Candela to Lux Converter that accounts for distance and beam tilt

Candela describes the source, lux describes the surface

The candela is an SI base unit for how strongly a source emits light in one particular direction. Lux describes something else entirely: how many lumens land on each square metre of a surface. Move a desk closer to the same fixture and the lux reading climbs while the candela figure never budges. That is why the datasheet talks in candela and the job spec talks in lux.

• Candela (cd) — beam strength in one direction. Independent of distance.
• Lumen (lm) — total light output in every direction. Usually the big number on a lamp box.
• Lux (lx) — light landing on a surface. One lux is one lumen per square metre.
• Foot-candle (fc) — the same idea per square foot, standard on US drawings and meters.

Without a distance there is nothing to convert

Turning intensity into illuminance means dividing by the distance squared, because the area the beam covers grows with the square of the distance. Double the distance and the surface gets a quarter of the light, not half. That is the inverse-square law, and the bars under the result show exactly that falloff for the distance you entered.

• 2× the distance → 1/4 the illuminance (25%)
• 3× the distance → 1/9 (about 11%)
• 5× the distance → 1/25 (4%)
• Half the distance → four times the illuminance

Tilt the surface and the cosine takes its cut

The inverse-square law on its own assumes the beam hits the surface head on. Wall washers, angled downlights and sloped work surfaces all break that assumption: the same beam spreads over a wider patch and the illuminance drops by the cosine of the angle. Put the angle in the tilt field and the correction is applied, with the status line reporting what percentage of the perpendicular value is left.

• 0 degrees — beam parallel to the surface normal, no loss
• 30 degrees — about 86.6% of the perpendicular value
• 45 degrees — about 70.7%
• 60 degrees — exactly 50%
• Approaching 90 degrees the beam only grazes the surface and illuminance falls toward zero.

One number is enough to get a result

Pick a direction, type a number in the value field, and the result updates on the keystroke. Leave the distance or the tilt blank and the tool falls back to the example distance of 6.5 ft and 0 degrees, saying so in the status line. Thousands separators are optional, and the preset chips fill in four common setups in one click.

• Choose cd → lx or lx → cd in the direction control.
• Type the intensity or the illuminance in the value field.
• Enter the distance and pick ft, in, m or cm.
• Add a tilt angle if the surface is not square to the beam. Blank means 0 degrees.
• Copy result puts the inputs, the formula, the result, the foot-candle figure and the reference band on your clipboard.

Worked example — a 2,000 cd fixture 8 ft above a desk

Say a fixture rated at 2,000 cd on axis is mounted 8 ft above a work surface, aimed straight down. Eight feet is 2.4384 m, and 2,000 divided by that squared gives 336.4 lx, shown next to 31.25 fc. That clears the 30 fc OSHA asks for in offices and first aid stations, but with almost nothing to spare, so any dirt on the lens or lumen depreciation over the year will push it under.

• Input — 2,000 cd · 8 ft · 0 degrees
• Calculation — 2,000 ÷ 2.4384² = 336.4
• Result — 336.4 lx (31.25 fc), the first aid stations and offices band
• Drop the mount to 4 ft and it jumps to 1,345 lx; raise it to 16 ft and it falls to 84.09 lx.
• Aim the same fixture at 45 degrees and 8 ft gives 237.9 lx instead.

The other direction — what intensity does 30 fc need

More often the target illuminance is fixed first. To hold the OSHA office minimum of 30 fc, which is 323 lx, on a desk 8 ft below the fixture, switch the direction to lx → cd and enter 323 with a distance of 8 ft. Multiplying by 2.4384 squared gives 1,920 cd. Now you can read a photometric report and keep only the fixtures whose on-axis intensity clears that figure, remembering this is the share carried by a single luminaire.

• Input — 323 lx · 8 ft · 0 degrees
• Calculation — 323 × 2.4384² = 1,920
• Result — 1,920 cd, and the 323 lx target is 30.01 fc
• Raise the ceiling to 10 ft and the requirement grows to 3,001 cd.

How the result is graded

The ladder on the right of the tool is the OSHA minimum illumination table for construction work, published in foot-candles. Whichever band your calculated illuminance lands in is highlighted as you type, so the number never sits there without a reference. Section 1926.56(b) sends you to the industrial lighting practice standard for anything the table does not name.

Lux equivalents are rounded from the exact factor 1 fc = 10.7639 lx. For areas the table does not cover, 1926.56(b) points to the industrial lighting practice standard.

Where lumens and watts fit in

Lamp packaging leads with lumens and watts. Watts are power draw and say nothing directly about output, while lumens count everything the lamp emits in every direction without telling you where it went. That is why lumens alone cannot give you lux: you need the beam distribution to get to candela first. It is also why this converter has no lumen field.

• Watts — power consumed. Efficacy varies enormously between lamp types.
• Lumens — total flux in all directions. Same lumens, different optics, very different floor readings.
• Candela — intensity in a stated direction. Narrow optics concentrate flux and push this number up.
• A tight spot with modest lumen output can beat a wide flood on axis for exactly this reason.

Moving between foot-candles and lux

US drawings, IES reports and most handheld meters sold in the States work in foot-candles. Because the international foot is defined as exactly 0.3048 m, one foot-candle equals one divided by 0.3048 squared, or roughly 10.7639 lux. Both figures appear in the result, so a metric datasheet and an imperial spec can be compared without a second calculation.

• 1 fc = 10.7639 lx (exactly 1 ÷ 0.3048²)
• 1 lx = 0.0929 fc
• 30 fc ≈ 323 lx — the OSHA minimum for offices and first aid stations
• 10 fc ≈ 108 lx, 5 fc ≈ 54 lx, 3 fc ≈ 32 lx

When the arithmetic stops matching the meter

The inverse-square law is exact only where the source can be treated as a point. Large housings and wide diffusers break that near the fixture, which is why lighting practice trusts the calculation from about five times the luminaire's largest dimension outward. If any of the following applies, treat the number as a design estimate and confirm it with a meter.

• The distance is under five times the fixture's largest dimension.
• A small room with bright walls and ceiling returning a lot of interreflected light.
• Several fixtures lighting the same spot. Calculate each one and add the results.
• An asymmetric distribution where only the on-axis intensity appears on the datasheet.
• A meter without cosine correction, or with the wrong spectral correction for LED sources.
• Aged lamps or dusty lenses, both of which cut real output well below the rated figure.

Before you order the fixtures

Running through this list once tends to be cheaper than re-aiming a lighting layout after installation. The biggest single source of surprise is a candela figure read off the datasheet without checking which angle it belongs to.

• Does the datasheet candela figure refer to the beam axis, or to the angle you actually plan to use?
• Was the distance measured from the light-emitting surface to the task plane rather than to the floor?
• If the task surface is sloped, has the tilt angle been entered?
• Is the target a maintained level or an initial one? A maintained level needs depreciation headroom on top.
• Have you added the contribution of other fixtures already lighting the space?
• Which row of the OSHA table does this area belong to?