The Colour rendering index (CRI) states how natural the reflection of colours is by the light. The CRI measuring unit is Ra. If bulbs have a low colour reflection index, colours will look unnatural. For comparison, sunlight reflects colours with a maximum natural CRI index of 100Ra. For LED bulbs this index constitutes 80-90Ra.
| Color rendering | Ra | Light source |
| Excellent | 90 – 100 | Incandescent bulb, halogen bulb |
| Very good | 80 – 89 | LED bulb |
| Good | 70 – 79 | Fluorescent bulb |
| Average | 50 – 69 | Metal halide bulb |
| Poor | <49 | Sodium and Mercury bulbs |
