1. Field of the Invention
The present invention relates to color quality measurement, and more particularly, to a system that can measure the color quality of a rounded surface, such as a two-piece can.
2. Discussion of the Prior Art
A reflectometer is a well known optical measurement instrument that is used for quality control in color printing production processes. The most common use of a reflectometer is to measure ink on paper to control a printing process, although reflectometers are also used to measure photographic prints and other images on various substrates such as paper, plastic and metal, in order to monitor and control the production of the images or to test the production equipment. There are a number of specific types of reflectometers, including reflection densitometers, reflection spectrophotometers, colorimeters, and glossmeters, all of which operate according to the same general principle.
In general, a reflectometer directs a beam of light having a known color quality, i.e., a balance of red, green and blue light in conformance with ANSI/ISO standards, at a printed sample. By using a filtration system, the reflectometer then measures the amount of light which is reflected from the surface of the sample and generates an output signal which is indicative of the reflectance of the sample. The color density of the sample may then be readily determined, since density is the inverse logarithm of reflectance.
Illumination and measurement systems of the type employed in reflectometers are commonly referenced to an axis which is perpendicular to the sample. For example, according to accepted standards for densitometers, the illumination angle is either 45.degree. or 0.degree. degrees, and the measurement angle is then 0.degree. or 45.degree., respectively. It is then assumed that the light absorbed by the sample is the difference between the incident light and the reflected light. The sample color density can then be calculated according to an accepted logarithmic relationship.
Presently, reflectometers are used to take color quality measurements of flat samples, such as printed lithographed sheets or exposed films. However, visual inspection is used for rounded surfaces, such as plastic or metal containers, and in particular, on a two-piece can of the type used for beer and soda cans. This is due to the difficulty in maintaining accurate illumination and measurement angles when working with rounded surfaces.
Thus, it would be desirable to have a device that could accurately measure the color quality of a set of measurement points on a sample having rounded surfaces, such as a two-piece can.