Daylight Factor vs Daylight Lux

Daylight factor and daylight lux answer different questions

Daylight factor is a relationship between an indoor reading and an outdoor reference reading taken under the same condition. Daylight lux is the measured or estimated illuminance at a point or plane under a named daylight condition. They are connected, but they are not interchangeable.

The daylight factor calculator owns the ratio check. The daylight factor examples table shows the note shape. The daylight lux values table keeps outdoor and indoor daylight conditions from being treated as constant values.

Term	What it notes	Typical question
Daylight factor	Indoor lux divided by outdoor lux, expressed as a percentage.	How much of the outdoor reference appears indoors at this point?
Daylight lux	Illuminance at a point or plane under a named daylight condition.	What did the meter read at this time and condition?
Electric-light lux	Illuminance from the electric lighting state being checked.	What does the installed or proposed lighting produce?
Combined reading	Daylight and electric lighting together.	What is visible in the actual occupied condition?

Ratio needs paired readings

A daylight factor note should keep the indoor and outdoor values together. The outdoor value is not a universal constant; it changes with sky condition, time, obstruction and measurement basis. The indoor value changes with room depth, glazing, shading, surface finishes and point location.

Daylight-factor field	Note note	Boundary
Indoor lux	Meter value at the assessed indoor point or plane.	The point must be named before comparison.
Outdoor lux	Exterior reference value taken for the same condition.	Local obstruction and sky condition should be noted.
Percentage	Indoor lux / outdoor lux x 100.	The percentage is a daylight relationship, not a compliance result.
Condition	Time, sky, shading, blinds and electric-light state.	A different condition creates a different note.

The daylight factor reading notes guide owns the paired-reading method. This page keeps the concept boundary clear so the percentage is not confused with a standalone lux target.

For electric-light checks, keep daylight values away from the lamp or luminaire calculation until the condition is named. The lumens to lux calculator checks an installed-output scenario, while the lux to lumens calculator works from a target illuminance. Daylight readings can sit beside either note, but they should not silently replace the electric-light case.

Choose the right daylight check

Daylight questions often sound similar, but the note changes once the user job is clear. A ratio question belongs with paired indoor and outdoor readings. A room-comfort question usually needs daylight lux, glare and shading notes. An electric-light design question still needs a maintained-light estimate for the electric case.

Search job	Better note	Supporting page	Boundary to preserve
Compare daylight factor with a daylight lux reading.	Concept boundary between ratio and illuminance.	Current guide, then the ratio or meter note owner below.	Do not read a percentage as a lux value.
Note how much outside daylight reaches a desk.	Daylight factor percentage from paired readings.	Daylight factor reading notes	Indoor point, outdoor reference and same-time condition stay together.
Log how bright the desk is right now.	Daylight lux at the named task plane.	Lux meter reading condition log	Time, sky, blinds, direct sun and electric-light state stay visible.
Explain harsh window-side light.	Daylight lux beside glare, view direction and shading notes.	Glare check lighting notes and daylight shading notes	Brightness, discomfort and shading are separate observations.
Compare daylight-only and electric-light states.	Separate daylight-only, electric-only and combined readings.	Daylight vs electric lighting notes	Do not subtract daylight from the electric-light note without naming the condition.
Consider dimming near windows or skylights.	Control-zone note with the daylight condition stated.	Lighting control zones and operating hours	The control note does not turn a daylight reading into an energy saving.

For Australian homes, offices, classrooms and retail interiors, that separation prevents a daylight factor percentage from being read as a room lux target, and prevents a bright one-time daylight reading from being treated as reliable operating light.

Search phrase	Start with	Fields that must be visible
Daylight factor percentage for a desk.	Daylight factor calculator	Indoor point, outdoor reference, same interval and percentage.
Daylight lux level near a window.	Lux meter reading condition log	Point label, plane, sky, direct sun, blind state and electric-light state.
Electric light reading after dark.	Daylight vs electric lighting notes	Active group, scene, dimming level, plane and no-daylight condition.
Before and after blind change.	Before and after lux comparison notes	Same point, same plane, two blind states and matched sky note.
Window or skylight daylight note.	Window and skylight daylight notes	Opening type, distance from opening, shading and point depth.

Daylight lux is condition-specific

Daylight lux can change quickly. A desk may read high near a window in direct sun and low in the same room on an overcast afternoon. A useful daylight lux note names the plane, point, time, sky, shading and electric-light state before the number is compared.

