Photometry unit conversions: lux, lumens, and display resolution

Photometric units describe light the way the human eye perceives it, which is why they differ from the pure radiometric units physicists use for raw energy, and why a photographer, a lighting designer, and a display engineer all reach for slightly different corners of the same toolkit. UnitSnap's Light group covers five categories: luminance, luminous intensity, illumination, image resolution, and frequency-wavelength, each tied to a specific everyday reference worth knowing.

Illumination: what lux actually measures

Illumination, measured in lux (lumens per square meter) or the older foot-candle (lumens per square foot, where 1 foot-candle equals roughly 10.76 lux), measures how much light falls on a surface, the figure behind every lighting-design recommendation. Direct sunlight measures around 32,000 to 100,000 lux, an overcast day around 1,000 lux, a well-lit office around 300 to 500 lux, and a moonlit night under 1 lux, a range of five orders of magnitude that explains why camera exposure settings swing so widely between a sunny afternoon and an indoor room.

Lighting codes and camera light meters both work in these units, so converting between lux and foot-candles matters most when comparing a US-sourced lighting spec, often in foot-candles, against an international one in lux: an office lighting standard requiring 50 foot-candles is asking for roughly 538 lux, a figure worth checking against actual fixture output before a room is signed off as adequately lit.

Luminance and luminous intensity: brightness from a source versus a surface

Luminous intensity, in candelas (cd), measures how much light a source emits in a specific direction, a figure printed on LED and flashlight packaging: a typical household candle is, unsurprisingly, close to 1 candela by the unit's original definition, while a modern LED flashlight can exceed 1,000 candela in its beam's center.

Luminance, in candelas per square meter (cd/m2, often called nits in display specifications), measures brightness as it appears from a surface rather than a point source, which is why it is the unit used for screens: a smartphone display typically runs 400 to 800 nits, high-end HDR displays exceed 1,000, and direct sunlight reflecting off white paper can exceed 20,000 nits, which is why screens struggle to stay readable outdoors. Luminance and luminous intensity are not directly interchangeable without knowing the emitting or reflecting surface's area, since one is a directional quantity and the other is a surface-brightness quantity.

Image resolution: DPI, PPI, and what print actually needs

Image resolution, in dots per inch (DPI) for print or pixels per inch (PPI) for screens, describes information density rather than light directly, but it lives in this category because it governs how a display or a printed page renders the light it emits or reflects. Standard print quality sits around 300 DPI; a smartphone screen typically runs 400 to 500 PPI; a large TV viewed from across a room can look sharp at well under 100 PPI simply because viewing distance changes how much resolution the eye can actually resolve.

The DPI-to-PPI conversion itself is usually one-to-one for digital purposes, since both describe the same idea, dots or pixels per linear inch, but the terms are not interchangeable in practice: DPI historically refers to a printer's physical dot placement, while PPI refers to an image or display's pixel density, and a 300 PPI image printed at 150 DPI will look soft even though no unit conversion was needed to produce it, since the mismatch is in output settings, not arithmetic.

Frequency and wavelength: two ways to describe the same light

Frequency-wavelength conversion sits at the physics end of this group: visible light runs from roughly 400 nanometers (violet, high frequency) to 700 nanometers (red, lower frequency), and frequency and wavelength are related by the speed of light, so frequency equals the speed of light divided by wavelength. This conversion matters most in optics, LED specification sheets, and color science, where a manufacturer might quote a wavelength in nanometers while a calculation downstream needs frequency in terahertz.

Camera exposure and the light values behind it

Photography ties several of these units together more directly than most other everyday uses. A camera's light meter is, at its core, reading illumination or the luminance reflected from a scene, and translating that reading into an exposure setting, aperture, shutter speed, and ISO together, is effectively a conversion from lux or candela per square meter into a combination that lets a fixed amount of light reach the sensor. This is why the same numerical settings that work outdoors at 50,000 lux fail completely indoors at 300 lux: the illumination has dropped by more than two orders of magnitude, and the exposure settings have to compensate by a comparable factor.

Studio lighting design works the same problem from the other direction, starting from a target illumination level, often specified in lux for a product shoot or foot-candles in older US-published lighting guides, and working out how many fixtures of a known luminous intensity, and at what distance, are needed to hit it. Since illumination falls off with the square of the distance from the source, doubling the distance to a light cuts the illumination to a quarter, a relationship worth knowing before assuming a conversion alone will explain why a shot looks under-lit.

Choosing the right photometric unit for the job

The fastest way to avoid a mismatched conversion in this group is to ask what is being measured, light falling on a surface (illumination, in lux), light leaving a source in a direction (luminous intensity, in candela), or brightness as seen from a surface (luminance, in nits), since these three are the ones most often confused for each other despite measuring genuinely different things. Once the right quantity is identified, the converters below carry the standard factors, including the lux-to-foot-candle and DPI-reference figures used throughout this guide. Keeping a small mental table of anchors, sunlight, an office, a moonlit night for illumination, a phone screen and a candle for luminance and intensity, makes it far easier to spot a converted number that is off by an order of magnitude before it ends up in a lighting plan or a camera setting.