The Coefficient of Utilization (CU) is a measure in lighting design that indicates how efficiently a lighting system uses the light it generates within a space. It is expressed as a value between 0 and 1 , where 1 means that all of the light is used effectively and 0 means that almost all of the light is wasted .
1. How is the CU calculated?
The CU is influenced by several factors, including the characteristics of the room and the lighting fixtures . The basic formula is:
whereby:
- CU = Coefficient of Utilization (how much light actually reaches useful places).
- Lumen output of fixtures = Total light emitted by the lamps.
- Work surface illumination = How much light actually reaches the work surface.
📌 A higher CU means more efficient lighting in the room.
2. Factors influencing CU
| Factor | Influence on CU |
|---|---|
| Reflection of walls, ceiling and floor | The brighter the surfaces, the higher the CU (more reflection and less absorption). |
| Space ratio (RCR, FCR, CCR) | A lower RCR means better light distribution and a higher CU. |
| Type and placement of luminaires | Direct lighting usually gives a higher CU than indirect lighting. |
| Luminaire reflection and light distribution | Well-designed luminaires with high light reflection enhance the CU. |
📌 Light-colored walls and ceilings help retain more light in the room, which increases the CU.
3. Why is the Coefficient of Utilization important?
✅ Determines the lighting efficiency → How well the light is used within a space.
✅ Helps optimize energy consumption → Less wasted light means less energy waste.
✅ Improves light uniformity → Prevents dark corners and shadow areas.
✅ Used in lighting calculations → Essential for correctly dimensioning lighting in offices, shops and industrial buildings.
📌 A low CU means that a lot of light is lost to reflections or absorption, which requires additional lighting and energy.
4. Applications of CU in lighting design
🏢 Offices → Maximum lighting efficiency for energy savings and productivity.
🏪 Retail & showrooms → Optimal lighting to highlight products without wasting light.
🏗 Industry & warehouses → Good light distribution for safety and working comfort.
🏡 Residential lighting → Avoid dark corners and increase light quality.
📌 Lighting designers use CU tables and simulation software to calculate the optimal lighting for different room types.
5. How can the CU be improved?
🔹 Use light-colored walls, ceilings, and floors → Less light absorption, more reflection.
🔹 Place fixtures strategically → Make sure they distribute light optimally and don't direct too much light into unusable areas.
🔹 Use luminaires with high reflection efficiency → Luminaires with well-designed reflectors can increase the CU.
🔹 Avoid deeply placed fixtures in high rooms → Light placed too high can be lost before it reaches the workspace.
📌 A good CU means that a room is optimally lit without unnecessary energy waste.
💡 In short:
The Coefficient of Utilization (CU) is an important metric that determines how efficiently light is used in a space . A high CU ensures energy savings, better light quality, and optimal illumination in offices, industry, retail, and homes . Lighting designers use the CU to make lighting systems as effective as possible .