Standard Deviation Colour Matching, SDCM

What is Standard Deviation Colour Matching, SDCM?

SDCM is an acronym which stands for Standard Deviation Colour Matching, which is to distinct LED color variation level where the human eye cannot perceive the color difference. It has the same meaning as a “MacAdam ellipse”. It plays an important role in LED technology and the commercial lighting field.
MacAdam Ellipse
The MacAdam ellipse is a system of colour measurement. It measures how much colour variation is possible around these axes, before the human eye detects a colour change. A series of ellipses can then be drawn around any target colour, and the closer any given lamp is to the target, the less colour deviation will be experienced when these lamps are placed side by side in an installation.

The spectrum of the standard light source changes with the color temperature, so the standard spectrum is different when the color temperature is different (generally, the detection equipment will automatically identify the color temperature range of the LED light source to be measured, and determine the corresponding color temperature value of the standard light source), and the color tolerance is different. At the same color temperature, the reference standard spectrum is the same, and the color coordinates x and y are different, then the color tolerance is different.
SDCM defines the tolerance in the color temperature with a number. If the LEDs all fall within 1SDCM (or a “1-step MacAdam ellipse”), the color difference between LED chips cannot be discerned by human eye. 2-3 SDCM means there is hardly any visible color difference while 4 or more SDCM is readily noticeable. The smaller the number is, the smaller tolerance with respect to the XY color space will be. But be noted that it only represents the color comparison under a certain light source, not under different light sources.

Most indoor environments require the high color Consistency,  this leading our indoor lighting's Leds are usually around MacAdam 2-3 SDCM(Vertex downlight, spotlight, cabinet light...etc).
For most exterior applications, a rating of MacAdam 5 SDCM is perfectly acceptable for flood light, outdoor garden light...etc.
Cheaper products will often use LED modules that have a range of MacAdam Ellipses beyond four, some going as high as eight. Fixtures using such modules need to be used with care. There may be general commercial of industrial areas where they are acceptable, but any requirement for colour sensitivity would rule them out. 
Why is it important?
The color consistency or chromaticity consistency of the light source is important because if you have more than one lamp in close proximity the difference in colour will be noticeable and reflect on the surroundings. Uniformity will not present which could create an undesirable lighting solution.
The problems are greatest when lighting a white surface, or placing a LED spotlight very close to a white wall.