Colour rendering deﬁnes the ability of a white light source to render object colours accurately. It is expressed by the general color rendering index (CRI) with values from 0 to 100, where 100 is the best (100 is provided by daylighting). Excellent color rendering index is achieved by lamps with a CRI above 90 which is e. g. needed in clinical areas in hospitals, health care buildings, museums, theatres, colour inspection/control/ selection and some types of retailers. In general, a CRI above 80 is considered sufﬁcient for accurate colour judgement in most indoor spaces.
The standard EN12464-1 speciﬁes minimum colour rendering requirements for practically all kinds of tasks by CRI (Ra). Table 1 shows concrete information on what CRI levels are recommended with minimum CRI 80 for nearly all types of rooms and applications except CRI 90 for health care examination and treatment.
The CRI is deﬁned by CIE as the average of the colour rendering indexes for eight test colours with low chromatic saturation. However there are 15 colour indexes. LED research has found that the index R9 (for the red colour) is very important for LED lamp colour rendering. It is recommended to use the IEA 4E SSL requirement of R9 > 0. Generally in lighting design, it is important to be aware that the colour rendering provided by the light source may be reduced by optics, glazing and coloured surfaces.
High-quality LEDs can maintain their light output for tens of thousands of hours. The built-in electronic driver, however, may show a sudden failure so the lifetime of the whole lighting system has to be considered.
The LED lamp lifetime is deﬁned as the period during which a given fraction of the total number of lamps (By) provide more than a pre-deﬁned percentage of the rated luminous ﬂux (lx), under standard test conditions,
e. g. L70B50 > 25,000 hours means that no more than 50 % of the lamps give less than 70 % of the rated luminous ﬂux after they have been used for 25,000 hour