Artificial Light & Colour Rendering

The first thing to decide about any lamp is its colour temperature, measured in kelvin (K) and printed on every decent bulb's box. The scale is a little counter-intuitive: low numbers are warm (yellow-orange), high numbers are cool (blue-white). A candle is around 1800K; a warm incandescent-style lamp about 2700K; a 'neutral' or 'natural' white around 4000K; and a cool 'daylight' lamp 5000-6500K. The name comes from heating a theoretical object until it glows - it runs red, then orange, then yellow-white, then blue-white as it gets hotter - which is why a hotter number looks cooler to the eye.

Colour temperature sets the mood of a room and the cast it throws over every surface. Warm light (around 2700K) is relaxing, flattering and domestic; it makes warm colours - terracotta, timber, warm whites, skin - glow, and is the natural choice for living rooms, bedrooms and restaurants. Cool light (4000K and up) is crisp, alert and clinical; it suits kitchens, bathrooms, workshops, offices and any task needing precision, and it keeps cool colours - blues, greys, greens - clean. Get it wrong and a room fights itself: a cosy bedroom under 6000K 'daylight' LEDs feels like an operating theatre, and a kitchen under dim 2700K can feel gloomy and make food and worktops look grubby.

> Colour temperature is the light's accent. Pick it for the room's job and for the colours you want to flatter - warm to make warm colours sing, cool to keep cool colours clean.

A practical warning: mixing colour temperatures carelessly across one space reads as a fault, not a feature. A room where a 2700K lamp sits beside a 4000K downlight and a 6500K bare fitting looks broken - one pool warm, the next cold - and it undermines any colour scheme. Choose a temperature per space (or per clearly separated zone) and hold to it.

Colour temperature tells you the light's cast, but not whether colours look true under it - and those are different questions. Two lamps can both be a pleasant 3000K and yet render colour completely differently, because one has a full, continuous spectrum and the other has gaps. The number that captures this is the colour rendering index, or CRI: a score out of 100 for how faithfully a light shows a set of reference colours compared with a full-spectrum reference (daylight or incandescent). 100 is perfect; a good modern lamp for interiors should be 90 or above; below about 80, colours visibly suffer.

CRI is genuinely useful but it has a famous blind spot, and it is exactly the one that matters most in interiors. The general CRI score is an average over eight fairly muted test colours - and it does not include the strong, saturated red sample known as R9. Deep red is precisely the wavelength region where cheap LEDs are weakest (recall the spiky SPD from Lesson 3.1), so a lamp can post a respectable CRI of 80-plus while quietly rendering reds, and therefore skin tones, terracotta and warm woods, badly. This is why a lamp can 'test fine' on the box and still make a room look sickly. For colour work, always ask for R9 as well as CRI - a high CRI with a poor R9 is a lamp that lies about red.

The stakes are highest wherever red lives, which is almost everywhere that matters emotionally. Skin under a low-R9 lamp looks grey, tired and unwell - fatal in a bedroom, a restaurant, a salon or anywhere people gather. Food looks unappetising. Timber, brick, terracotta and warm textiles go flat and lifeless. Meanwhile the blues and greens might look fine, so the fault is easy to misdiagnose as a paint problem when it is really a lamp problem. For any space where colour and people matter, specify CRI 90+ and a solid R9 (say 50+), and treat a bargain bulb with no published R9 as a risk.

LEDs are the default now, and rightly so - efficient, long-lived and dimmable. But they created a new colour problem precisely because of how most of them make white light. A common LED starts with an intense blue emitter and coats it with a yellow phosphor; blue plus yellow reads as white to the eye and to a light meter, but the underlying spectrum is spiky, with a real dip in the cyan-greens and, crucially, weak deep red. That is the technical reason so many otherwise-modern rooms feel subtly wrong: the light is bright, efficient and 'white', and still starving the colours that carry warmth and life.

The fix is not to avoid LEDs but to read them properly and buy the good ones - the difference between a poor LED and an excellent one is now enormous, and often not much more expensive. On the box or datasheet, check four things in order: colour temperature in kelvin (does it suit the room?); CRI (aim 90+); R9 if published (aim 50+, and be suspicious if it is hidden); and consistency, expressed as a binning tolerance in 'MacAdam steps' - a low number means one batch of lamps will actually match each other, which matters when several light one wall.

Reading a lamp for colour - the quick checklist
  CCT (kelvin)   -> warmth: 2700 cosy / 3000 warm-neutral / 4000 neutral / 5000+ cool
  CRI            -> honesty: 90+ good, 95+ excellent, under 80 avoid for colour work
  R9             -> the red it hides: 50+ good; if not published, be wary
  Binning steps  -> consistency: fewer MacAdam steps = lamps that match each other
  Dimming        -> good LEDs 'warm-dim' toward 2200-2400K like incandescent; cheap ones just get dim and grey

One more subtlety worth knowing: many premium warm LEDs are designed to warm-dim, dropping toward 2200-2400K as you dim them, mimicking the cosy glow of a dimmed incandescent. Cheap LEDs just get dimmer and, if their R9 is poor, greyer and sadder. If evening ambience matters - and in living and dining spaces it usually does - warm-dim behaviour is worth specifying, because it keeps warm colours and skin flattering right down to candle-level brightness.

Pulling it together, choosing artificial light for colour is a short, disciplined sequence, and it belongs in the colour decision, not in a separate electrical package handed off and forgotten. First, decide colour temperature by room and mood: warm (2700-3000K) for living, dining, bedrooms and hospitality where you want warm colours and skin to glow; neutral to cool (3500-4000K, occasionally higher) for kitchens, bathrooms, studies and workshops where precision and clean cool colours matter. Then hold that temperature consistent within each space or clearly zoned area, so the room does not read as patched together.

Second, demand colour honesty everywhere people and finishes matter: CRI 90+ as a floor, a published R9 (50+) wherever red, skin, wood or terracotta appear, and matched binning where several lamps wash one surface. Treat an unbranded bulb with no colour data as unknown until proven otherwise - it is the single most common way a good scheme dies after dark. Third, think about layers, not one ceiling light: a mix of general, task and accent light lets you use warmth and brightness deliberately, and a good dimmer (ideally warm-dimming) turns one fitting into several moods.

Finally - and this connects straight to the next lesson - approve colour under the actual lamps that will be installed, not the site's temporary work lights. A colour signed off under a bare, cool, low-CRI construction bulb is a colour you have not really seen. The safest practice is to view final samples under the specified lamps and in daylight, because a colour has to survive both. Two colours that match beautifully under one light can diverge under another - the phenomenon of metamerism - which is exactly why on-site approval under real, final light is non-negotiable, and exactly where Lesson 3.4 begins.