1. Field of the Invention
Embodiments of the present invention generally relate to optical Commission Internationale de l'Eclairage (CIE) color-matching filters and, more specifically, to a method for determining optical colorant thicknesses for deposited filters designed through the use of varying densities of a controlled set of colorants, such that, when weighted by an available illuminant spectrum, the deposited filters approximate CIE-like color-matching filters weighted by a colorimeter illuminant spectrum.
2. Description of Related Art
Optical filters are used in many color-related applications, including various color measurement systems, such as colorimeters. There are many types of filters, including absorptive filters, interference filters, and others. A photoelectric tristimulus colorimeter is used to measure the color of light incident on the device. If such light is emitted from a light source, such as a computer display screen, the colorimeter is performing an emissive color measurement. However, the subject of this invention is a reflective photoelectric colorimeter. Such a colorimeter has a light source that directs light through an optical system against a color sample. The light is reflected by the color sample toward three or more photoelectric detecting devices. A primary filter is positioned in front of each photoelectric detecting device. In one technology, each primary filter substantially conforms the spectral sensitivity of the photoelectric detecting device to a linear combination of the color-matching functions. A measuring device, which is connected to the photoelectric detecting devices, reads or measures the amounts of the respective primaries or tristimulus values in response to the incident light. It may, however, be desirable to estimate the tristimulus values that would be produced by the sample under a reference light (such as a CIE standard illuminant), even when the actual colorimeter light has a known but different spectrum. In that case, each primary filter substantially conforms to a linear combination of the color-matching functions, weighted by the ratio of the spectrum of the reference light to the spectrum of the actual colorimeter light.
Although it is theoretically possible to design primary filters exactly corresponding to an ideal, it is practically impossible to manufacture primary filters having transmission factors exactly corresponding to the ideal. This is because an error is inherent in measuring primary or tristimulus values of the color sample. This error is caused by differences between actual and theoretical transmission factors of the primary filters.
Past attempts to correct this error have involved attempts to alter the transmission factor characteristics of the primary filters by forming the primary filters using a number of superimposed colored plates. However, because the spectral characteristics of the colored plates depend upon the components of the materials used in the plates—normally glass—it was generally impossible to exactly match the theoretical transmission factors. It was prohibitively difficult to accurately duplicate the theoretical transmission values over the complete wavelength range of the primaries or tristimulus values. These past attempts that increased the number of plates, undesirably decreased the amount of light received or passed through the primary filter. In addition, past attempts to fabricate primary filters by carefully superimposing a number of plates in an attempt to match theoretical transmission factors were time consuming and expensive to make.