Color Matching and Metamerism

Color control and color matching are important to vacuum metallizers. Customers will often present a metallizer with a sample and ask him to color match it using a metal alloy, dip dye, or colored lacquer. In this TIP we will discuss the considerations that can assist the metallizer in determining if a successful match can be achieved.

Let’s start with some basic definitions:

Hue – pure spectrum colors. There are three primary (red, blue, yellow) and three secondary (orange, green, violet). Different color systems exist for printing, painting, or projecting light.

Saturation – The amount of color present. Lower saturation moves toward gray. High saturation has vivid coloration.

Value – the relative lightness or darkness of a color. A black & white photograph has no hue or saturation, only value.

Tint a pure color to which some amount of white has been added.

Shade – a pure color to which some amount of black has been added.

Gloss (Luster) – How shiny a surface is. Measured by a gloss meter. Do not confuse with brightness or lightness. A black surface can be glossy.

Light – can be reflected, emitted, or transmitted. The colors we perceive are influenced not only by the color of an object but also by the color and quality of the light falling on it and by the color of any substance the light is transmitted through.

In fact, metallizers make use of the ability of transmitted light to change color on a regular basis. Dyed lacquers change the color of the light transmitted through them, changing a silver reflection into one that appears as gold, copper, or another color.

When a customer asks for a color match, it is important to consider the use and display environments and attempt to duplicate those conditions when making the match. Remember, light influences color. We will concern ourselves mainly with 4 types of light:

  1. Daylight – when testing, indirect north light is preferred. A slightly overcast day is ideal. This light has a slight blue cast
  2. Incandescent light – will vary somewhat depending on wattage and operating temperature of the bulb. Generally contains more red and yellow than other sources.
  3. Fluorescent light – usually labelled as cool white or warm light. It is important to note that this light contains several narrow bands of light at different wavelengths, rather than a full continuous spectrum.
  4. LED light – is a newcomer to the scene. LEDs emit very narrow bands of light. White is obtained either by combining the light from red, green, and blue LEDs or by using blue LEDs to excite a light-emitting phosphor similar to thatwhich coats fluorescent tubes.

The concept of metamarism is very important. Similar colors can be obtained with different combinations of hues, saturations, and values. However, two combinations that match in daylight, may appear different under fluorescent 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. The interior will be a neutral shade and will 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 the figure below for a practical example.

Ocular Fatigue

Demonstration of Ocular Fatigue. Stare at the black dot for 30 seconds without moving your eyes. Colored dots will gradually disappear into the gray background.

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, Click to View