LED-spectrum shaping by optical filter achieves tunable daylight simulator

The many advantages of LEDs—such as compactness, long life, high brightness, saturated color, low power consumption, and short response time^1,2—are helping the industry to develop rapidly. Indeed, LEDs are being considered as a possible standard light source. However, at present they cannot replace white light because of their poor color rendition, usually evaluated by the color-rendering index (CRI). The CRI was established by the Commission Internationale de l'Eclairage to assess how closely colors subjected to a test illuminant resemble their appearance under a reference illuminant.³ The maximum CRI is 100.

Recently, a proposal was made to create a daylight simulator using LEDs.⁴ However, the electric control gear for the number of LEDs required—40, for example, or even 15—is complicated, and the heat generated in each device makes the colors susceptible to shifting.⁵ Consequently, we have devised a daylight simulator that reduces the number of LEDs to six. The key advance was to place an optical filter at the light exit, and to tune the electric driving current of each LED so that the daylight simulator matches a range of correlated color temperatures (CCTs). The CCT is the temperature (in degrees Kelvin) of a Planckian radiator (a natural emitter) whose perceived color most closely resembles that of a given stimulus seen at the same brightness and under specified viewing conditions.³

The optical filter takes advantage of the phenomenon of thin-film interference. The thin film consists of a stack of at least two materials with high and low refractive indices, enabling a desired wavelength to pass through. The materials we chose were titanium dioxide (TiO₂) and silicon dioxide (SiO₂) (high and low refraction indices, respectively) to design an optimized filter consisting of 19 layers using Essential Macleod simulation software. We employed physical vapor deposition with ion-beam assist to coat the optical filter at 180^°C. The coating chamber was pumped to 1×10⁻⁵torr, and a proper amount of O₂ was fed into it during deposition. The layer thickness of the TiO₂ and SiO₂ was controlled by an optical monitor. The error of thickness between the coated and designed optical filter was within 1%, as shown in Figure 1.

Figure 1. Spectrum of the optical filter for color correction.

Figure 2. Spectra for comparing standard daylight, LEDs, the optical filter, and LEDs with the optical filter. The spectrum of LEDs with the optical filter matches standard daylight very well under 5000, 6500, and 7500K CCT conditions, respectively. The background to each spectrum shows the color at that CCT. a.u.: Arbitrary units.

The daylight simulator created with our optical filter comprised six LEDs—warm white, deep red, green, blue, royal blue, and UV—to reduce the complications of circuit design and the heat problem of the LEDs. Modulating the driving current of each LED combined with the optical filter (i.e., the tunable daylight simulator) successfully achieved CCTs that ranged from 4000 to 10,000K, which covers the needs of most practical applications. The resulting CRI easily exceeds 97. Figure 2 shows some of these results.

Shaping the spectrum of LEDs using optical filters thus improves this source of light for applications such as photography, color-industry measurement, diamond testing, art lighting, and general lighting. The technology can also supply convenient and reliable daylight illumination for a number of fields of study, including green energy and the so-called Helmholtz-Kohlrausch effect.^6–9 This is the name given to a phenomenon where two color stimuli have the same luminance (a measure of brightness), but the stimulus with the greater saturation is perceived to be brighter. Consequently, it is possible to provide some spaces with perceptually brighter illumination while keeping energy consumption low.¹⁰

The coated filter can still be improved, since transmittance at some wavelengths is lower than that of the design shown in Figure 1. Several of the filter layers are too thin (only a few nanometers) to be deposited in appropriate thickness by ion-beam-assisted evaporation. Instead, we intend to try deposition using ion-beam sputtering with tantalum and silicon targets. In our experience, this approach is more controllable than evaporation in both thickness and refractive index, although the coating process is slower.

This work was sponsored by the National Science Council, Taiwan, under grant NSC95-2221-E-008-057-MY3.