Which light metrics should you specify for your application?
Correlated colour temperature and colour rendering index are becoming outdated as lighting metrics such as LED technology develops and customers become discerning.
Selecting the correct LED lighting products for both exterior and interior spaces has become essential in today’s fast-growing built environment and, with the advancement of LED, it is now easier to select suitable products that will complement the requirements of lighting projects.
Correlated colour temperature (CCT) and colour rendering index (CRI) are traditionally the standard industry terms used to specify the light produced by a luminaire.
CCT defines the general colour appearance of a white LED and is defined in Kelvin; warm light is around 2700 K, moving to natural white at around 4000 K, and to cool white at 5000 K or more.
Fig. 1: CRI is a quantitative measure of the ability to reveal the colours of the illuminated object.
Note that CCT does not tell you the exact colour of the light or anything about the colour quality of the light. CRI is a quantitative measure of the ability of a light source to reveal the colours of various objects faithfully in comparison with an ideal or natural light source (see Fig. 1).
While these two measures have been used in the lighting industry almost exclusively for decades, they are fast becoming increasingly outdated as LED technology and knowledge improve and customers become more discerning. More specific metrics are being introduced and LED manufacturers are creating more customised and optimised outputs.
Specialised CCTs, such as for meat and bread displays, were carried over into the LED age from fluorescent technology, and their uses are self-explanatory.
Fig. 2: Photopic and scotopic vision.
However, almost every LED manufacturer has developed a different approach to optimising lighting. Even when there are similarities, their unrelated terms give little indication as to their actual performance. “Brilliant Mix”; “Class A”; “Vivid”; “Décor”; “Pearl”; “Clear”; “CRI97” and “Utra-White” are a few of the terms used to describe specialised LEDs currently available. Collectively, these LEDs are often referred to as “below BBL” (black body locus).
LEDwise Lighting’s development engineers stay abreast with the performance of all current LEDs and adhere to the following five general rules to help with the selection of the correct luminaires:
Exterior lighting
During the day, humans use their highly colour sensitive photopic vision. At night however, colour becomes less important and more lux sensitive scotopic vision takes over. The eye’s sensitivity curve (known as the luminosity function or V(λ) curve see Fig. 2), also shifts its peak sensitivity wavelength from yellow to blue. This is why higher colour temperatures are mostly used for exterior LED lighting – usually 5000 K and above, with 5700 and 6500 K being common.
Fig. 3: Lower CRI is suited to outdoor lighting.
Exterior lighting is largely used for safety reasons, and high lux levels are therefore deemed to be more important than light quality. Therefore, monochromatic sodium vapour lamps (up to 200 lm/W) are traditionally used for streetlights despite their lower scotopic performance than higher CCT polychromatic light sources.
High CRI is therefore also usually not required for exterior lighting (see Fig. 3). LEDs can now have good CRI and high efficacy, so a CRI greater than 70 is usually considered to be sufficient, with luminaire manufacturers taking advantage of LEDs’ higher efficacy.
Interior domestic lighting
LED lighting for homes must try to replicate the “feel” of incandescent bulbs, so good CRI is essential (incandescent bulbs have a CRI of 100).
CRI greater than 80 is standard and the CCT of the luminaire is largely determined by the room to be illuminated and personal taste. Rooms such as living rooms and bedrooms which require an inviting, cosy ambience will use a CCT of 3000 K or less (see Fig. 4). Kitchens and bathrooms, on the other hand, require a brighter, crisper feel and will use a CCT of 4000 K and over (see Fig. 5).
Fig. 4: A CCT of 3000 K or less is perfect
for creating warm ambience.
Interior retail lighting
Retail lighting design is much more technical, with much more scope for optimising lighting based on specific applications. The typical choice of CCT is still 4000 K with CRI >90 becoming the accepted norm.
More and more LEDs which boost the vividness of colours and are therefore ideal for the retail environment are currently becoming available. They also maintain the “whiteness” of white at the same time, regardless of CCT.
Retailers have in the past felt that 3000 K produces whites which are too “creamy” while 4000 K can create too much of a “cool” appearance. A correlated colour temperature (CCT) 3500 K was therefore often selected as a middle ground – with the change in CCT being the only way to alleviate the perceived problems.
Fig. 5: CCT of 4000 K and over creates a brighter, crisper feel.
The choice of CCT will still be down to the individual retailer but the new generation LED (termed “Vivid”; “Brilliant”; “Class A”, “Décor” etc.) allows 3000 K to have bright whites and 4000 K to have bright, rich colours (see Fig. 6). The excellent overall performance of these LEDs, both in colour performance and energy efficiency, therefore makes them an ideal choice for the retail environment.
Fig. 6: With the new generation LEDs, 4000 K now provides “bright” whites and “rich” colours.
Commercial office lighting
The extended periods of time people spend in offices, often with very little and sometimes no natural light present, add extra complexities to lighting requirements. Minimising glare and temporal light artefacts is very important in creating a comfortable, productive work environment. Typically, a Unified Glare Rating (UGR) less than 19 is required, as well as excellent flicker performance.
The incoming acceptable metrics for flicker are Stroboscopic Visibility Measure or SVM <1 and a Short-term Flicker Indicator PstLM <1 as the currently-used current ripple indication (for example <10%) is not sufficient to describe flicker performance adequately.
The human body’s natural Circadian rhythm can be disrupted by extended exposure to artificial light and can lead to poor work performance and, more importantly, poor health. Office lighting which changes to reflect the natural changes in CCT during the day (lower CCTs in the early morning and late afternoon) can help alleviate these problems (see Fig. 7).
Fig. 7: Higher CCTs of 5700 and 6500 K with CRI >70 are generally
acceptable for interior industrial lighting.
Interior industrial lighting
For industrial illumination such as task and warehouse lighting where 24-hour per day operation is common, the focus is primarily on high lux levels and lm/W performance.
Higher CCTs of 5700 and 6500 K are popular with CRI >70 and are usually found acceptable. Some retailers like to have a constant CCT from warehousing through to despatch and, finally, in the store, with 4000 K being the most appropriate choice. Appropriate lens selection and custom aisle beams for example, can improve W/m2 performance vastly.
Contact LEDwise, Tel 021 447-3325, www.ledwise.co.za
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