What is the difference between luminous efficacy and luminous efficiency?

Luminous efficacy is an absolute measure in lumens per watt (lm/W). Luminous efficiency is the ratio of a source's luminous efficacy to the maximum possible efficacy (683 lm/W for monochromatic 555 nm light), making it a dimensionless number between 0 and 1 (or expressed as a percentage).

What light source has the highest luminous efficacy?

Theoretically, monochromatic green light at 555 nm has the maximum possible luminous efficacy of 683 lm/W. Practically, modern white LED light sources currently offer some of the highest efficacies for general lighting, exceeding 200 lm/W in labs and around 150 lm/W in commercial products.

Is a higher luminous efficacy always better?

Not necessarily in practice. While higher efficacy means more light per unit of energy, other factors like colour rendering index (CRI), colour temperature, lifespan, and cost are also critical for the suitability of a light source for a given application.

Can 'luminous efficacy' apply to natural light sources like the sun?

Yes, in principle. The luminous efficacy of daylight (integrated across the solar spectrum) is approximately 100-120 lm/W, which is lower than the theoretical maximum because the sun emits much of its energy in non-visible infrared and ultraviolet wavelengths.

luminous efficacy

Very Low

UK/ˌluːmɪnəs ˈɛfɪkəsi/US/ˌlumənəs ˈɛfɪkəsi/

Technical/Scientific

My Flashcards

Definition

Meaning

The ratio of the total luminous flux (visible light output) emitted by a light source to the total electrical power consumed by that source.

A measure of how efficiently a light source produces visible light, expressed in lumens per watt (lm/W). In a broader technical context, it can also refer to the efficiency of the human eye's perception of light from a given source.

Linguistics

Semantic Notes

A standardised, quantitative metric in lighting engineering and physics. It is not a subjective measure of brightness. Often contrasted with 'luminous efficiency', which is a dimensionless ratio relative to the theoretical maximum.

Dialectal Variation

British vs American Usage

Differences

No significant lexical differences. Spelling of related terms may vary (e.g., metre vs meter).

Connotations

Purely technical, no connotative differences.

Frequency

Equally rare outside specialised fields in both dialects.

Vocabulary

Collocations

strong

high luminous efficacyoverall luminous efficacyluminous efficacy ofimprove luminous efficacy

medium

measure the luminous efficacytheoretical maximum luminous efficacyluminous efficacy rating

weak

discuss luminous efficacycalculate the luminous efficacyreport on luminous efficacy

Grammar

Valency Patterns

The luminous efficacy of [LIGHT SOURCE] is [VALUE] lm/W.Researchers measured/calculated/determined the luminous efficacy.

Vocabulary

Synonyms

Neutral

lighting efficiency (in non-technical contexts)luminous efficiency (related but distinct)

Weak

light output per watt

Vocabulary

Antonyms

inefficiency (general)luminous inefficacy (non-standard)

Usage

Context Usage

Business

In product specifications and marketing for energy-efficient lighting, e.g., 'This LED boasts a luminous efficacy of 150 lm/W.'

Academic

In physics and engineering papers comparing light source technologies, e.g., 'The study compared the luminous efficacy of OLEDs and traditional LEDs.'

Everyday

Virtually never used in casual conversation. A simplified term like 'energy efficiency' or 'brightness per watt' might be used.

Technical

The primary context. Used in datasheets, research, standards (e.g., ISO/CIE), and lighting design calculations.

Examples

By Part of Speech

adjective

British English

The luminous-efficacy figures were impressive.
A high-luminous-efficacy lamp is preferable.

American English

The luminous efficacy data was compelling.
We need a high luminous-efficacy solution.

Examples

By CEFR Level

A bulb with high luminous efficacy saves electricity.
This chart shows the luminous efficacy for different lights.

Engineers aim to improve the luminous efficacy of commercial LEDs.
The luminous efficacy of sodium-vapour streetlights is quite high, but their colour rendering is poor.

While the photopic luminous efficacy peaks at 555 nm, the scotopic efficacy curve is shifted towards shorter wavelengths.
The overall luminous efficacy of a lighting system must account for driver losses and optical efficiency, not just the bare source.

Learning

Memory Aids

Mnemonic

Think: 'Luminous Efficacy' = Light Output (Lumens) / Electrical Input (Watts). It's the light-bulb's 'miles-per-gallon' rating.

Conceptual Metaphor

EFFICIENCY IS A RATIO; LIGHT IS A PRODUCT.

Watch out

Common Pitfalls

Translation Traps (for Russian speakers)

Do not translate 'efficacy' as 'эффективность' in isolation. The established technical term is 'световая отдача'.
Avoid confusing with 'освещённость' (illuminance) or 'светимость' (luminance).

Common Mistakes

Confusing 'luminous efficacy' (lm/W) with 'luminous efficiency' (a dimensionless fraction).
Using it to describe perceived brightness, which is also affected by color temperature and distribution.
Pronouncing 'efficacy' as /ɪˈfɪʃənsi/ (like 'deficiency') instead of /ˈɛfɪkəsi/.

Practice

Quiz

Fill in the gap

The new prototype achieved a record of 200 lumens per watt.

Multiple Choice

What is the primary unit of measurement for luminous efficacy?

FAQ

Frequently Asked Questions

: Luminous efficacy is an absolute measure in lumens per watt (lm/W). Luminous efficiency is the ratio of a source's luminous efficacy to the maximum possible efficacy (683 lm/W for monochromatic 555 nm light), making it a dimensionless number between 0 and 1 (or expressed as a percentage).
: Theoretically, monochromatic green light at 555 nm has the maximum possible luminous efficacy of 683 lm/W. Practically, modern white LED light sources currently offer some of the highest efficacies for general lighting, exceeding 200 lm/W in labs and around 150 lm/W in commercial products.
: Not necessarily in practice. While higher efficacy means more light per unit of energy, other factors like colour rendering index (CRI), colour temperature, lifespan, and cost are also critical for the suitability of a light source for a given application.
: Yes, in principle. The luminous efficacy of daylight (integrated across the solar spectrum) is approximately 100-120 lm/W, which is lower than the theoretical maximum because the sun emits much of its energy in non-visible infrared and ultraviolet wavelengths.