Phosphorescent Acrylates Could Permit Nonelectric Illumination

Michael J. Lander

Lights and signs that phosphoresce brightly for an extended period could provide individuals with a sense of security when the electricity fails. Establishment of a full-color spectrum in these systems, however, requires red, green and blue sources. Green and blue phosphors exhibit a high brightness that diminishes slowly over time, but available red phosphors possess neither of these characteristics.

Green and blue phosphors were combined with three dyes to yield a full-spectrum phosphorescent array, seen here under dark conditions. Violet phosphor was used only in the three resins in the lower right-hand corner. Reprinted with permission of Optics Express.

Researchers led by Mitsunori Saito at Ryukoku University in Otsu, Japan, have used dye-doped acrylic resins mixed with green and blue phosphors to obtain warm-color illumination without the use of red phosphors.

The scientists first conducted tests to determine the properties of various phosphorescent materials. After exposing them to fluorescent light for five minutes, they used a Soma Optics Ltd. multichannel spectrometer to measure the emission spectrum of a metal-doped red phosphor and of green and blue phosphors of europium- and dysprosium-doped strontium aluminum oxides.

Although the former displayed stronger emission at a specific wavelength, its total irradiance was comparatively low. Furthermore, the green and blue phosphors continued to emit light for up to three hours — six times as long as their red counterpart. The investigators also found a violet phosphor to have good intensity, although its short-wavelength emission was hard to see.

The group went on to combine the green phosphor with the dyes kiton and rhodamines 6G and B — selected for their high red-conversion efficiency — in photocurable acrylic resin to create mixtures with a red or orange afterglow. The three combinations as well as the red phosphor were excited with blue, white or daylight from Matsushita Electric Industrial Co. Ltd. and Toshiba Corp. fluorescent lamps. Ninety minutes later, measurements with an Ando Electric Co. Ltd. (now Yokogawa Electric Corp.) optical power meter revealed that afterglow intensities from the dye-doped resins ranged from 0.004 to 0.009 nW/mm², compared to 0.0012 nW/mm² for the red phosphor. They also shone visibly for two to three hours.

On the basis of these results, the researchers used green, blue and violet phosphors with various proportions of the three dyes to form an array of 49 resins with violet to red emission colors. Since most of the mixtures’ afterglow intensities decayed at a similar rate, displays created from them would maintain their color balance until they go dark. Because the resins appeared a different color under bright conditions, the same displays could show different images as light conditions change.

The researchers are working with partners in industry to develop phosphorescent films for application to signs and walls for emergency illumination. They also are examining the human eye’s response to light conditions to design a better method of evaluating afterglow brightness.

Optics Express, Feb. 19, 2007, pp. 1621-1626.