FAQ: "This is an example of the reject we are trying to detect. Correlated color temperature CCT is the main measurement I think I will need. We have been getting between 4000 and 6700 kelvin. How would you approach this application?"
This automotive headlamp is a combination of the LED lamp itself; a metalized reflector (typically sputter-coated aluminum on a thick, plastic substrate) and the transparent lens in front.
The image illustrates the color problem well. The yellowness could come from any of the three elements in the headlamp but the metallized reflector and the transparent lens are the most common sources.
CCT - Correlated Color Temperature is a measurement of the self-illuminating headlamp assembled from all 3 components. CCT does not correlate to visual but an acceptable CCT range for the headlamp color could be established. A range of 4000 kelvin (warm white, yellowish; bad or "NG" no good headlamp) to 6700 kelvin (slightly bluish white; cooler, more neutral; "good" headlamp) is a wide range, likely greater than a visible difference.
This CCT measurement using a spectroradiometer (not an instrument HunterLab sells) of the entire headlamp assembly, powered up, could determine that a "good", visibly acceptable headlamp was between, say, 5500 and 6700 kelvin. Headlamps representing the extremes of the good acceptance range would have to be visually verified to be an acceptable match when mixed together on the same car.
The problem with this CCT approach is that the measurement is taken on the final assembled headlamp. What do you do with a fully assembled, full-cost headlight if the color is unacceptable - throw it out? Also, how do you know which of the 3 components is causing the yellowness. A better option is measure the color of the individual headlamp components prior to assembly.
An UltraScan VIS can measure the yellowness (Yellowness Index per ASTM E313 D65/10 or C/2) of the metallized reflector in RSIN reflectance mode and the transparent lens in TTRAN transmission mode. Limits of acceptance can be easily defined by the client and YI measurements directly quantify the visual yellowness that is at issue here. Depending on the shape of the headlamp, cut-outs of the metallized reflector and transparent lens may be required.
The self-illuminating LED lights themselves would have to be measured by a spectroradiometer, but the LEDs are typically the most consistent color component of an automotive headlamp, and are not likely causing the problem.