The Wrong Bulb Changes the Picture

How Temperature Bends Colour

Light has a colour, even when it looks white. Physicists measure it in Kelvin: a low number is warm (orange-red), a high number is cool (blue-white). A standard old-fashioned tungsten bulb runs around 2,700 K. Midday daylight sits near 5,500–6,500 K. Most LED bulbs sold for domestic use land somewhere in between, though the range varies wildly by brand and label.

That number matters because paint pigments do not reflect a fixed colour — they reflect the wavelengths available in the light that hits them. Hang a painting under a warm tungsten source and the light is already weighted toward orange. Yellows deepen and intensify; blues go dull and slightly greenish; cool greys turn beige. Switch to a daylight-balanced source at 5,500 K and the same surface looks flatter, cooler, more truthful to how the painter mixed those colours — since most historical painters worked by north-facing window light, which is diffuse and sits in that cooler range.

The practical gap between a 2,700 K domestic bulb and a 5,000 K gallery-grade LED is not subtle. Earth pigments — the ochres, siennas and umbers — glow under warm light; they were laid in by painters for whom warm, earthy tones were the foundation of the palette, so warm sources can feel period-appropriate. But a Prussian blue or a lead white mixed toward the cool end of the palette will look like a different paint under tungsten than under daylight. The painter made decisions at a specific colour temperature. You are either honouring those decisions or overwriting them.

What to Look for on the Box

When choosing a bulb for a picture, ignore the wattage and read the Kelvin rating, usually printed on the front of the box or listed in the product spec. For most paintings — oils, watercolours, acrylics — a lamp rated between 3,500 K and 5,000 K will treat the surface most fairly: warm enough to avoid clinical coldness, cool enough not to flood the palette with orange. Many museums use sources around 3,000–3,500 K for period paintings precisely because the slight warmth flatters the surface without distorting it badly.

The second number worth reading is the CRI — Colour Rendering Index. Rated out of 100, it measures how accurately a light source renders colour compared to natural daylight. Cheap LEDs often score 80 or below; a CRI of 90 or higher makes a visible difference, particularly in mid-tones and shadow passages where colour distinctions are subtle. Together, Kelvin and CRI are the two controls you actually have.