Optical Characteristics of Glow-in-the-Dark Materials

Jill Chen, John Noé, Harold Metcalf

Laser Teaching Center, Department of Physics and Astronomy

Photoluminescence is the emission of light from some material after that material has been stimulated by light. There are two kinds of photoluminescence: one is fluorescence, where the emitted light is released in a very short time; the other is phosphorescence, where the light is released after a long time (over seconds, minutes, hours, or even days). Fluorescence is well known from ordinary fluorescent lights and also from the ``black light effect'' that creates different colors of light when materials are exposed to ultraviolet light. Similarly, phosphorescence is familiar from ``glow-in-the-dark'' toys such as yo-yo's and decorative stars. Less well known is that phosphorescence also has many serious applications of considerable value to society, such as in safety products that require no electricity to operate.

The purpose of this project is to develop ways to characterize glow-in-the-dark materials by measuring their light output over time under different conditions (temperature, duration and type of activation). This information is important to guide the development of better materials and also can give insights into the complex physics and chemistry of how the materials work. The initial measurements [1] were done with a sample provided by Shannon Luminous Materials, Inc. in Santa Ana, California. It was labelled B-901 and described as ``mixed oxides; containing, we think, Al, B, Sr, Eu, and possibly, Hf.'' The glow light emitted by the sample after activation in room light was recorded in a dark environment with a silicon photodetector connected to a computerized voltmeter. Readings were taken once per second for 6,000 seconds, and then plotted in a spread-sheet program. The graph below shows the result.

We initially expected that the decay of the light would be an exponential function of time, which would give a straight-line plot on a log-linear graph. Instead we see that, after about the first minute, the decay curve is a straight line on a log-log plot, which corresponds instead to a power law, I(t) ~ 1 / t n. The exponent n in these data is very close to n = 1, as shown by the parallel red line with this slope. Future plans are to perform similar measurements with this sample under different conditions and with other samples, to see whether or by how much the exponent of the power law varies from unity. Samples will also be activated with specific colors of light of known intensity, and the absolute intensity of the glow light measured with a calibrated detector.

[1] This research began as a recent short project in the Introduction to Research course (WSE187) in the Women in Science and Engineering program, as described at http://laser.physics.sunysb.edu/~jill/report/