) is the standard way to characterize how bright a screen is.
The distinction between lux and nits trips people up. Lux describes how much light is landing on a surface; nits describe how much light is leaving or reflecting off a surface toward your eye. When you read a book outdoors, the lux level is determined by the sun. The luminance of the page in nits depends on how reflective the paper is and how much of that sunlight it sends back toward you.
Radiometric Units for Raw Energy
Photometric units are weighted by the human eye’s response, but many applications need to know the actual energy content of the light regardless of how it looks to us. Radiometric units handle this. The watt is the fundamental radiometric unit of power. Irradiance, measured in watts per square meter, describes how much radiant power reaches a surface. Radiance, measured in watts per square meter per steradian, captures how much energy is emitted or reflected from a surface in a specific direction, much like luminance does in photometric terms but without the eye-sensitivity weighting.
These units matter in fields where ultraviolet, infrared, and other non-visible wavelengths carry the action. Dermatologists quantify UV exposure in radiometric terms because the eye does not detect UV, yet it still causes sunburn and skin damage. Thermal engineers designing building insulation care about infrared irradiance from the sun, again invisible to the eye but a major source of heat. Astronomers routinely measure the flux of starlight in watts per square meter across wavelengths from radio waves to X-rays, well outside the narrow band that photometry covers.
How Light Is Measured for Plants
Plants do not see the world the way humans do. Photosynthesis is driven primarily by wavelengths between about 400 and 700 nanometers, a range called photosynthetically active radiation, or PAR. Because the photopic curve used to define lumens peaks at 555 nanometers and tapers off sharply toward blue and red, a lumen rating can badly misrepresent how useful a light source is for a plant. A warm white LED might produce plenty of lumens but relatively little blue and red light compared to a horticulture LED tuned for plant growth.
The unit growers use instead is photosynthetic photon flux density, or PPFD, measured in micromoles of photons per square meter per second. Rather than weighting by the eye’s sensitivity, PPFD counts photons within the PAR range, giving each wavelength roughly equal weight (since each photon can drive a photosynthetic reaction regardless of color, at least to a first approximation). Research on indoor cannabis cultivation, for instance, tested canopy-level PPFD ranging from 120 to 1,800 micromoles per square meter per second and found that dry flower yield increased linearly with increasing PPFD all the way up to 1,800, even though leaf-level photosynthesis saturated well below that intensity.7Frontiers in Plant Science. Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment That finding underscores why PPFD at the canopy level, not just at a single leaf, is the relevant metric for growers making lighting decisions.
If you are setting up a grow light for houseplants, PPFD is the number to look for on the spec sheet. Low-light tropicals do fine around 50 to 150 micromoles. High-light crops and flowering plants often need 400 or more. Lumen ratings on the same fixture can be misleading because they overweight green light that the plant largely reflects.
Melanopic Lux and the Biology of Your Body Clock
Conventional lux measures how bright a light appears to your visual system. But since the discovery of intrinsically photosensitive retinal ganglion cells, or ipRGCs, a growing body of research has focused on how light affects the body’s non-visual systems: circadian rhythm, melatonin production, alertness, and sleep. These cells contain a photopigment called melanopsin that peaks in sensitivity around 480 nanometers, squarely in the blue range. Standard lux was never designed to capture this effect, and it does a mediocre job of predicting how a given light source will shift your circadian clock.
A newer metric called melanopic lux (or more formally, melanopic equivalent daylight illuminance) weights the light spectrum according to the sensitivity of melanopsin rather than the eye’s brightness channel. An analysis pooling data from nineteen laboratory studies on melatonin suppression, circadian phase resetting, and alerting responses found that melanopic illuminance consistently provided the best available predictor for responses of the human circadian system, outperforming standard photopic lux across a wide range of stimulus types and intensities.8PubMed. Melanopic illuminance defines the magnitude of human circadian light responses under a wide range of conditions
This has practical consequences. Two lights with identical lux readings can have very different effects on your melatonin suppression if their spectra differ in the blue region. A warm, yellowish lamp at 200 lux might deliver far less melanopic lux than a cool white LED at the same brightness, and thus have a smaller effect on your alertness or your sleep timing. Researchers have argued that melanopic lux should be applied in settings like nursing homes, workplaces, and homes to better manage the non-visual effects of electric lighting.9PubMed. Applying Melanopic Lux to Measure Biological Light Effects on Melatonin Suppression and Subjective Sleepiness Some lighting standards bodies have begun incorporating melanopic metrics, though widespread adoption in consumer products is still early.
Color Temperature and Color Rendering
Two additional metrics frequently appear on lighting product labels: correlated color temperature (CCT) and color rendering index (CRI). Neither measures brightness or energy in the same sense as the units above, but both describe qualities of light that affect how we perceive a lit environment.
CCT, measured in kelvins, describes the hue of white light by comparing it to the color emitted by a theoretical heated object. Lower values like 2,700 K correspond to warm, yellowish light (similar to an incandescent bulb), while higher values like 5,000 to 6,500 K match the cooler, bluish tones of daylight. CCT is a useful shorthand, but it tells you nothing about the full spectral composition. Two lights with identical CCT can have very different spectra, which matters both for color rendering and for the melanopic effects discussed above.
