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THE LIGHTING REFERENCE

LIGHTING · SCIENCE · PRACTICE

A shared language for light.

20 source-linked concepts, explained in original language with their practical limits.

Primary-source research · Original explanations · Reviewed October 4, 2026

01

Use terms precisely

A specification becomes clearer when quantities, perceptions, methods and operating conditions are named separately. The entries below are short editorial explanations checked against public IES definitions and catalog pages. For normative wording and complete definitions, follow the official source.

02

Read the qualification

Each concept includes a limitation because many lighting mistakes come from extending a valid measurement beyond what it actually describes. A glossary entry is a starting point for a decision, not a universal design rule.

Luminous flux

Luminous flux measures radiant-energy flow after weighting it for a standardized visual response; its unit is the lumen.

Ordinary luminous-flux values use the photopic response unless another response is specified.

Illuminance

Illuminance describes the density of luminous flux arriving at a surface.

It describes incident light, not subjective brightness or the light returned toward a viewer.

Lux

One lux corresponds to one lumen arriving per square metre.

This is a unit relationship, not a recommended design target.

Luminous intensity

Luminous intensity describes luminous flux per unit solid angle in a particular direction and is expressed in candelas.

The point-source definition is applied in practice when source dimensions are small relative to observation distance.

Luminance

Luminance is a directional photometric quantity associated with light leaving, passing through, or arriving at a surface.

Do not use it as a synonym for illuminance.

Brightness and luminance

Perceived brightness depends partly on measurable luminance and partly on observation conditions, including the eye's adaptation.

A subjective sensation cannot be replaced completely by a single physical measurement.

Photopic vision

Photopic vision is mediated primarily by the retina's cones.

The source gives approximate adaptation conditions; this text intentionally does not turn them into design thresholds.

Scotopic vision

Scotopic vision is mediated primarily by the retina's rods.

The source describes very low adaptation luminance; it is not an instruction for lighting design.

Mesopic vision

Mesopic vision occupies the adaptation range between scotopic and photopic conditions.

The fully adapted-eye condition matters; avoid presenting boundaries as universal instantaneous changes.

Correlated color temperature

CCT associates a source's chromaticity with the temperature of the closest-matching blackbody chromaticity.

It is a chromaticity descriptor, not the physical temperature of an LED.

TM-30 color rendition

TM-30 evaluates overall color fidelity and gamut area together with hue-specific fidelity, chroma shifts, and hue shifts.

Public catalog description verified as ANSI/IES TM-30-24+E1; the full calculation standard was not reproduced.

CCT and Duv

TM-40 describes computation of CCT and distance from the Planckian locus, Duv, using a light source's chromaticity coordinates.

Public catalog description identifies ANSI/IES TM-40-24. Do not present full methods or restrictions without consulting the standard.

Discomfort glare

Discomfort glare causes discomfort but does not necessarily reduce visibility or visual performance.

Keep separate from disability glare, although both can occur together.

Disability glare

Stray light inside the eye can reduce retinal-image contrast and thereby reduce visibility or visual performance.

A source that causes discomfort can also cause disability glare.

Unified glare rating

UGR is a measure of discomfort produced by a lighting system.

This definition does not establish a universal acceptable UGR or a product-independent rating.

Flicker terminology

The common word flicker can refer both to temporal light modulation and to visual artifacts perceived in response to it.

IES cautions that the term can be non-specific; distinguish the physical modulation from the response.

Integrated daylighting

Daylighting design must address occupant comfort, lighting quality, and energy efficiency together across changing daylight and weather conditions.

Public catalog synopsis only; no numerical criteria or proprietary tables reproduced.

Daylight and electric lighting

IES daylighting guidance includes shading, glazing, electric-light integration, switching, dimming, and photosensing strategies.

Supported by the public table of contents; it is a scope description, not a detailed control algorithm.

Lighting control strategies

IES education addresses occupancy-based controls, daylighting controls, and lighting-reduction strategies in interior and exterior design.

Public course objectives establish the topics; local adopted energy codes determine actual compliance requirements.

Tunable white

Tunable-white lighting varies the correlated color temperature of a white-light source through spectral tuning.

Changing CCT does not by itself demonstrate a health outcome.

Keep building your understanding.

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