When an automotive "chrome" mirror or headlamp enclosure is referred to, it is typically sputter-coated aluminum on a plastic substrate to simulate a chrome surface.
The “Reflectivity” or "Color Reflectivity" of an automotive chrome mirror is measured as:
Color Reflectivity = [(L* x L*)/100] + offset
Where L* is the CIE L* Lightness value for D65/10
The offset (default is 0) is a nominal value that may be used for to improve inter-instrument agreement.
A High Chrome Mirror would have a L* = 82, a* = -0.73, b* = 0.76 D65/10
Color Reflectivity = [(L* x L*)/100] = [(82 x 82)/100] = 67.24
A Blue Tint Mirror would have a L* = 76.24, a* = -8.27, b* = -7.63 D65/10
Color Reflectivity = [(L* x L*)/100] = [(76.24 x 76.24)/100] = 58.12
The Blue Tint Mirror has a lower Color Reflectivity than the normal High Chrome Mirror due to higher chromatic absorption.
Targets and tolerances will vary with the product, but the overall goal is lot-to-lot consistency for a given mirror surface type.
Further Notes on Color/Reflectivity/Yellowness measurement of Automotive Mirrors or Headlamp Enclosures
The highly reflective, metallic chrome color is inherently neutral but can develop trace amounts of yellowness or "gold tint", due to process variation or trace contamination. This can become important when automotive mirrors or headlight enclosures are mixed from different lots.
Being metallic in nature, the color is visually seen in the specular reflectance (shine) of the mirror surface. To measure metallic color, a CIE d/8 sphere instrument measuring in the RSIN (Reflectance Specular Included) mode is required.
A portable MiniScan EZ d/8 SAV sphere offers the ability for placement of the measurement head on a flat surface within the headlamp. It does not matter which interior facet is measure, just as long as the measurement head is in solid contact with the metallic surface. The 8-mm area of view will average the signal well. A single reading is sufficient, but averaging 2 readings per measurement is more robust.
The most repeatable measurements for this type of sample will be obtained on a flat surface with the measurement head of the instrument in solid contact with the mirror sample.
Sometimes, a shape of the headlight reflector enclosure does not allow solid contact with the mirror surface. Two alternatives are:
- Make a cut-out of the reflector enclosure that allows solid placement at the measurement port.
- Create a flat "witness coupon" representative of the process. A witness coupon is a 35 mm x 35 mm flat piece of the same polymer composite, run through the same vacuum-coating metallic process. This would give a representative, flat, uniform sample with the same coating as the headlight reflectors. This type of mirror sample would be easy to measure on a variety of HunterLab sphere instruments.
As head light reflectors are mirror-like, it is recommended that an aluminum mirror (very stable over time; verify mirror surface is clean and not abraded) be measured before beginning sample measurements. This aluminum mirror serves as an PQ (Performance Qualification) step to ensure the instrument is set up correctly and measuring consistently over time. Baseline the mirror in the color scales used for the application, then measure the mirror daily (can SPC results over time) to verify that the measured values are in close agreement to the original baseline values.
There are several color metrics that can provide process color information.
- Y reflectance is an overall indicator of the reflection efficiency of the headlamp surface; the higher the better.
- CIE L*, a*, b* provide a full numerical descriptor of the color and dL*, da*, db* quantify the degree of color difference between a standard color and sample.
- L* represents Lightness with 0 being a perfect black; 50 a middle gray; 100 a perfect white. L* is also used as a basis for Color Reflectivity measurements.
- a* represents redness-greenness. Positive values of a* are red; negative values of a* are green; 0 is neutral.
- b* represents blueness-yellowness. Positive values of b* are yellow; negative values of b* are blue; 0 is neutral
- Elliptical (DEcmc or dE00) color differences greater than 1 can suggest there may be a perceptible difference between a sample and a standard, or can be expanded to define an acceptable limit.
- Yellowness Index (YI E313 D65/10) might be a suitable companion index to CIE L*, a*, b* values to quantify small lot differences in yellowness from lot-to-lot.
- While full L*, a*, b* values quantify the color fully and should be recorded, I prefer to use a single metric like Yellowness Index E313 D65/10 as a single number for quantifying the trace degrees of "gold tint" in a neutral chrome color. Delta Ecmc or dE00 elliptical differences are good metric to predict when the yellowness difference is visually significant.
FAQ: "Is the SAV (Small Area of View) model the best choice of MiniScan EZ d8 sphere sensors? My client said he had flat plaques. Would the LAV (Large Area of View) give a better reading?"
The diameter of the MSEZ diffuse sphere is the same - 75 mm (3 in), for both LAV (Large Area of View) and SAV (Small Area of View) models but the collection lenses, port diameters and viewed sample areas are different.
- MSEZ Diffuse Sphere LAV views a 20.0 mm sample area within a 25.0 mm port.
MSEZ Diffuse Sphere SAV views a 8.0 mm sample area within a 14.3 mm port.
As a general rule it is always best to measure the largest area of sample view possible to better area average the reflected signal. In this case, the MSEZ 4000L will provide a 3:1 area averaging advantage over the MSEZ 4000S.
Use the MSEZ 4000L for all automotive mirror surfaces unless the size of the sample is very small, requiring the MSEZ 4000S model.