CRI, scored on a scale up to 100, rates how faithfully a light source reveals the colors of objects compared to a reference illuminant. A CRI of 100 means colors look just as they would under the reference. In one study on how lighting affects meat color assessment, researchers controlled for a CRI above 95 while varying the color temperature of LED lighting between 2,900 and 7,800 K, finding that even at high CRI, changes in CCT shifted the perceived color of red meat.10PubMed. Impact of Correlated Color Temperature on Red Meat Color Assessment via Image Processing Systems For grocery store displays, art galleries, and anywhere color accuracy matters, both CRI and CCT need to be considered together.
How Vision Changes in Dim Light
All the photometric units discussed so far rely on the photopic luminous efficiency function, which describes how the eye responds to light under bright, daytime conditions using cone photoreceptors. But in dim environments, vision shifts to scotopic mode, dominated by rod cells that have a different spectral sensitivity peaking around 507 nanometers rather than 555. In between these two states lies mesopic vision, which is where most nighttime urban environments and many indoor settings actually fall.
Mesopic vision is genuinely complicated. The transition from rod-based to cone-based vision involves not just a swap of photoreceptors but also changes in the neural pathways carrying the signals. Even before the transition to cone vision is complete, the rod system itself switches from a slow, sensitive pathway to a faster, less sensitive one.11PubMed. Into the twilight zone: the complexities of mesopic vision and luminous efficiency As a result, any measure of visual performance in the mesopic range depends on the illumination level, the spectral content of the light, where on the retina it falls, and how fast it changes. Standard photometric units, built on the photopic curve alone, do not capture these complexities well. Researchers continue to work on mesopic luminous efficiency models, but no single replacement has been universally adopted for practical lighting standards.
This gap matters for road and street lighting. Drivers at night operate almost entirely in mesopic conditions. Two streetlights with identical lux ratings but different spectra can produce different levels of visual performance at nighttime because the eye’s sensitivity curve is effectively shifted. Some municipalities have already begun evaluating streetlight designs using mesopic-weighted metrics in addition to standard photopic lux.
Measuring Light Pollution
Quantifying how much unwanted artificial light spills into the night sky is a measurement challenge in its own right. Light pollution researchers draw on many of the same units, including lux and luminance, but apply them to unconventional targets: the brightness of the night sky, the illumination of natural habitats, the glow visible from space. A systematic review of 140 articles on light pollution measurement methods found that no single device meets all the measurement objectives researchers face, and opportunities exist for further development, particularly in refining low-cost imaging tools.12ScienceDirect (Elsevier). How to measure light pollution—A systematic review of methods and applications
Common tools range from handheld lux meters and sky brightness meters to satellite imagery and calibrated all-sky cameras. Some researchers express sky brightness in magnitudes per square arcsecond, an astronomical unit that works well for comparing observations across sites but means nothing to most people. Others report illuminance at ground level in lux, which is more intuitive but misses the directionality and spectral content of the skyglow. This fragmentation in metrics makes it harder to compare studies and set regulatory thresholds, an ongoing frustration in the field.
Measuring Light for Other Species
Lux is built around the human visual system, but most animals see differently. Many mammals have different sets of photoreceptors with different spectral sensitivities. Birds can detect ultraviolet light that humans cannot. Insects respond to polarized light. Using human-centric lux to describe the light environment of a nocturnal rodent or a reef-dwelling fish is, at best, a rough approximation.
A photoreceptor-specific metrology system has been proposed that adapts the logic of melanopic lux to any species. The approach weights light energy across the spectrum according to the sensitivity of each class of rod, cone, or melanopsin photoreceptor in the target animal. Defining those spectral sensitivities is the critical step, and researchers have begun calculating species-specific equivalent daylight illuminances for laboratory animals like mice.13PubMed Central. Beyond Lux: methods for species and photoreceptor-specific quantification of ambient light for mammals The goal is to let biologists describe the light environment an animal actually experiences, not just the one a human would perceive standing in the same spot. This is relevant for animal welfare standards in labs, for conservation biology, and for ecological studies on how artificial lighting disrupts wildlife behavior.
How Instruments Capture These Measurements
Behind every lux reading or luminance measurement is a physical instrument, and the choice of instrument introduces its own complications. Two of the most common laboratory tools for characterizing light sources are spectroradiometers and integrating spheres. A spectroradiometer measures the spectral power distribution of a source at specific angles, while an integrating sphere collects all the light emitted in every direction to give total output. A comparison of these two methods for organic LED performance measurements found that with careful mathematical conversion, spectroradiometer data from multiple viewing angles could be brought into agreement with integrating sphere results within a reasonable range of deviation, suggesting that the more accessible spectroradiometer measurement can sometimes substitute for the more involved integrating sphere setup.14AIP Advances. Comparison of organic light emitting diode performance using the spectroradiometer and the integrating sphere measurements
At the consumer level, smartphone light sensors have made basic lux measurements widely accessible. Many apps claim to turn your phone’s ambient light sensor into a lux meter, and while these readings are not lab-grade, they can give a useful ballpark for checking whether your office lighting is adequate or your grow lights are delivering the expected intensity. For anyone needing real precision, calibrated instruments traceable to national metrology standards remain essential, but the barrier to entry for casual light measurement has dropped dramatically.