Colour Temperature And Colour Rendering Are Separate Claims

Lighting specifications combine two numbers that sound related and describe different things. One states the tint of the light, the other states how faithfully objects appear under it, and a source can score well on one and badly on the other.

Colour temperature describes the light itself

The figure comes from the colour a theoretical object would glow at a given temperature, which is why higher numbers correspond to bluer light and lower numbers to warmer light.

It is a description of appearance rather than of heat, so a cool white lamp is not colder in any physical sense and may consume more power than a warm one.

Two lamps sharing a colour temperature can still look different, because the figure locates the light along one line and says nothing about how far off that line the source sits.

Colour rendering describes what the light reveals

An object has no colour without illumination. It reflects the wavelengths present in the light striking it, so wavelengths missing from the source cannot appear in the reflection.

A rendering index compares the appearance of reference colours under the source against their appearance under a reference light, and reports how closely they match.

Sources with gaps in their spectrum score poorly, which is why some lighting makes skin and fabric look flat or wrong while the light itself appears perfectly white.

The spectrum behind the numbers is the real story

Many white light sources produce a narrow blue emission converted to broader yellow light by a coating, and the deep red end of the spectrum is often weak as a result.

A general index averaged across sample colours can hide that weakness, since one poor sample has limited effect on an average of many.

This is why supplementary figures reporting performance on strong red specifically exist, and why they are quoted in applications where reds matter.

Consistency between units is a third property

Manufacturing variation means lamps from the same batch differ slightly in colour, and producers sort them into bins to keep the variation within a stated tolerance.

A tight binning specification is expensive because it discards or downgrades more product, and it is the reason two lamps sold as the same colour can visibly differ.

The mismatch shows most in continuous surfaces such as ceilings, where two adjacent fittings are compared directly by the eye.

Colour also shifts with dimming and age

Some sources warm as they dim and others hold their colour, and which behaviour occurs depends on the driver design rather than on the lamp technology alone.

Over a long service life the conversion coating degrades, so the light gradually shifts in both colour and output, typically without the user noticing the change.

Maintained output figures address the second half of this, describing the fraction of initial brightness remaining at a stated point in life rather than at switch-on.