{"id":3800,"date":"2026-10-01T10:53:09","date_gmt":"2026-10-01T02:53:09","guid":{"rendered":"https:\/\/www.ihylight.com\/?p=3800"},"modified":"2026-10-01T10:55:03","modified_gmt":"2026-10-01T02:55:03","slug":"cob-led-color-shift-at-operating-temperature-understanding-%ce%b4uv-duv-and-sdcm-from-25c-to-85c","status":"publish","type":"post","link":"https:\/\/www.ihylight.com\/ko\/cob-led-color-shift-at-operating-temperature-understanding-%ce%b4uv-duv-and-sdcm-from-25c-to-85c\/","title":{"rendered":"COB LED Color Shift at Operating Temperature: Understanding \u0394u’v’, Duv, and SDCM from 25\u00b0C to 85\u00b0C"},"content":{"rendered":"

\"COB<\/p>\n

A COB LED that meets its color target at 25\u00b0C may move to a different chromaticity point at operating temperature. The shift can come from temperature<\/p>\n

dependent changes in the blue pump, phosphor conversion, encapsulant, drive current, and thermal path. CCT alone may hide that movement. A useful 25\u00b0C-to-85\u00b0C evaluation records chromaticity coordinates and interprets three quantities: \u0394u’v’ for total movement, Duv for tint relative to the Planckian locus, and SDCM for position within a color\u00a0tolerance region.<\/p>\n

Define which temperature is controlled, keep electrical and optical conditions consistent, allow the COB to stabilize, and compare the same metrics at every setpoint. Otherwise, the temperature and color results cannot be connected reliably.<\/p>\n

Why COB LED Color Shifts as Temperature Rises<\/strong><\/h2>\n

A white COB combines many LED dies with a phosphor\u00a0conversion system. As junction temperature rises, the blue emission spectrum and phosphor conversion efficiency can change at different rates. The resulting spectral balance may move the measured chromaticity even when input current remains nominally constant. Optical materials and package construction can add further temperature dependence.<\/p>\n

The magnitude and direction are product\u00a0specific. One COB may move mainly along the CCT direction, another across the green\u00a0magenta axis, and another along a curved path. Drive current, thermal resistance, phosphor formulation, optics, and measurement geometry influence the result. The U.S. Department of Energy’s LED color stability research<\/u><\/strong><\/a>\u00a0also identifies operating temperature and current as important influences.<\/p>\n

An 85\u00b0C chamber ambient, 85\u00b0C case temperature, and 85\u00b0C junction temperature are not equivalent. The report should name the controlled temperature and record case temperature at the manufacturer’s defined point. Estimate junction temperature only with applicable thermal data. IHY Lighting’s guide to COB LED thermal management<\/u><\/strong><\/a>\u00a0explains why case, ambient, and junction temperatures must remain separate.<\/p>\n

What \u0394u’v’, Duv, and SDCM Actually Tell You<\/strong><\/h2>\n

These metrics describe different relationships. They should be reported together rather than treated as interchangeable limits.<\/p>\n

\"CIE<\/p>\n

\u0394u’v’ Measures Total Chromaticity Movement<\/strong><\/h3>\n

In the CIE 1976 uniform chromaticity scale, a source is located by u’ and v’. The point\u00a0to\u00a0point difference between a baseline and a hot measurement can be expressed as:<\/p>\n

\u0394u’v’ = \u221a[(u’\u2082 – u’\u2081)\u00b2 + (v’\u2082 – v’\u2081)\u00b2]<\/em><\/p>\n

This unsigned distance shows how far the color point moved, but not its direction. Retain the coordinate pairs or a chromaticity plot because two COBs can have the same \u0394u’v’ while moving in different directions.<\/p>\n

Duv Shows Tint Relative to the Planckian Locus<\/strong><\/h3>\n

Duv is a signed distance from the Planckian, or blackbody, locus at the source’s CCT. In normal white\u00a0light interpretation, positive Duv lies above the locus and tends toward a greener tint; negative Duv lies below it and tends toward a pinker or more magenta tint. Duv is especially useful when a small CCT change produces an unacceptable\u00a0tint change.<\/p>\n

Duv is not the same as the \u0394u’v’ distance between 25\u00b0C and 85\u00b0C. A COB can move largely along the locus, producing measurable \u0394u’v’ with little change in Duv. It can also cross the locus, changing the sign of Duv even when the total movement is modest. Record both Duv at each setpoint and \u0394Duv from the 25\u00b0C baseline.<\/p>\n

SDCM Describes Tolerance Around a Target Color<\/strong><\/h3>\n

SDCM, commonly discussed through MacAdam ellipses, expresses how far a chromaticity point sits from a target region in perceptual step units. A lower step specification defines a tighter color\u00a0tolerance region. Because the ellipses change in size and orientation across the chromaticity diagram, SDCM does not have a universal one\u00a0to\u00a0one conversion to \u0394u’v’ or Duv.<\/p>\n

SDCM shows whether a hot\u00a0state color point remains inside the agreed bin or application tolerance. It does not replace the coordinate path. Retain the target chromaticity, ellipse definition, baseline point, hot point, and measurement uncertainty.<\/p>\n

A Repeatable 25\u00b0C to 85\u00b0C Test Method<\/strong><\/h2>\n

A controlled test controls thermal, electrical, and optical variables that can imitate color shift.<\/p>\n

    \n
  1. Define the temperature basis. State whether 25\u00b0C and 85\u00b0C refer to ambient, case, or another controlled reference. Identify the case measurement point and the method used to estimate junction temperature, if required.<\/li>\n
  2. Fix the operating point. Use the intended driver, dimming state, thermal interface, mounting, heatsink, optics, and enclosure. Record actual current and voltage at every setpoint.<\/li>\n
  3. Establish a stabilized baseline. At 25\u00b0C, wait until both thermal readings and photometric values stop changing materially. Measure spectral power distribution, CCT, x and y, u’ and v’, Duv, flux, and relevant colorquality metrics.<\/li>\n
  4. Increase temperature in defined steps. Intermediate points such as 40\u00b0C, 55\u00b0C, and 70\u00b0C can reveal nonlinearity that an endpointonly test would miss. Stabilize before each measurement and use the same instrument geometry.<\/li>\n
  5. Measure at 85\u00b0C. Repeat the complete optical and electrical record. Calculate \u0394u’v’ from the baseline, calculate \u0394Duv, and plot the coordinate path against the agreed SDCM boundary.<\/li>\n
  6. Return to 25\u00b0C. A recovery measurement helps distinguish reversible temperature behavior from permanent drift or damage caused during the test.<\/li>\n
  7. Test more than one unit. Compare units and production lots when color matching matters. A single sample cannot characterize manufacturing variation.<\/li>\n<\/ol>\n

    Instrument calibration, chamber uniformity, stray light, self\u00a0heating, stabilization time, and fixture repositioning contribute uncertainty. State these controls so teams can compare supplier data on the same basis.<\/p>\n

    Turn the Measurements into an Acceptance Specification<\/strong><\/h2>\n

    Avoid a requirement such as \u201cno visible color change from 25\u00b0C to 85\u00b0C.\u201d It is subjective and omits the thermal reference, initial bin, observer conditions, and measurement method. Use testable requirements instead:<\/p>\n