Display omitted •Vertically-aligned multi-walled carbon nanotube(VA-MWCNT) was synthesized by low-pressure chemical vapour deposition(LPCVD).•Effect of synthesis parameters on the near-to-mid infrared absorption of the forest was measured at various incident angles.•As synthesize time and temperature rising, height of the MWCNT forest was increased and other properties were changed.•Increased height of the forest results in increased absorption and reduced absorption coefficient.•Circular graphite waveguide system was proposed to model the absorption characteristics. In this paper, the growth of optimized vertically aligned multi-walled carbon nanotube (VA-MWCNT) forests by LPCVD method for use in a large-area absorber in infrared detectors is presented. The effect of synthesis temperature (500−700 °C) and time (1−10 min) on the optical absorption coefficient in the infrared (2−20 μm) is investigated by FT-IR measurement at various incident angles (15-80°). The structural properties of VA-MWCNT are characterized by SEM, TEM and Raman spectroscopy. Spectral measurements show an increasing absorption with the height of the forest that results at increased synthesis time and temperature. However, the absorption coefficient decreases with increasing synthesize time and temperature, while it is also affected by other properties, such as diameter, density, alignment, and uniformity. Moreover, the reduction in absorption at oblique incident angles demonstrates the relevance of surface properties. Finally, a circular graphite waveguide system is used to model the absorption characteristics of an MWCNT forest.