Illuminance, CCT, and CRI

What is Illuminance ?

Illuminance refers to the total luminous flux incident on a surface, per unit area. It measures how much light falls on a surface, weighted by the human eye’s sensitivity to different wavelengths (the luminosity function). In simple terms: Illuminance is the amount of light that falls on a given surface.

The SI unit of illuminance is lux (lumens per square metre). A foot-candle, used in regions following United States customary units, is another unit of illuminance and is equal to one lumen per square foot—approximately 10.764 lux.

What is CCT (Correlated Color Temperature)?

Correlated Color Temperature (CCT) describes the color appearance of a light source, based on the temperature (in Kelvin) of an ideal black-body radiator emitting a similar hue. In simpler words: CCT tells us whether a light source appears warm (yellow/red tones) or cool (blue tones), based on its temperature.

The SI unit is Kelvin (K).

🔹Warm and Cozy (2200K–2700K): Creates a relaxed, inviting atmosphere. Ideal for homes, bedrooms, and dining spaces.

🔹Neutral White (3000K–3500K): A balanced light used for general illumination in offices, kitchens, and retail spaces.

🔹Cool White (4000K–4500K): Sharp, focused lighting suited for tasks, workstations, and garages.

🔹Daylight/Outdoor (5000K–6500K): Mimics daylight; perfect for outdoor lighting, parking areas, and sports facilities.

What is CRI (Color Rendering Index)?

The Color Rendering Index (CRI) quantifies how accurately a light source reveals the true colors of objects compared to a reference light, such as natural daylight. The CRI is determined by the light source’s spectrum.

The index is measured from 0 to 100, with a perfect score of 100 indicating that the colors of objects under the light source appear the same as they would under natural sunlight. Typically, light sources with a CRI of 80 to 90 are considered good, and those with a CRI of 90+ are considered excellent.

General rule: The higher the CRI, the better the color rendering capability.

The most commonly used CRI value is CIE Ra. The color rendering index Ra (general reference) is calculated from the mean of the first eight test colors (R1–R8). According to the EN 12464-1 lighting standard, a minimum Ra value of ≥ 80 is required for most activities. Other test colors (R9 to R15) are not included in the Ra calculation. These include: R9 – saturated red, R13 – skin color (light), and R15 – skin color (medium). This means a light source could have a high CRI score but still perform poorly when rendering red tones. That’s why lighting professionals often look at the R9 value in addition to the CRI.

The average of all 15 test colors is expressed as the Re value (reference extended), which enables a more precise evaluation of light quality.