The distribution of light across wavelengths. From that it derives correlated colour temperature, colour rendering and fidelity indices, chromaticity coordinates and the deviation from the black-body locus, alongside illuminance. A lux meter collapses the whole spectrum into a single weighted number, so it can tell you a space is bright enough while being unable to distinguish a high-quality light source from a poor one.

When a colour property has been specified, promised or disputed. Checking delivered LED luminaires against their stated colour temperature and rendering index, investigating why two fittings look different, verifying lighting in retail, museum, medical or inspection settings where colour judgement matters, and horticultural applications where the spectrum is the functional requirement. For confirming light levels alone, an illuminance meter is the correct and cheaper instrument.

Colour temperature describes the appearance of the light itself, from warm to cool, expressed in kelvin. Colour rendering describes what the light does to the appearance of objects lit by it, compared with a reference source of the same colour temperature. They are independent: two sources can share a colour temperature and render colours very differently, which is why both are specified and both need measuring.

Because two sources can share a correlated colour temperature while sitting on opposite sides of the locus, one tinted green and the other pink, and they will look mismatched side by side even though their specification sheets agree. The deviation figure captures that difference. It is the measurement that most often explains a complaint about fittings looking different when the headline colour temperature is identical.

Simpler colour meters report a limited set of derived values without capturing or presenting the full spectral curve. They are cheaper, quicker and adequate for screening and for confirming that a batch is broadly consistent. A full spectroradiometer provides the underlying spectrum, supports the wider set of metrics and carries more weight in a dispute. Choose by whether the result needs to be indicative or defensible.

Many can, and it is increasingly a reason to buy one. Flicker and stroboscopic effect from LED drivers cause discomfort, headaches and, around rotating machinery, a genuine safety hazard, and they cannot be assessed by eye at higher frequencies. Instruments that support it report modulation depth, frequency and standard flicker metrics. Confirm the specific metrics an instrument provides against whatever your specification or client requires.

Stray light from other sources, which is why measurements are best made with competing sources off or shielded. Distance and aiming, since these instruments measure what falls on or enters the sensor. Temperature, both of the instrument and of the luminaire, because LED output and colour shift as a fitting warms, so allow it to stabilise before recording. And calibration, which should be traceable and current for the wavelength range in use.