Light booth

The function of the light booths is not simply to shine light on a sample. It is to produce a particular spectrum of light that will reflect the proper colors off the sample. If there are any shifts in the spectral output, the samples may or may not match. This is due to an effect known as Metamerism. This effect will cause samples to match under one lighting condition, but not... 

The conditions surrounding the light booth are important factors to consider during color matching. As you are about to see, these conditions can and should be controlled. 

Dimensions - The hanging dimensions should be set consistently with the rest of the industry. The distance of the hanging light booth to the surface of the perch must be set to 31" /- 2". The Light — Booth and the perch should be at a 30 degree angle and perpendicular to each other (Figure 2). 

Smoking should not be done in the location of the Light Booth. Tar residue sticks to the Daylight Filters, prismatic lens, reflectors and lamps. This will change the Color Temperature of the reflected light. 

Maintenance - Light Booth maintenance should be performed periodically between scheduled PMs. These are in accordance with the manufacturers cleaning instructions. After disconnecting input power to the unit the following items should be cleaned ... 

With the exception of the Observer Differences, all of these conditions can and should be controlled. If the light booth or the surround need to be painted, the paint formula is as follows PPG 80-110 Interior Latex Flat, B-46, L-2Y, 0-12. This will make one gallon of paint. Cleaning and maintenance of the reflectors and filters will keep the colors more vivid between scheduled calibration. 

Annual calibration and certification is not only a requirement for ISO and QS 9000, but is good practice for maintaining precision instrumentation in which the light booth is consid-... 

When we started this work, we had hoped to address the correlation of illumination/mea-surement geometry for the multi-angle instrument versus the visual evaluation conditions. This was suggested to us by the fact that we had customers with different conditions for visual assessment as many molders still utilize the conventional light booth. The two systems have different illumination geometiys and intensity levels and we have seen this affect visual assessment in the past. This work would require precise measurement of the angles involved and we concluded this was beyond the scope of our work. In the end, we came to view the multi-angle unit as a tool to measure differences and help us correct those that we saw as visually important. ... 

Color labs are outfitted with laboratory size equipment that simulates the larger machines used for production internally and by their customers. Typical processing equipment found in the lab are small extruders, two-roll mills, ban-burry mills, and media mills. Small rotational, injection and blow molding machines are used to duplicate the customers process. Instruments and computers are required for testing physical properties and color. Most labs have a computer-controlled color measuring system and a light booth to evaluate color. The spectrophotometer with computer is initially used to assist in colorant formulation and later as a quality control (QC) tool to provide certification of the quahty of match to standard. The light booth provides a standardized set of conditions to visually observe color and appearance. 

The principal use of the light booth is to compare a colored specimen with a control sample to obtain a visual assessment of the color match. The match and the standard can be compared under a variety of light sources to determine if metamerism is present. Metamerism occurs when a pair of samples appears to match under one light source but not under another. Metamerism is a common phenomenon and often occiurs when the two samples are colored with blends of different colorants. 

Figure 6-14. Light booth for visual color evaluation. (Courtesy of Gretag Macbeth.)...

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