June 8, 2017
Four Reasons to Use a Light Booth for Visual Assessment of Textiles
This article was published in the May 23, 2017 AATCC E-Newsletter.
A light booth is used to simulate various lighting conditions and to provide a consistent and accurate viewing environment for the visual evaluation of color. Light booths are designed to control the amount and balance of spectral energy, enabling a user to easily see how the selected light source will interact with a viewed object. If color is an important aspect of your product, then an industry-compliant light booth is a must-have.
The color and appearance of materials is dependent on the geometry, quantity, and spectral nature of the illumination, as well as the surround (viewing environment). ASTM D1729 specifies the conditions for visual color appraisal and matching, the use of different light sources to detect for a conditional color match, and it allows for effective communication of color among parties working together. Therefore, the key factor to consider when selecting a light booth is whether it meets the ASTM D1729 standard. Learn more about the ASTM D1729 standard.
Metamerism occurs when a person views two samples under one light source (e.g., daylight) and they appear to match but when the same person views the same samples under a different light source (e.g., incandescent) they do not. A color matching booth is the ideal tool to use for determining if metamerism exists. Color matching booths are designed with a minimum of three light sources and each source puts out the same intensity of light (foot candles or lux). In a booth, the sides are a spectrally neutral color (gray) to eliminate color contamination. A red/yellow source (i.e. incandescent lamps), a blue source (a daylight source at 6500 Kelvin is the standard choice), and a greenish source (e.g. cool white fluorescent) are the primary sources with which to start. Additional sources can be used to make certain products will have the proper appearance in the lighting environment under which they will be used or selected. An example of this would be using the same source as that used in a specific retail store chain. Read more about detecting metamerism.
Various Light Sources
The lower the color temperature of the light source, the warmer or redder the source will be. Inversely, the higher the color temperature of the source, the cooler or bluer it will be. The primary light sources used in color evaluation are:
- Daylight D65 (6500K) (blue source) A light bluish colored light source used in color matching applications of paints, plastics, textiles, inks, automotive, and other manufactured products. It accentuates blue and subdues green and red. D65 is commonly used as a primary light source in color measurement instrumentation
- Incandescent (yellow/red source) Simulates typical home or retail accent lighting. It is a standardized illuminant described in the international standard, CIE Publication 15.2004 and specified for use in color matching applications in ASTM D1729-2016. It is used where a yellowish-red source is required. It is the predominant source/illuminant used for both instrumental and visual color matching applications.
- Cool white fluorescent (CWF) (green source) It is a wide band fluorescent source commonly used in commercial lighting applications in North America. It is characterized by emitting high amounts of green and very little red energy. It has a color temperature of approximately 4150K and a CRI of approximately 62.
The additional sources are:
- Light energy not visible to the human eye, but is present in natural daylight. UV energy has the ability to excite optical brightener agents (OBAs) and fluorescent dyes and pigments within a sample causing them to emit light in the visible spectrum, usually in the blue region. These substances are used in various products to “brighten” colors, particularly whites. Ultraviolet primarily is used to detect the presence of optical brighteners in materials.
- An optional source (D50, D75, TL83, Horizon, LED). Used to ensure products will have the proper appearance in the lighting environment under which they will be used. An example of this would be using the same source as that used in a specific retail store chain.
- As more retail, office, and home environments switch to LED lighting it is becoming increasingly necessary to evaluate color in LED viewing conditions. The increased use of LED lamps is primarily being driven by the fact that they use less energy than other light sources. It should be noted that LED lamp technology is still rapidly advancing. As a result, it is difficult to ensure consistency of color temperature from lamp-to-lamp, batch-to-batch, and manufacturer-to-manufacturer. There is no official LED lighting standard for color matching. LED lamps are best utilized as an optional light source to gauge how the product may appear in an environment illuminated by a similar LED source. Learn more about various light sources and their use in color matching.
Trust Your Eyes
Spectrophotometers and colorimeters are valuable color evaluation tools. But they should not be the only way you evaluate color. When evaluating textiles, fabric weave and texture can affect a measuring device’s accuracy, therefore a visual match in a light booth should be part of the evaluation process. When the piece to be used as the standard for the color match has a different texture than the sample to which it is compared, it is not possible to rely on the data from an instrument and a visual color match must be made.
The fact is, instruments and light booths should co-exist in color management. Companies that use a light booth experience shorter production cycles, less redos, and better efficiency. When their suppliers and customers also use a light booth, the experience is further enhanced. With a standardized light booth, the supply chain is able to specify a light source under which to view the sample—resulting in better communication.
Keys to Success
The two keys to being successful with a light booth are using it and maintaining it. Booths that use older filtered technology are more expensive to maintain. These booths use high intensity filament type lamps and special glass filters, which change over time. To maintain proper color quality, these filters need to be occasionally changed and the booth needs a service call to be recalibrated and to confirm that it is still in specification.
Light booths that use newer fluorescent technology require much less maintenance and are more affordable to operate. To maintain conformance with industry specifications they simply need to be relamped after every 2500 hours of use.
When used regularly and properly, a light booth will improve your workflow, reduce cycle time, and show a positive return on investment.