Color Matching & Metamarism

MIDWEST TUNGSTEN Tips SERVICE Color Matching & Metamarism

Published now and again by MIDWEST TUNGSTEN SERVICE

Color control and color matching are important and quality of the light falling on it and by the to vacuum metallizers. Customers will often color of any substance the light is transmitted present a metallizer with a sample and ask him through. to color match it using a metal alloy, dip dye, or colored lacquer. In this TIPs we will discuss In fact, metallizers make use of the ability of the considerations that can assist the metallizer transmitted light to change color on a regular in determining if a successful match can be basis. Dyed lacquers change the color of the achieved. light transmitted through them, changing a silver refl ection into one that appears as gold, copper, Let's start with some basic defi nitions: or another color.

Hue - pure spectrum colors. There are three When a customer asks for a color match, it primary (red, blue, yellow) and three secondary is important to consider the use and display (orange, green, violet). Different color systems environments and attempt to duplicate those exist for printing, painting, or projecting light. conditions when making the match. Remember, light infl uences color. We will concern ourselves Saturation - The amount of color present. mainly with 4 types of light: Lower saturation moves toward gray. High saturation has vivid coloration. Daylight - when testing, indirect north light is preferred. A slightly overcast day is ideal. This Value - the relative lightness or darkness of a light has a slight blue cast color. A black & white photograph has no hue or saturation, only value. Incandescent light - will vary somewhat depending on wattage and operating temperature Tint - a pure color to which some amount of of the bulb. Generally contains more red and white has been added. yellow than other sources.

Shade - a pure color to which some amount of Fluorescent light - usually labelled as cool black has been added. white or warm light. It is important to note that this light contains several narrow bands of Gloss (Luster) - How shiny a surface is. light at different wavelengths, rather than a full Measured by a gloss meter. Do not confuse with continuous spectrum. brightness or lightness. A black surface can be glossy. LED light - is a newcomer to the scene. LEDs emit very narrow bands of light. White is Light - can be refl ected, emitted, or transmitted. obtained either by combining the light from red, The colors we perceive are infl uenced not only green, and blue LEDs or by using blue LEDs to by the color of an object but also by the color excite a light-emitting phosphor similar to that which coats fl uorescent tubes. ASTM D4086 - Standard Practice for Visual Evaluation of Metamerism The concept of metamarism is very important. Similar colors can be obtained with different ASTM E1499 - Standard Guide for Selection, combinations of hues, saturations, and values. Evaluation, and Training of Observers However, two combinations that match in daylight, may appear different under fl uorescent or LED light. Colors that appear different under varied lighting conditions are called metameric colors. This metameric effect makes it necessary to view parts under a variety of lighting conditions, especially those conditions that most closely simulate the use or display environment of the part.

A metallizer can build or purchase a cabinet which simulates a variety of lighting conditions in a controlled manner. These are known as color light booths or color assessment cabinets. Figure 1- Comparison of Light Source Spectra Original image created The interior will be a neutral shade and will by Typozon for Popular Mechanics' Great Lightbulb Test. contain a variety of standard light sources. It is important for the metallizer and the customer to agree on the protocol for determining color match. The ASTM standards listed below can help.

Another topic of consideration is that of ocular fatigue. When staring at a certain color for long period of time, your eyes adjust in such a way as to attempt to normalize that color. See Figure 2 for a practical example. Inspectors must take frequent breaks from their work in order to minimize ocular fatigue. Otherwise, their work will become suspect over time.

There are several industry standards which can be referenced for more information:

ASTM D1729 - Standard Practice for Visual Appraisal of Colors and Color Differences of Figure 2- Demonstration of Ocular Fatigue Stare at the black dot for 30 seconds without moving your eyes. Colored dots will gradually disap- Diffusely-Illuminated Opaque Materials pear into the gray background.

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