Illuminance Converter

Convert between illuminance units including lux, foot-candle, phot, nox, and lumen per square meter.

Understanding Illuminance

What is Illuminance?

Illuminance measures how much luminous flux (light) falls on a surface per unit area. It tells us how bright a surface appears when lit by a light source. The SI unit is the lux (lx).

Think of it as "light density" on a surface.

Lux vs Lumen vs Candela

These photometric units measure different aspects of light:

• Lumen (lm): Total light output from a source (luminous flux)
• Candela (cd): Light intensity in a particular direction (luminous intensity)
• Lux (lx): Light received on a surface (illuminance = lm/m²)

Example: A 1,000 lumen light bulb might produce 500 lux on a desk 1 meter below it, but only 125 lux on a desk 2 meters away (inverse square law).

Inverse Square Law

Illuminance decreases with the square of distance from a point light source.

Where E is illuminance (lux), I is luminous intensity (candela), and d is distance (meters). Double the distance → quarter the illuminance.

Foot-candle (fc)

The foot-candle is the imperial unit of illuminance, defined as the light received on a surface one foot away from a standard candle. It's commonly used in the United States for lighting design.

A 100 lux office is approximately 9.3 foot-candles.

Phot (ph)

The phot is the CGS (centimeter-gram-second) unit of illuminance.

The phot is quite large—direct sunlight is about 10 phots. It's rarely used today, with lux being preferred.

Nox

The nox is a non-SI unit sometimes used in astronomy for very low light levels.

Despite the name similarity to lux, the nox equals one millilux, making it useful for describing dim lighting like moonlight or starlight.

Illuminance vs Luminance: A Critical Distinction

Similar to other unit confusions, illuminance and luminance are fundamentally different:

Recommended Illuminance Levels

Natural Light Levels

• Starlight: 0.001 lx (1 mlx) - Minimal visibility
• Quarter moon: 0.01 lx (10 mlx) - Objects barely visible
• Full moon: 0.1-0.3 lx - Can see outlines and shapes
• Deep twilight: 1-10 lx - Outdoor activities become difficult
• Civil twilight: 10-50 lx - Can read outdoors
• Overcast day: 1,000 lx - Comfortable outdoor visibility
• Full daylight: 10,000-25,000 lx - Bright but not harsh
• Direct sunlight: 32,000-100,000 lx - Maximum natural light

Photography and Exposure

In photography, illuminance affects exposure settings. The relationship between illuminance, ISO, aperture (f-stop), and shutter speed follows the exposure triangle. Higher illuminance allows:

• Faster shutter speeds (freeze motion)
• Lower ISO (less noise)
• Smaller apertures (greater depth of field)

Light meters measure illuminance (incident meters) or luminance (reflected meters) to determine proper exposure settings.

Safety and Regulations

OSHA and Building Codes specify minimum illuminance levels for workplace safety:

• Corridors/Stairways: 50-100 lx (5-10 fc)
• Warehouses: 100-200 lx (10-20 fc)
• General Office: 300-500 lx (30-50 fc)
• Detailed Office Work: 500-750 lx (50-75 fc)
• Precision Assembly: 1,000-2,000 lx (100-200 fc)

Insufficient lighting increases eye strain, reduces productivity, and can create safety hazards.

Measuring Illuminance

Illuminance is measured with a light meter or lux meter. These devices contain a photosensor (usually a silicon photodiode) with a cosine corrector to properly weight light arriving at different angles. The meter displays the illuminance directly in lux or foot-candles.

Many smartphones now include apps that estimate illuminance using the phone's ambient light sensor, though these are less accurate than dedicated instruments.

Color Temperature vs Illuminance

It's important to note that color temperature (measured in Kelvin) and illuminance (measured in lux) are independent properties:

• Color temperature: Describes the color appearance (warm/yellow vs cool/blue)
• Illuminance: Describes the quantity of light

A dim warm lamp (2700K, 50 lx) and a bright cool office light (5000K, 500 lx) have different colors AND different intensities.

Most Common Conversions

Quick Reference Cards

Typical Illuminance Levels