Significance: Spatial frequency domain imaging (SFDI) is a diffuse optical measurement technique that can quantify tissue optical absorption (μ_a) and reduced scattering (μs′) on a pixel-by-pixel basis. Measurements of μ_a at different wavelengths enable the extraction of molar concentrations of tissue chromophores over a wide field, providing a noncontact and label-free means to assess tissue viability, oxygenation, microarchitecture, and molecular content. We present here openSFDI: an open-source guide for building a low-cost, small-footprint, three-wavelength SFDI system capable of quantifying μ_a and μs′ as well as oxyhemoglobin and deoxyhemoglobin concentrations in biological tissue. The companion website provides a complete parts list along with detailed instructions for assembling the openSFDI system.

Aim: We describe the design of openSFDI and report on the accuracy and precision of optical property extractions for three different systems fabricated according to the instructions on the openSFDI website.

Approach: Accuracy was assessed by measuring nine tissue-simulating optical phantoms with a physiologically relevant range of μ_a and μs′ with the openSFDI systems and a commercial SFDI device. Precision was assessed by repeatedly measuring the same phantom over 1 h.

Results: The openSFDI systems had an error of 0  ±  6  %   in μ_a and −2  ±  3  %   in μs′, compared to a commercial SFDI system. Bland–Altman analysis revealed the limits of agreement between the two systems to be   ±  0.004  mm^−  1 for μ_a and −0.06 to 0.1  mm^−  1 for μs′. The openSFDI system had low drift with an average standard deviation of 0.0007  mm^−  1 and 0.05  mm^−  1 in μ_a and μs′, respectively.

Conclusion: The openSFDI provides a customiz