Converting Lux to Foot-Candles: Easy Illuminance Guide

Why Illuminance Units Matter More Than You'd Think

If you've ever designed a light sensor circuit, calibrated an ambient light sensor for a display backlight controller, or tried to spec a photodiode for outdoor use, you've run into illuminance. And if you've done this across datasheets from different manufacturers or different countries, you've probably noticed the units are all over the place.

American lighting specs love foot-candles. European datasheets stick to lux. Older photometry literature throws around phots. And when you're trying to figure out whether your sensor has enough dynamic range to handle both a moonlit parking lot and direct sunlight, you need to convert between these units without making a mistake that costs you two orders of magnitude.

The math isn't hard. But it's easy to mess up when you're tired, and the consequences can be expensive.

The Basics: What Is Illuminance?

Illuminance measures how much luminous flux hits a surface per unit area. It's the "how bright does this surface look" metric, weighted by human eye sensitivity (the photopic response curve). The SI unit is the lux (lx), defined as one lumen per square meter.

where $E$ is illuminance in lux, $\\Phi$ is luminous flux in lumens, and $A$ is area in square meters.

A few reference points that are burned into my brain:

• Moonlight on a clear night: about 0.25 to 1 lux (our calculator uses 1 lux as a round number)
• Typical office lighting: 300 to 500 lux
• Overcast day outdoors: around 1,000 lux
• Bright sunny day in the shade: 10,000 lux
• Direct sunlight: 100,000 lux or more

That's a dynamic range of about 100,000:1 from moonlight to direct sun. If you're designing a sensor that needs to work across this range, you need to think carefully about your ADC resolution, gain stages, and saturation limits.

The Unit Zoo

Let's get the conversions straight.

Lux to Foot-Candles

The foot-candle (fc) is the imperial cousin of the lux. One foot-candle equals one lumen per square foot. Since there are approximately 10.764 square feet in a square meter:

Or going the other way:

So 500 lux (typical office lighting) works out to about 46.5 foot-candles. American building codes and IESNA recommendations are usually given in foot-candles, so if you're designing lighting controls for the US market, you'll be doing this conversion constantly.

Millilux and Kilolux

These are just metric prefixes applied to lux. Nothing tricky here:

Millilux shows up when you're dealing with very dim conditions—astronomical twilight, for instance, or testing the dark current behavior of photodiodes. Kilolux is convenient for outdoor measurements so you don't have to write "87,000 lux" when "87 klx" will do.

The Phot

Here's where it gets a bit obscure. The phot (ph) is a CGS unit equal to one lumen per square centimeter. Since there are 10,000 square centimeters in a square meter:

You'll occasionally encounter phots in older scientific literature, particularly papers from the mid-20th century. I've also seen it in some vintage photographic equipment specs. For modern work, it's mostly a historical curiosity, but if you're reverse-engineering an old design or reading a classic paper on photometry, knowing this conversion saves you a trip to Wikipedia.

Worked Example: Specifying an Ambient Light Sensor

Let's say you're designing a display backlight controller for an industrial HMI panel. The spec says it needs to work from "dim warehouse conditions" (about 50 lux) to "outdoor kiosk in direct sunlight" (up to 100,000 lux). The customer's test procedure document, written by their US-based quality team, specifies test points in foot-candles.

You're evaluating an ambient light sensor with a datasheet that gives sensitivity in counts per lux and a maximum measurable illuminance of 88,000 lux. Will it work?

First, let's convert the test points. The customer wants to test at:

• 5 fc (dim conditions)
• 50 fc (normal indoor)
• 500 fc (bright indoor / outdoor shade)
• 9,000 fc (direct sunlight)

Converting to lux:

That last number is a problem. The sensor maxes out at 88,000 lux, but the customer wants to test at nearly 97,000 lux. You have three options: push back on the test spec, find a different sensor, or add an optical attenuator (like a neutral density filter) in front of the sensor for the highest brightness range.

If you'd eyeballed "9,000 foot-candles is probably around 90,000 lux," you might have missed this. The actual number is about 8% higher than that rough estimate, and 8% is the difference between passing and failing the test.

Let's also express the full range in kilolux for the design review presentation:

Dynamic range: about 1,800:1. That's roughly 10.8 bits of range, so a 12-bit ADC should be adequate with some headroom.

Common Mistakes and Gotchas

Confusing Illuminance with Luminance

Illuminance (lux) measures light arriving at a surface. Luminance (candela per square meter, or nits) measures light leaving a surface toward your eye. They're related but not interchangeable. If someone asks for the "brightness" of a display in lux, they're confused—displays are specified in nits. If they ask for the ambient light level hitting the display, that's lux.

Forgetting the 10.764 Factor

I've seen engineers use 10 as a "close enough" conversion factor between lux and foot-candles. It's not. The error is 7.6%, which doesn't sound like much until you're trying to hit a regulatory threshold or match a calibration standard. Use 10.764, or better yet, open the Illuminance Unit Converter and let it do the work.

Mixing Up Phots and Foot-Candles

Both are non-SI units, both have "ph" or "f" in them, and both are about 10× different from lux. But they're different 10× factors in different directions. A phot is 10,000 lux. A foot-candle is about 10.764 lux. If you swap these, you're off by a factor of nearly 1,000. I've seen this happen in translated datasheets where someone "corrected" a unit they didn't recognize.

Assuming Linear Sensor Response

Most ambient light sensors have reasonably linear response across their specified range, but cheap sensors and photodiodes can saturate, have nonlinear regions, or exhibit temperature-dependent sensitivity. When you're converting units to figure out if your sensor can handle a particular illuminance level, remember that the conversion is exact but the sensor might not be.

Ignoring Spectral Mismatch

Lux is defined for the photopic response of the human eye, which peaks around 555 nm (green). If your light source has a weird spectrum—like a narrow-band LED or a sodium vapor lamp—the lux reading from a sensor might not match what a calibrated lux meter would show, depending on how well the sensor's spectral response matches the photopic curve. This isn't a unit conversion problem per se, but it bites people who assume that converting units is the hard part.

When to Use Each Unit

Stick with lux for anything international or any new design. It's the SI unit, it's what modern sensors are calibrated in, and it's what you'll find in IEC and ISO standards.

Use foot-candles when you're working with American lighting codes, OSHA requirements, or customers who spec things in imperial units. Don't fight it—just convert accurately.

Millilux is useful for low-light applications: night vision systems, astronomical instruments, or characterizing sensor noise floors. Kilolux keeps your outdoor measurements readable.

Phots? Honestly, unless you're reading old literature or working with legacy equipment, you can probably ignore them. But when you do encounter one, now you know: multiply by 10,000 to get lux.

Try It

Next time you're staring at a lighting spec in foot-candles and your sensor datasheet is in lux, don't do the mental math. Open the Illuminance Unit Converter and get the exact number in seconds. It handles the common reference points—moonlight, office lighting, bright outdoors, direct sunlight—and converts between lux, foot-candles, millilux, kilolux, and phots. One less opportunity to drop a decimal place or use the wrong conversion factor.