--- title: "Colour rendering and CRI in luminaire selection" subject: "Industrial Power Distribution and Illumination" chapter: 6 key: "semester_6/industrial_power_distribution_and_illumination:colour-rendering-cri" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/colour-rendering-cri site: BCT Engineering --- # Colour rendering and CRI in luminaire selection Industrial Power Distribution and Illumination · Chapter 6: System Components for Industrial Illumination - Asked in: 2080 Chaitra, 4 marks - Video: [What Is Color Rendering Index CRI In Lighting Design?](https://www.youtube.com/watch?v=NWh-dEuxaDQ) — Engr. Ahmed / Khadija Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/colour-rendering-cri - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-6 ## Question Why is color rendering important and how CRI help in selecting luminaries? ## Short answer Colour rendering is the ability to show true colours; poor CRI leads to errors. CRI (0–100) measures it; select luminaires with CRI ≥90 for colour-critical tasks (hospitals, printing), ≥80 for offices, and lower for warehouses, balancing task needs with efficacy. ## Answer **Colour rendering** is the ability of a light source to show the true colours of objects compared with a reference source (daylight or an incandescent lamp). It is measured by the **Colour Rendering Index (CRI or Ra)** on a scale of 0–100; 100 means colours look exactly as under the reference source. #### Why colour rendering is important - **Correct identification:** colour-coded wires, pipes, safety signs, and medicines must be recognised correctly; poor CRI can lead to errors and accidents. - **Quality control:** printing, textile, paint, food grading and hospital work need accurate judgement of colour. - **Visual comfort and appearance:** skin tones, goods in shops and interiors look natural and pleasant under high-CRI light. - **Productivity:** better colour contrast reduces eye strain and mistakes. #### How CRI helps in selecting luminaires | CRI (Ra) | Quality | Typical sources | Suitable application | |---|---|---|---| | 90–100 | Excellent | Halogen, high-CRI LED | Colour matching, art galleries, hospitals | | 80–89 | Good | LED, triphosphor fluorescent | Offices, schools, shops, homes | | 60–79 | Moderate | Metal halide, standard fluorescent | Industrial halls, sports areas | | 20–59 | Poor | Mercury vapour, HPS | Warehouses, street lighting | | < 20 | Very poor | Low-pressure sodium | Roads, security lighting | So the designer first fixes the minimum CRI the task needs (e.g. Ra ≥ 80 for offices as per EN 12464 / IS 3646, Ra ≥ 90 for colour inspection), then chooses among the lamps meeting it the one with the highest efficacy and suitable colour temperature. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does CRI measure? A. Lamp life in hours B. How truly a source shows object colours compared with a reference source, on a 0-100 scale C. Lamp efficacy in lm/W D. Colour temperature in kelvin Answer: B. CRI (Ra) compares colour appearance with a reference source; 100 is ideal. (Key point: CRI meaning) 2. Why is good colour rendering important? A. It reduces power consumption B. Correct identification of colour-coded items, quality control and visual comfort C. It makes lamps last longer D. It increases voltage Answer: B. Colour-coded wires, signs, printing, textile and hospital work need accurate colour judgement. (Key point: Importance) 3. How is CRI used in selecting luminaires? A. Choose the lowest CRI lamp always B. Fix minimum Ra for the task (e.g. 80 for offices, 90 for colour inspection), then choose the most efficient lamp meeting it C. Ignore CRI for offices D. Pick low-pressure sodium for colour matching Answer: B. The designer sets the required Ra first, then picks the highest-efficacy lamp that meets it. (Key point: Selecting luminaires) ## Related in Industrial Power Distribution and Illumination - [Flood lighting design and luminaire selection](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aflood-design-luminaire-selection) — Industrial Power Distribution and Illumination, chapter 8 - [Cable selection against fire, halogen-free cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-selection-fire-halogen-free) — Industrial Power Distribution and Illumination, chapter 3 - [Selecting main panel board and DB rating](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Apanel-board-db-rating) — Industrial Power Distribution and Illumination, chapter 5, asked 2× - [Selecting size and number of lifts](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alift-size-number-selection) — Industrial Power Distribution and Illumination, chapter 5 - [Selecting MCCB and MCB for MDB and SDB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amccb-mcb-selection-mdb-sdb) — Industrial Power Distribution and Illumination, chapter 5 ## Related in other subjects - [d-d transition and origin of colour](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ad-d-transition) — Engineering Chemistry, chapter 5 - [Material properties for selection](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Amaterial-properties-selection) — Machine Design I, chapter 2 - [Hardware selection for IS](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Ahardware-selection-factors) — Information System, chapter 6 - [Knee point voltage and CT selection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-selection) — Switchgear and Protection, chapter 2