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Color In A New Light


Color in a New Light   

 

By Jean Hoskin, PhD


Part 1: Why change to LEDs?

 

LEDs are replacing incandescent and fluorescent lamps due to environmental regulations, energy efficiency, and cost savings.  The Department of Energy projects the market share to be 48% of lumen-hour sales with a reduction of lighting energy consumption of 15% by 2020. 

 

Advantages of LEDs

·      Greater efficiency, less energy

·      Instant brightness

·      Longer life

·      Color stable over time

·      Shock resistant, durable

·      Versatile formats

·      Beams can be focused, directional, or linear

·      Cool light, radiates little heat

·      Dimmable

·      Glare managed by lamp design

·      Can be digitally managed to manipulate brightness, color, and mood

·      Environmentally friendly

Disadvantages of LEDs

·      Initial price is more expensive

·      Do not cast light evenly in all directions

·      Performance dependent on ambient temperature

·      Color objects perceived differently than in daylight and traditional sources

·      Blue hazard may interfere with sleep patterns

·      Production color quality is inconsistent 

·      No Ultra Violet energy for neon or fluorescent colors

 

LEDs have many advantages, but expect the appearance of colored objects to change.  The color of LEDs is different from traditional light sources. Ours eyes see color as light reflected from an object.  If a color is missing in the light source illuminating an object, we cannot see the color. The strong blue in an LED has diminished the brightness of warmer colors. 

 

Retail lighting has significant importance on the store image and the color of all merchandise.  Changes in lighting have been driven by LED energy savings and long life.  Since the color of merchandise is critical to sales, consistent color quality is equally relevant. An advantage for retailers is that LEDs are rapidly changing and improving. For example, the addition of a red component to some LEDs has improved the saturation of warmer colors. 

 

Resources:

·      Department of Energy  www.energy.gov

·      Energy Star

 

Color in a New Light    Part 2: What are LEDs?

 

LEDs are light emitting diodes. This is a solid-state semi-conductor that converts electrical energy directly into visible light.  The material used in the diode determines the color of the light. Because LEDs emit a single color, they were initially used for signal lights, such as the green power indicator on computers or household appliances.  Advancements have created a spectral power distribution (SPD) that appears as white light.  LEDs are very small and grouped in an array to create unique effects, providing flexibility in color and lighting design. In addition to differing in color from traditional light sources, LEDs have a lot of variation among themselves.   

 

Placing red, green and blue LEDs together and mixing the output creates a perception of “white” light. Another method relies of the mixing of complementary light to achieve white. Using a blue LED with a phosphor that emits complementary yellow light, the result is white light. However, these LEDs have a bluish cast because of high energy in the 450 - 470 nm wavelength region of the spectrum.   In this rapidly changing field, new phosphors are being developed to improve color quality. Some LEDs have the addition of red to provide greater color saturation in the warm region.  LEDs are continually improving, so it is important to understand the impact of developments on perceived color.  

 

The variations in the LEDs can be characterized by their spectral power distribution (SPD). SPD is the amount of radiant energy at each wavelength of the visible spectrum.  It clearly shows the peaks and valleys at specific wavelengths, indicating the impact it will have on perceived color. For LEDs it is also important to compare sources that provide an equivalent quantity of lumens. Lumens are a measure of the total quantity of visible light emitted by a source. Information on SPDs is available from the LED manufacturer or the resource True Colors from the Department of Energy.  (www1.eere.energy.gov/buildings/ssl/pdfs/true-colors.pdf)

 

CIE (Commission Internationale de L’Eclairage) is creating data for standards, which will be available in 2017.  Five SPD curves are proposed to represent LEDs of different color quality.  These curves are created by a tested method of averaging measurements and are intended for use as CIE Standard Illuminants in instrumental colorimetry. Understanding the SPD of the different LEDs will be critical for color evaluation. 

 

Lighting Manufacturer Resources:

·      Cree   www.cree.com

·      General Electric  www.gelighting.com

·      Philips  www.lighting.philips.com

·      Sylvania   www.sylvania.com

 

Color in a New Light   Part 3: Color Quality of Light Sources

 

Light color quality is described by appearance and color accuracy or fidelity. Color appearance is measured using Correlated Color Temperature (CCT). Color accuracy is measured using Color Rendering Index (CRI). 

 

CCT: A warmth or coolness of light along a continuum from red to yellow to white to blue. The progression is similar to the way a black body changes color when heated using temperatures in Kelvin (K). The higher the CCT is the cooler the appearance. Think of the expression “white hot”.  Bluish D65 has a CCT of 6500K. Less than 4000K are considered warm, such as Incandescent A with a CCT of 2850K. The CCT gives a general appearance of the light, but does not indicate the peaks and valleys along the visible spectrum. As a result multiple light sources can have the same CCT but significantly different spectral power distribution (SPD).   Although one can generalize that blues will appear duller in warm light sources, the CCT does not specifically indicate the appearance of individual colors.      

 

CRI: It is the ability of a light source to accurately display a group of 8 or 9 standard color samples compared to a reference light source. The smaller the average chromaticity difference the higher the CRI with 100 being the maximum rating.  The reference sources approximate incandescent and daylight depending on the CCT of the light tested.  The CRI is an average measurement, so color difference information about specific colors is not provided.  

