How do you measure the color of bare metal materials?

Bare Metal Brass Disks
Bare metal with a brightened, polished surface showing both color and gloss characteristics in the specular reflectance with a rapid fall-off to near black at the aspecular angles.

Bare metals are classified as opaque metals and do not allow any light to pass through them. In opaque metals, the specular reflection and diffuse reflection are key to defining the light interaction of this unique sample type. For bare metals both color and gloss are seen in the specular reflection which makes up a large portion of the reflection. A small amount of diffuse reflection results from imperfections in the metals surface that causes the specular reflection to scatter. This phenomenon can be seen in the picture above.

Since the color of a bare metal is contained predominately in the specular reflection these types of samples can only be measured in reflectance specular included (RSIN) mode. RSIN mode is only available on sphere instruments. Brushed metals that have a roughened surface behave in a similar way, however in this case color is also seen in the diffuse reflection. The roughened surface allows the light to scatter more resulting in a larger diffuse reflection and smaller specular reflection component then bare metals. In the case of brushed metals a 45/0 directional instrument could be used to take measurements.

HunterLab sphere instruments are excellent for measuring samples such as chrome faucets, aluminum pans, or gold candlesticks. The picture above shows brass disks that were successfully measured on an UltraScan VIS instrument in the RSIN mode.

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