45°/0°, 0°/45°, and Annular Ring Illumination Optics

To illuminate an object, spectrophotometers have one of two types of illumination optics: sphere or 45°/0° ("forty-five zero" or 0°/45°). Illumination optics are selected to include or exclude the specular component (glare). The colors of roughened surfaces of some textiles (flannel), paints (sand paints), and plastics (textured polystyrene) depend on the angle at which they are viewed.

When measuring color, we are generally measuring a surface that has been colored by a pigment or dye. White light passing through the colorant has some wavelengths removed, resulting in the sensation of color. Specular reflections represent light that has reflected from the surface and thus not passed through the colorant. Generally we wish to exclude this specular component from the measurement, as it is not related to the color of the object.

Using 45°/0° viewing delivers a good approximation of the typical visual evaluation technique. 45°/0° includes the effect or appearance of gloss and texture in the measurement. This appearance measurement simulates how the human eye would see a sample.

The 0°/45° design is based on the setup of a visual evaluation where the incident light is overhead at 0 degrees and the viewing angle is 45 degrees (or the optics are reversed as in 45°/0°). It has been shown through experimentation that the same results can be achieved with 45°/0° as with 0°/45°. An object can be evaluated with the light source directly overhead and viewed at 45 degrees (preferred), or the same evaluation can be obtained by illuminating an object at 45 degrees and viewing directly overhead.

If an object has a specular component of reflection (glare), it will be excluded by 45°/0° illumination automatically. Why? Light that is reflected specularly will be reflected at the same angle as it strikes the object. With 45°/0° illumination optics, light that is reflected away at 45 degrees will not be "viewed" by the spectrophotometer.

An advantage of 45°/0° instrument design is that it can provide annular (ring) illumination of the object. Annular illumination optics light the object at 45 degrees in a complete circle. To visualize this, imagine a cone whose apex angle is 45 degrees to the cone's axis. The object is placed at the tip of the cone. The sides of the cone represent light striking the object. The object is "viewed" by the spectrophotometer along the cone axis, through the base of the cone