The xyz’s of Color Correcting Coatings

The perceived color of objects is easily affected by the spectral distribution of the illuminating light source. While this may be merely inconvenient when it comes to matching your clothes, it is extremely important in the surgical suite, and in the art and architectural worlds. We'll show you how AccuCoat works with the lighting industry to modify the color temperature or specific xy coordinates of light sources by applying color correcting filter coatings to achieve the perfect color every time.

Color correcting coatings alter the color of a light source upon transmission, sometimes just enough to be barely perceived with the human eye. These adjustments are important, however, as they ensure that artwork, architecture, and surgical suites are illuminated with the optimal light for the task.

In the lighting industry, luminaires are described most often by their color temperature, which assumes an incandescent bulb with the spectral distribution of a blackbody source. As lighting transitions to more efficient LEDs, this designation becomes both more arbitrary and difficult to achieve. Still, it is the standard, as our eyes associate a more blue-shifted “cooler” tint with natural sunlight (D65, or T_c=6500 K), while indoor lighting is typically “warmer”, having more intensity in the red and a lower color temperature (T_c=3000-4000 K).

Color correcting filters can be designed to shift color temperature up or down the scale, or to achieve a desired color rendering index (CRI) or specific set of chromaticity coordinates (xy). This allows lighting manufacturers more latitude to achieve the desired output intensity, then correct for the spectral distribution in front-end optics. This is particularly important in medical applications, in which doctors depend upon lighting to be both bright and consistent from one environment to another in order to provide the desired contrast when looking at tissues.

Dielectric-based interference coatings are well-suited to the task, reflecting a portion of the undesirable wavelengths while efficiently transmitting the others. They are robust, environmentally stable, and both light- and heat-tolerant. They can be designed with arbitrary transmission spectra to carefully shape the spectral distribution of a light source for use in medical or architectural applications, museums, and artwork – anywhere color rendering needs to be precise.

When a manufacturer of lighting assemblies comes to us at AccuCoat for a color correcting coating, they are looking to achieve a very specific color temperature and brightness. Doctors are very particular about color accuracy, whether of lamps in the surgical suite or an otoscope in the exam room. The color temperature of an illuminant can change how colors are rendered significantly – when controlled, it can improve contrast and reduce the variability and subjectivity of visual assessments.

The light source to be modified may be close in color, but need minor adjustments; or close in energy, but need a more dramatic color shift. Variations in the power supply may affect light output or color as well. If given the initial xy coordinates or color temperature and a set of target coordinates, AccuCoat can add and subtract different wavelengths as needed to rebalance the spectral profile to achieve the desired color.

It is often desirable to work in xy coordinates, as they provide a more accurate description of hue than does color temperature. Starting with the luminaire’s initial color temperature or CRI, we convert to xy. From there, we define the transmission curve needed to shift to the desired color, setting a range of x and y tolerances to be hit by the coating. This may translate into a box, triangle, parallelogram, or other shape on the chromaticity diagram, depending on the customer’s needs. Shifts of up to ±0.1 or ±0.2 in x and y can be easily achieved, as well as very small adjustments. Hitting the tolerance desired is never an issue, as we find that we are typically able to generate color correcting coatings with less variability in color than the luminaire’s manufacturing distribution.

Since both efficiency and cost are of paramount importance in the lighting industry, most of the color correcting coatings that AccuCoat applies are to an existing optical element in the system, such as a lens, reflector, or window in the optical assembly. We work with our customers to look at their needs on a system level, often satisfying more than one need to streamline the manufacturing process and maximize final radiant output of the luminaire.

This may be as simple as applying a color correcting filter coating to one side of an optic, and an anti-reflection coating to the other. Our ability to work with curved surfaces and unusual shapes on a wide variety of substrate types allows us to flexibly adapt to unique requirements. We can also incorporate color correction into a reflector coating to minimize system losses and reduce cost.

When needed, we can even provide some compensation for spectral variations of the light source – the spectral output of LEDs are known to vary with the angle from normal, and we can design a color correcting coating that helps to remedy that to generate a more spectrally uniform output.

By working with our lighting industry customers to understand the behavior and design of their optical system fully, we’re able to provide customized and often multi-faceted solutions that meet and exceed the desired targets for color temperature, output, and consistency – on time, and on-budget.

Contact us to discuss your unique needs for color correction today!