Design and performance of a variable spectrum synthesizer

Abstract

We present the design and performance of a broadband variable spectrum synthesizer (VSS). The VSS system is capable of synthesizing multiple arbitrary spectra using an arc lamp as a source, a spectrograph to disperse the light, a digital micromirror device to select wavelengths and adjust intensities, and optical components to focus and direct the light. The VSS can provide radiation with a spectrum of selected wavelengths dispersed over an area or recombined into a narrow line. The area mode is most useful when imaging the spectrum across a target area is desired. The wavelength dispersion in this mode provides a spatially modulated spectral profile for comparing responses to the wavelengths in the selected bands. The line mode has utility in cases where a light signal with many superimposed spectra is desired. Both modes can simulate the spectra with a very specific wavelength, intensity, and exposure time characteristic. The current VSS system provides intensity and wavelength variations over a range from 250 to 1100 nm and can level wavelength intensities variations to within 15%. The instrument wavelength resolution was measured as 2.0 nm±0.5 nm and the maximum power density delivered was 220 μW/cm2 nm (365 nm). Although the VSS has applications in a number of physics and engineering disciplines, the control software and application described are tailored for measurement of wavelength-induced apoptosis/necrosis of skin cancer cells in the field of clinical phototherapy. An experiment using psoralens coupled with UVA light and an experiment using visible light in photodynamic therapy are discussed. The experimental results obtained for radiation of wavelengths between 300 and 700 nm at various exposures on living cells are presented.