We describe two mechanical light chopper blade designs that can be used, with a monochromator and broadband light-source-based spectroscopy setup, to perform wavelength modulation spectroscopy. The left and the right half of a beam emerging from a monochromator would have spectral distributions that are skewed to longer and shorter wavelengths about the central wavelength for positive grating orders. Our two designs, the alternating double-slot blade and the vertical right-angled blade, allow switching between the two halves of the beam front to generate a small periodic variation in the wavelength distribution, enabling wavelength modulation spectroscopy. In contrast to existing methods of wavelength modulation, this technique does not require modification of the monochromator or the light source unit. We analyze the signal waveforms and show how, and under what conditions, wavelength modulation data can be extracted from the alternating signal components measured by the detector. To demonstrate their functioning and applicability, we have performed wavelength-modulated reflectance spectroscopy on epitaxial semiconductor samples using these chopper blades. © 2005 American Institute of Physics.