In this work we propose the use of twisted-nematic liquid-crystal spatial light modulators
(TN-LC-SLM) as a useful tool for training students in the manipulation of light beams with
phase-only masks. In particular, we focus the work on the realization of phase-only gratings and
phase-only spiral phases for the generation of vortex beams, beams carrying orbital angular momentum
(OAM). Despite the extensive activity in this field, its experimental implementation for educational
purposes is limited because it requires the use of very expensive high-resolution liquid-crystal on
silicon (LCOS) SLMs. Here, we show that a low-cost experimental implementation can be done with
older TNLC technology. However, these devices, intended for display applications, exhibit rather
limited optical phase modulation properties in comparison with modern LCOS devices, such as a very
low range of phase modulation and a general coupled intensity modulation. However, we show that a
precise characterizati…