Correlated Colour Temperature and CRI: What the Driver Does Not Control

When a customer complains that two strips look different, the first instinct is to blame the driver. In most cases the driver is innocent. The apparent difference in the light comes from the LEDs themselves, from their correlated colour temperature (CCT) and their colour rendering index (CRI). Understanding the boundary between what a driver controls and what it does not saves a great deal of misdirected effort.

Correlated colour temperature describes whether light appears warm or cool, expressed in kelvin. A 2700 K strip looks warm and amber like an incandescent; a 4000 K strip looks neutral; a 6000 K strip looks cool and blue. The CCT is set by the phosphor coating on the LED chip and by the chip itself. A constant-voltage driver delivers a fixed DC rail and cannot change the colour temperature of the LEDs on that rail. If the strip is 3000 K, it stays 3000 K whether the driver dims it to 100 percent or to 5 percent.

What a driver can influence at the low end is subtle. With PWM dimming at a frequency that keeps the LED chip at its nominal current in short pulses, the colour temperature stays stable because the chip never cools enough to shift its emission. With analog dimming, the chip current falls and the colour temperature can drift slightly warmer at low levels. This is a driver-related effect, but it is small, usually tens of kelvin, and it is not the cause of two different strips looking different.

Colour rendering index, CRI, measures how faithfully a light source reproduces colours compared with a reference. A CRI of 80 is the common commercial minimum; 90 or better is the standard for retail, hospitality, and photography. Like CCT, CRI is fixed by the LED phosphor and the chip selection. The driver does not change it. A strip with CRI 80 will never render a red as accurately as a strip with CRI 95, no matter how the power supply is adjusted.

The practical consequence for a specifier is that colour consistency is a strip-buying decision, not a driver-buying decision. Two reels of strip from different production batches, or from different suppliers, will differ in CCT and CRI even when they are fed by identical drivers on the same 24 V rail. The way to control this is to specify the CCT and CRI explicitly, buy reels from the same batch when matching is critical, and accept that the driver's job is to deliver a clean, stable voltage, not to fix a strip that was purchased without a colour specification.

This division of responsibility is worth stating on a project file. The driver guarantees the electrical conditions: a regulated rail, dimming range, flicker performance, and protection. The strip guarantees the photometric outcome: CCT, CRI, lumen output, and colour consistency over time. When a job fails a colour requirement, the investigation should start at the strip's datasheet and batch, not at the driver, because the driver was never able to change the answer.

There is one colour parameter the driver does affect indirectly, and it is the reason the distinction is worth drawing so carefully. If a driver has high output ripple or a PWM frequency in the low band, it can cause a barely perceptible flicker, and flicker can interact with the eye's perception of colour in a way that makes two otherwise identical strips appear to differ. This is a quality problem, not a colour problem. Fix the flicker, and the strips match again because their true CCT and CRI were never the issue. For buyers, this is the practical test: when the light looks wrong, first confirm the strip's spec and batch, then check the driver's flicker and ripple, and only then conclude which component was at fault.