Filter Wheel Against Full Spectrum
A colorimeter is fast, cheap, and wrong under the wrong light source. A spectroradiometer is slow, expensive, and right. Knowing which you need is half of calibration.
How Each Instrument Sees Light
A colorimeter works by passing light through a set of physical or interference filters — typically three to five, shaped to approximate the CIE colour-matching functions — and summing the signal from each photodiode. The result lands directly in tristimulus XYZ, which is exactly what colour science needs. The filters are fixed at manufacture, the electronics are simple, and the whole thing can cost less than a good lens. Speed is the other payoff: a reading takes milliseconds, which makes real-time profiling practical.
A spectroradiometer does the opposite: it disperses the incoming light across a diffraction grating or prism and samples the full spectral power distribution (SPD), typically in 1–5 nm increments from around 380 nm to 780 nm. XYZ is computed afterward by integrating the measured SPD against the colour-matching functions in software. You get the complete spectral fingerprint of the source, not a filtered approximation of it — which means accuracy is limited only by the instrument's detector sensitivity and wavelength calibration, not by how closely its filters happen to match the target illuminant.
Where Colorimeters Break Down
The problem lives in a word calibration engineers use carefully: matrix. Every colorimeter ships with a correction matrix (sometimes several, selectable by display type) that compensates for the fact that its physical filters cannot perfectly replicate the CIE functions. Those matrices are derived from spectral measurements of specific reference displays — typically white-LED-backlit LCD panels or P3 OLED units measured at the factory. Apply the right matrix to the right display type and the error is small: ΔE below 1 is achievable on a modern wide-gamut LCD or conventional WRGB OLED.
Now point that same colorimeter at a quantum-dot LCD, a blue-pump OLED with an unusual emitter blend, or a prototype microLED panel, and the spectral mismatch between the display's actual SPD and the reference SPD used to derive the matrix produces errors that the instrument cannot self-correct. The reading looks plausible; it is wrong by amounts that matter. A quantum-dot backlight's narrow-band green peak sits in a spectral region where small filter deviations produce large tristimulus errors. The colorimeter doesn't know it's lying.
A spectroradiometer has no such dependency on the source spectrum. It measures whatever arrives. The white point it reports is derived from the actual SPD, making it the reference-class tool whenever the display technology is non-standard, the target illuminant is tight, or the result will be submitted to a standards body.
Choosing the Right Tool
The practical decision tree is short. For routine profiling of production LCD monitors against sRGB or DCI-P3 with a known-good colorimeter matrix: use the colorimeter. Readings are fast, repeatability is excellent, and the economics of profiling hundreds of units a day demand it.
For any of the following, use a spectroradiometer: validating a display against a specification that will be signed off externally; measuring a panel whose spectral output has no established colorimeter matrix; characterising a novel emitter technology; or diagnosing why two displays that profile to the same primaries still look different. The last case matters more than people expect — two displays can share identical XYZ coordinates while having divergent SPDs, and only the spectroradiometer will show you why.
Cost does not go away. Entry spectroradiometers from makers such as Konica Minolta, Photo Research, or Jeti start at a multiple of what a professional colorimeter costs, and the highest-accuracy instruments are substantially more again. For a calibrator running consumer work, a well-characterised colorimeter with per-technology matrices is the defensible choice. When the tolerance on a deliverable tightens past what the matrix can guarantee, the spectroradiometer is not optional — it is the only honest instrument in the room.