45c:0 Correlation to Multi-Angle Spectrophotometers Reply

When selecting a spectrophotometer to use for measuring and comparing color, it is particularly important to use the right geometry for the application. Sphere instruments are preferred for weathering studies and tinting strength for example; 45c:0 is preferred when harmonizing multi-component objects that may contain parts with different gloss levels; and multi-angle is preferred for materials that change appearance based on observation angle such as coatings with metallic or pearlescent pigments. One question that sometimes comes up regarding multi-angle spectrophotometers is whether the 45° measurement angle is the same geometry as a 45/0 or 45c:0 instrument. The answer is yes, and no.

The diagram below shows the geometry of a 45c:0 spectrophotometer:

 

The sample is illuminated circumferentially at 45°, and the reflected light is measured normal to the surface, referred to here as 0°.

This diagram shows the illumination and measurement angles of a multi-angle spectrophotometer:

 

The surface is illuminated at 45° off the normal, and the measurement angles are given in relation to the specular angle (opposite angle to the light source). Thus the 45° measurement angle in a multi-angle spectro corresponds to the 0° measurement angle of a 45/0 spectro. In both cases the sample is illuminated at 45 degrees off the normal, and the color is measured normal to the surface. So what’s the difference? Circumferential illumination.

45c:0 instruments illuminate the sample using a homogeneous 360° ring of light, with all light hitting the sample at 45° off the normal. This eliminates the influence of directionality, meaning the spectro can be oriented in any direction and the results of the measurement will be the same. Multi-angle spectrophotometers do not use circumferential illumination, instead illuminating the sample with a single beam of light from one direction. This is done by design; Multi-angle spectrophotometers are primarily used for measuring painted vehicles such as cars, boats and airplanes, all of which are most often viewed outdoors. Since typical viewing conditions involve light coming from a single direction (the sun) the light source in multi-angle spectros is designed to mimic these viewing conditions. Thus the 45° measurement angle in a multi-angle spectro will produce similar results to a 45c:0 spectro but may differ slightly due to the influence of directionality. Note also that the notation “45c:0” is relatively new, with the “c” meant to indicate circumferential illumination. Many spectros that are labeled as 45/0 or 45:0 also use circumferential illumination.

Panel Match – When Matching to a Standard Isn’t Enough Reply

If you’ve ever used a spectrophotometer for color QC, you probably have a general idea of how it works: measure a standard or use a stored one, then measure the sample. The color difference is calculated, and if that difference falls below a certain level it means the sample is a good color match to the standard, and the product is ready to be shipped. The assumption here is that any parts that are a good color match to the standard will also be a good color match to each other, and this is unfortunately not true. Two samples can be a good match to the standard, both visually and instrumentally, but may not be an acceptable match to each other. This is where the panel match function comes in.

Take a look at the set of three flooring tiles below:

 

 

 

 

 

 

 

Each tile is labeled on the back with a 1, 2, or 3, and when placed in numeric order as above, they appear to be a good color match to each other. But look what happens when we switch Tiles 2 and 3:

 

Suddenly the tiles don’t appear to match! Nothing has changed about the tiles themselves, only the configuration in which we’re viewing them. To understand the cause of the apparent mismatch, we look to the color data:

 

Here we see that Tile 2 is slightly lighter, less green, and less yellow than Tile 1. This trend continues when comparing Tile 3 to Tile 2. However, the color differences between Tiles 1 and 2 are not large enough to be discerned visually, nor are the differences between Tiles 2 and 3. We can see from the table and color plot below that when Tile 2 is used as the standard, Tiles 1 and 3 both fall within tolerance:

 

However, if we set Tile 1 as the standard:

 

Suddenly Tile 3 is no longer within tolerance! This explains why the colors appear to match when lined up in numerical order, but when the order is switched such that we are directly comparing tiles 1 and 3, they no longer appear to match. Now suppose Tile 2 is used as the master standard and Tiles 1 and 3 are production parts. Tile 1 would be measured against Tile 2 and found to be an acceptable match, as would Tile 3. When the customer goes to install Tiles 1 and 3 side-by-side however, they would be upset to find that the tiles do not match! This is why when testing parts that will end up being viewed next to each other, it is necessary to compare those parts not only to the master standard, but to each other as well. This is where the panel match function comes in; by using Organizers in BYK-Gardner’s smart-chart software, when a color reading is taken of a sample to compare to the standard, another sample can be selected to compare the reading with as well. This ensures that not only will production parts match the desired color standard, but will also be a good visual match to other samples when viewed in a real-world setting.