Question 1

Illuminance

Accepted Answer

is the density of those lumens once they land on a surface. As mentioned, it is measured in lux (lx) (lumens per square meter) or foot-candles (fc) (lumens per square foot). It is crucial to distinguish illuminance from luminance. While illuminance measures the light arriving at a surface, luminance measures the light reflecting off that surface and traveling to the human eye. Luminance is measured in candelas per square meter ($cd/m^2$), often referred to as "nits." If you shine a 1,000-lumen flashlight onto a piece of black velvet and a piece of white paper from the exact same distance, the illuminance on both surfaces is identical. However, the luminance of the white paper will be drastically higher because it reflects the light, whereas the black velvet absorbs it. Finally, the working plane is an imaginary horizontal surface at the height where visual tasks are performed—typically defined as 30 inches (0.76 meters) above the floor for an office desk—which serves as the standard height for measuring illuminance.

Question 2

What is the difference between lux and lumens?

Accepted Answer

Lumens measure the total amount of visible light emitted by a light source in all directions, representing the raw output of a bulb. Lux measures the density of that light when it lands on a specific surface area, technically defined as one lumen per square meter. If you place a 1,000-lumen bulb in a tiny closet, the lux will be extremely high; if you place that same 1,000-lumen bulb in a massive warehouse, the lux will be extremely low because the same amount of light is spread over a much larger area.

Question 3

How many foot-candles are in a lux?

Accepted Answer

One lux is equal to approximately 0.092903 foot-candles. Conversely, one foot-candle is equal to exactly 10.7639 lux. This specific mathematical relationship exists because a foot-candle is based on one square foot, and a lux is based on one square meter; there are 10.7639 square feet inside a single square meter. To convert quickly in your head without a calculator, you can simply divide a lux value by 10 to get a rough estimate of the foot-candles.

Question 4

Can I accurately measure illuminance with a smartphone app?

Accepted Answer

While smartphone light meter apps are highly convenient, they are generally not suitable for professional, legal, or scientific measurements. Smartphones use their front-facing camera or ambient light sensor to estimate lux, but these sensors are designed to adjust screen brightness, not to act as calibrated photometric devices. They lack the physical cosine diffusers required to accurately measure light coming from sharp angles, and their software algorithms can be off by as much as 30% to 50% depending on the brand of the phone and the color temperature of the light source.

Question 5

Why do indoor plants need specific lux levels, and is lux the best measurement for them?

Accepted Answer

Plants require specific volumes of light energy to drive photosynthesis, and different species have evolved to thrive under different canopy conditions (e.g., ferns need low light, tomatoes need high light). While many growers use lux to gauge light intensity (often aiming for 15,000 to 50,000 lux for high-yield vegetables), lux is actually a flawed metric for botany. Lux is weighted to human vision (green/yellow light), whereas plants primarily use red and blue light. Professionals prefer measuring PPFD (Photosynthetic Photon Flux Density), which counts the exact number of photons in the specific color spectrums that plants actually consume.

Question 6

How does distance affect the illuminance of a surface?

Accepted Answer

Illuminance decreases exponentially as the distance from the light source increases, a principle governed by the Inverse Square Law. If you measure 1,000 lux on a desk that is 1 meter away from a lightbulb, moving the desk to 2 meters away will not cut the light in half; it will cut it to one-quarter (250 lux). Moving it 3 meters away will reduce the illuminance to one-ninth (111 lux). This exponential drop-off is why ceiling height is the single most critical variable when designing architectural lighting systems.

Question 7

What is the minimum recommended illuminance for reading and office work?

Accepted Answer

The widely accepted international benchmark for standard office work, reading printed documents, and computer data entry is 300 to 500 lux (approximately 30 to 50 foot-candles) measured directly at the desk surface. If the tasks involve highly detailed visual work, such as drafting, proofreading fine print, or intricate mechanical drawing, the standard increases to 750 to 1,000 lux. Sustained reading in environments with less than 200 lux forces the ciliary muscles in the eyes to overwork to maintain focus, leading to rapid visual fatigue and headaches.

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