 

Due to limitations of the CRI metric, in 2015 the IES (Illuminating Engineers Society) published a new measurement called TM-30-15. This metric better describes the characteristic of the light source by fidelity and gamut using a sample of 99 color standards from the full spectrum. Included in the results is a color vector diagram, which illustrates the shifts in hue and chroma in regions of color space. This provides color difference information about specific colors. 

 

Other color quality ratings factors included in Energy Star are: 

·      Color angular uniformity, the amount of variation of chromaticity across the beam angle of the bulb.

·      Color maintenance - change in chromaticity during operation

Chromaticity is the quality of a color determined by its dominant wavelength and purity.  The most obvious attribute of a product is color, which depends on the light color quality.  Information on CCT and CRI is available from the manufacturer. 

 

Additional Resources:

·      Department of Energy webinar on TM-30.

·      Lighting Research Center   www.lrc.rpi.edu

·      www.edisontechcenter.org/LED

  

Color in a New Light   Part 4: LEDs and Color Evaluation

 

Changes in retail lighting impacts product development, especially color selection and evaluation.   It is necessary to align the store lighting with visual and instrumental color evaluation throughout the supply chain for control and consistency. 

·      If you have the opportunity to collaborate with store and lighting designers on the selection of LEDs:  determine metrics, develop and test prototypes with a lighting manufacturer, then pilot and analyze selections. If the color team is able to participate early in the decision-making, it is possible to select store lighting that balances color quality with energy efficiency and cost. 

When the stores have installed LEDs, there are several action points for the color team.   Large retail teams who have already been through this process have found that the store design/lighting team is not aware of the product development color team and the color approval process. 

·      It is important to find a lighting expert. 

·      Determine store lighting. How are the LED’s being used?  Some light is for general use and some for accent.  What type of light is used? There are several types of light in the store environment, including daylight. 

·      If possible, create a store simulation showroom at your office to demonstrate the appearance of product under new lighting conditions. If done well this can show a number of lighting conditions, such as ambient, wall fixtures, and spot lights.

·      Another option is a light box that simulates the different lighting situations in the store. This requires measuring the lighting in the store and working with a light box manufacturer to verify and simulate the lighting. Although this light box may not be used for critical color approval, it does provide a model of how the colors will look in the store. 

·      Resource for inspiration: www.telelumen.com

·      When the garment is on the hanger in the store, it is all customer perception. 

 

 Color in a New Light   Part 5: LEDs and Instrumental Color Evaluation

 

It is not possible to reproduce store lighting with one standard.   Experts recommend selecting a set of standard illuminants including daylight.  For decision-making choose a set of standard illuminants that will reveal anomalies and metamerism.    

·      Currently there is no standard illuminant for LEDs. CIE is creating 5 standards available this year. When released, these illuminant curves can be installed in color measurement programs for spectrophotometers. It will be important to select one that best represents your color evaluation criteria and store experience. 

·      If the same merchandise appears in several stores, then the best solution is to select the CIE standard illuminants and stay with them. It makes a more consistent supply chain process.  

·      An alternative to these standard illuminants is to professionally measure the store lighting and create data to add to the color measurement software. This requires increased control over supply chain compliance. A change in the store lighting may require a change in your data set. In other words, constant monitoring and work. A better solution would be use the information as a reference for selecting the best CIE standard illuminant. 

·      As of the end of 2016, retailers updating their color process for LEDs had not rolled out new SPDs to the supply chain and were relying on existing CIE standard illuminants for approval decisions.  This is due to the cost and complexity of changes, the lack of consistency in LED production, and need for process accuracy and repeatability. 

Color Standards

·      After selecting the CIE standard illuminant, it will be necessary to evaluate the color difference of color standards between the current illuminant for evaluation and the LED illuminant.  Research currently done by retailers, who have switched to LEDs, indicated that color inconstancy was significant. Only 16% of the color standards tested had DE less than 1.0 and almost half… 45% had DE greater than 3.0.  This is a case for “inspect what you expect!!”

·      Future color standard selection will require review in multiple lighting conditions and the updated lighting specifications will have to be communicated to color standards suppliers. 

·      If you are innovation savvy and have crossed the threshold into more instrumental approval than visual, use technology to optimize the curve of your color standard for approval with curve matching and the reduction of metamerism issues. 

  

Color in a New Light   Part 6: LEDs and Visual Color Evaluation

 

There are several options for lighting for visual evaluation, but be aware that not all solutions are equal in color quality or cost.  Regular certification will be critical. 

 

Issues:

·      There are variations in peak wavelength of the diode. 

·      Different methods are used to create “white” light, there is no standard.

·      Due to large production tolerances, consistency varies

·      No UV

 

Options are positive, but will need to be balanced with cost.  

·      It is possible to retro fit a light box with LEDs, since they are compatible with T8 fluorescent ballasts; however, off the shelf LEDs are not all color critical. 

·      After the CIE illuminants are released, light boxes may be configured with light sources that represent these curves.

·      Hybrid light boxes are available which combine fluorescent and LEDs

·       Custom light boxes can be created by partnering with major lighting manufacturers

·      Software controlled LED for unlimited light simulations are available

·      16 tunable channels for daylight LED shows close tracking to the filtered tungsten daylight

 

Impact on the supply chain

·      The more complex the specifications are, the more initial costs are increased and supply chain compliance and training must be maintained. 

·      The more general the specifications, such as CCT, the more variations there may be in supply chain compliance.

·      SPD is the best solution, but it needs to be reproducible in a physical environment for visual evaluation. 

 

 

 

 

 

 

 

 

 

 

 

 

 

Posted: March 29, 2017

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