Daylight lux note	Example	Why it matters
Point and plane	Desk centre, 720 mm task plane.	The same room can have several daylight levels.
Time and sky	Morning, bright overcast, no direct sun at the point.	Later readings may not match.
Shading state	Blinds open, tilted or closed; awning or eave noted.	Shading changes the reading and glare condition.
Electric-light state	Off, on, dimmed or scene state noted.	Daylight-only and combined readings should not be merged.

The lux meter reading notes table is the compact owner for recording those fields.

Practical point and condition labels

Short labels make daylight factor, daylight lux and electric-light notes easier to compare without turning them into one blended number. Keep the label stable, then change only the condition being checked.

Label	Plane	Daylight condition	Electric-light condition	Opening or control note
W1	Desk task plane beside window.	Bright overcast, no direct sun at point.	Off for daylight-only reading.	Blinds open, eave shade present.
W1-B	Same desk task plane.	Same interval where possible.	Off for daylight-only reading.	Blinds tilted or partly closed.
I1	Internal desk row task plane.	Same sky note as W1.	Off, then later electric scene if checked.	Distance from window noted.
S1	Bench plane below skylight.	Sky and sun-at-opening note recorded.	Off or combined state named.	Diffuser, shaft or blind state written.
E1	Same task plane after dark.	No daylight.	Electric scene normal or dimmed.	Control group and scene state written.

Where a daylight factor percentage is needed, pair the indoor point with an outdoor reference from the same short interval. Where a lux note is enough, keep the number attached to the point, plane and condition instead of turning it into a general room result.

Keep owner notes separated

The same site note can mention daylight, electric light, controls, load and visual quality, but each item needs its own note owner. A clear handoff is better than a blended note that looks complete while hiding the basis of the reading.

Note slice	Keep with	Owner page	Do not blend with
Daylight ratio	Paired indoor and outdoor readings, percentage and shared condition.	Daylight factor reading notes	Electric-light output, colour quality or load.
Daylight lux	Point, plane, time, sky, shading and electric-light state.	Lux meter reading condition log	A daylight-factor percentage unless the outdoor reference is recorded.
Window or skylight context	Opening type, obstruction, shading and room-depth note.	Window and skylight daylight notes	A whole-room conclusion from one bright point.
Electric-light lux	Electric-only or named scene reading, with daylight state stated.	Daylight vs electric lighting notes	A daylight-only reading treated as installed lighting output.
Energy or connected load	Circuit, load, hours and operating assumption.	Lighting energy baseline notes and connected load to annual kWh	Daylight availability recorded as a saving by itself.
Colour quality	Appearance, colour temperature and rendering notes.	Colour temperature vs CRI	Illuminance or daylight ratio.
Control behaviour	Sensor zone, operating hours and manual override context.	Occupancy sensor lighting control notes	A daylight condition treated as proof of control performance.

Do not subtract daylight casually

It can be tempting to read a daylight value and subtract it from an electric-light target. That can be useful as a planning note only when the daylight condition is clearly named. Daylight availability is not constant, and a bright reading at one time does not prove that electric lighting can be reduced across all use periods.

Estimate question	Safer note	Related page
How much daylight is present now?	Daylight lux at a named point and condition.	Lux meter reading notes
What percentage relationship exists?	Paired indoor and outdoor daylight-factor readings.	Daylight factor calculator
What electric lighting remains?	Electric-light case with daylight condition stated.	Lighting control notes
What is the maintained-light estimate?	Luminaire output, UF, MF and target plane.	Room lighting calculator

Window-side and room-depth readings should not be merged

Daylight falls away across many rooms, and the useful comparison is often between the window-side zone and the deeper zone. A single average can hide the fact that one desk is bright and glary while another desk still depends on electric light.

Zone	Daylight note	Lighting decision it supports
Window-side desk row	Daylight lux, view direction and shading state.	Whether dimming or glare control needs a separate note.
Internal desk row	Daylight lux under the same time and sky condition.	Whether electric-light output still carries the task.
Display or counter face	Vertical daylight reading at the viewed surface.	Whether daylight helps or creates contrast problems.
Circulation path	Floor reading and direct-sun note.	Whether the path condition differs from the task plane.

Measurement terms to keep apart

Term	Keep with	Do not treat as
Lux	A measured or calculated illuminance value.	A lighting quality result by itself.
Illuminance	Light arriving on a surface.	Luminaire output or wattage.
Daylight factor	Indoor/outdoor daylight ratio.	An electric-light performance measure.
Measured illuminance	Meter reading tied to a point and condition.	A whole-room or all-day result.