Lighting products, specifically LED, often make reference to an ANSI specification C78.377-2008, which is associated with Correlated Colour Temperature, or CCT. The specification outlines standards / tolerances for colour consistencies among light sources. I.e. the acceptable degree of variation in colour temperature. These tolerances are identified by applying MacAdam Ellipses.
David Lewis MacAdam was a colour scientist working for Eastman Kodak in the 1940’s. In test conditions he measured how closely individuals could match one colour against another. He altered the colour controls with a method similar to RGB or CMYK, starting from different colour directions.
With the results MacAdam was able to deduce zones or (boundaries) around target colours. The average observer could not distinguish a difference in colour within the spectrum of these zones. When plotted on a XY chromacity colour space these zones took on the form of ellipses, hence the namesake of this unit of tolerance
As can be seen in the XY chromacity colour space above, some ellipses are larger than others. The average individual is more sensitive to colour changes in blue hues than green hues, thus the ellipses in the blue hues are considerably smaller. The size of the ellipses is based on standard variance called “steps”, 1 step is where the boundary of the ellipse is always just one standard variance from the target colour, 2 steps when the boundary is always 2 away from the target colour, etc.
The measurement of intolerances is very important when ensuring a lighting installation’s aesthetic is uniform. If lighting in a gallery or showroom was noticeably inconsistent it would compromise the overall appeal of the space.
The application of MacAdam Ellipse tolerances has increased in correlation with the adoption of LED light sources. Incandescent and compact fluorescent are subject to fewer variables which effect resultant CCT, as a result their ellipses tend not to exceed 7 steps. As LED light sources are subject to more variables their CCT is liable to stray further from the target colour and as a result a stronger emphasis is placed on MacAdam ellipses when regarding LED. For solid state light source like LED ANSI standard C78.377-2008 states that CCTs must be within 4 steps meaning, a much lower colour variance.
By opting for high quality LEDs that undergo the scrutiny of quality control, as well as “binning” (ensuring one batch matches another), 3 MacAdam ellipse tolerances are achievable with LED.