A series of optical spectral sensors is based on a combination of solid-state sources (illumination) and detectors housed within an integrated package that includes the interfacing optics and acquisition and processing electronics. The focus is on low cost and the fabrication of the sensor is based on techniques that favor mass production. Materials and components are selected to support low-cost, high volume manufacturing of the sensors. Spectral selectivity is provided by the solid-state source(s) thereby eliminating the need for expensive spectral selection components. The spectral response covers the range from the visible (400 nm) to the mid-infrared (25,000 nm/25.0 μm), as defined by the availability of suitable low-cost solid-state source devices. A refractive optical system is employed, primarily in an internal reflection mode, allowing a selection of sample handling tools, including, but not restricted to internal reflectance and transmittance. A secondary channel allowing for light scattering or fluorescence methods is an option. The targeted applications of the sensing devices are for lubricants and functional fluids in the heavy equipment, automotive and transportation industries. A source reference channel is included to provide measurement stability and temperature compensation.