Scenario workplanesLight pool, target contour, power, and lux/W
Unit Converters
Illuminance Comparison Calculator
Keep light level and energy input separate while comparing two measured or modeled scenarios against the same workplane target.
TWO-SCENE WORKPLANE COMPARISON
Compare delivered light and connected power without confusing the two
Use measured or modeled workplane illuminance for two lighting scenes. The calculator keeps lux, represented area, incident lumens, connected watts, and target attainment separate.
Comparison boundary: lux per watt is a workplane scenario metric, not luminaire efficacy. Measurements must use compatible workplane definitions and operating conditions.
Illuminance difference—
Higher workplane efficiency—
Scenario A lux per watt—
Scenario B lux per watt—
Scenario A target state—
Scenario B target state—
PAIRED LIGHT POOLS
See target coverage and power together
Each scene uses a luminous pool, a target contour, and a power label. This prevents the brighter scene from automatically being called the more efficient one.
| Scene | Workplane lux | Incident lumens | Lux/W | Target |
|---|
HOW TO USE
Compare scenes on a common decision boundary
- Define the workplane and measurement or model method used by both scenes.
- Enter representative illuminance, area, and connected power for Scenario A.
- Enter the same evidence for Scenario B without forcing the areas to match.
- Set one target that actually governs the comparison.
- Read target state first, then lux difference, incident lumens, and lux per watt.
COMPARISON FUNDAMENTALS
Six measures prevent a false winner
CALCULATION METHOD
Evaluate adequacy before efficiency
ΦA = EA AAΦB = EB ABqA = EA/PAqB = EB/PBΔE = EB − EAE is lux, A m², P watts, Φ incident lumens, and q workplane lux per watt.
ADEQUACY GATE
A low-power scene is not useful if it misses the required task
Target attainment is reported separately for each scenario. Choose among scenes that satisfy the governing workplane criterion, then compare power and distribution. The target itself must come from the project or applicable guidance and remain tied to maintained conditions.
AREA BOUNDARY
Average lux over different areas changes the meaning of incident lumens
Five hundred lux over a small desk and four hundred lux over an entire room are not interchangeable. Incident lumens provide one area-aware quantity, but only if each lux value is representative of its entered area. A single-point reading cannot justify a whole-room flux claim.
EFFICIENCY PROXY
Workplane lux per watt is not luminaire efficacy
Luminaire efficacy relates emitted lumens to electrical watts under defined test conditions. This page’s ratio depends on geometry, reflectance, maintenance, controls, and sampling. It is useful for a controlled scene comparison, not as a product label.
DETAILED CALCULATION PROCESS
Default A/B substitution
| Symbol | A | B | Unit |
|---|---|---|---|
| E | 500 | 450 | lx |
| A | 20 | 20 | m² |
| P | 200 | 150 | W |
| Et | 400 | lx | |
- A incident lumens = 500 × 20 = 10,000 lm.
- B incident lumens = 450 × 20 = 9,000 lm.
- A workplane ratio = 500/200 = 2.5 lx/W.
- B workplane ratio = 450/150 = 3.0 lx/W.
- Difference B−A = −50 lx.
- Both 500 lx and 450 lx exceed the 400 lx target.
- B has higher workplane lux per watt while A is brighter.
- Reverse check: 3.0 lx/W × 150 W = 450 lx.
RESULT INTERPRETATION
There may be no single winner
A can deliver more light while B uses power more effectively. When both meet target, the decision may favor lower power, better uniformity, controls, or visual quality. When only one meets target, an efficiency ratio cannot rescue the inadequate scene.
EVIDENCE TO MATCH
Compare like with like
Retain workplane geometry, point or grid method, averaging rule, daylight state, fixture output state, dimming level, connected-power boundary, ballast or driver inclusion, area, occupancy state, maintenance condition, timestamp, and target authority.
LIMITS AND EXCLUSIONS
What this comparison does not rank
- Uniformity and localized dark points are not inferred.
- Glare, color rendering, spectral quality, and flicker are excluded.
- Controls, operating hours, and annual energy are not calculated.
- Fixture life, maintenance, embodied cost, and heat gains are excluded.
- Target compliance is only the entered arithmetic rule.
WORKED DECISION CASES
Different projects choose differently
LED retrofit
Both scenes exceed target; the retrofit uses less power and has higher lux/W. The team still checks uniformity and glare before selection.
Accent scene
A low-power scene has high lux/W at one focal point but misses the surrounding workplane. The ratio is not representative of the required area.
TECHNICAL GLOSSARY
Terms for the scene record
- Workplane
- Plane where illuminance is evaluated.
- Connected load
- Electrical power inside the selected boundary.
- Incident lumens
- Uniform-equivalent lux times area.
- Luminaire efficacy
- Emitted lumens per watt under defined test conditions.
- Target
- Entered project illuminance level.
- Maintained illuminance
- Illuminance expected after defined depreciation.
- Scene
- One controlled lighting configuration.
- Boundary
- Definition of included area and power.
IMPORTANT NOTE
Do not choose from one-point lux and whole-system watts
The numerator and denominator must describe the same scene boundary. Use a representative workplane statistic when power covers an entire room.
Frequently asked questions
Is higher lux always better?
No. Use the governing task target and visual-quality requirements.
Is lux/W the same as lm/W?
No. Geometry affects lux/W.
Can areas differ?
Yes, but interpret incident lumens and representativeness carefully.
Why use one target?
A shared target makes adequacy directly comparable.
Does power include controls?
Include every component inside the chosen connected-load boundary.
Can measured and modeled lux be mixed?
Only with an explicit method caveat; matched methods are preferable.
Does this calculate annual energy?
No. Add schedules and control states separately.
Can a below-target scene still win?
Not for a decision that requires the entered target.
AUTHORITATIVE BASIS