Accurate　measurement　of　instantaneous　rotational　speed　(IRS)　over　a　wide　range　is　critical　in　aerospace,　precision　manufacturing,　and　other　engineering　fields.　The　paper　proposes　a　novel　method　for　measuring　IRS　based　on　the　angle　of　a　cam　detected　by　an　optical　coherence　system.　The　optical　path　difference　(OPD)　is　modulated　by　the　variation　in　diameter　of　the　continuous　rotary　cam.　By　establishing　a　mapping　relationship　between　the　OPD　and　the　rotation　angle,　IRS　across　a　wide　range　can　be　accurately　measured　through　demodulation　of　the　optical　coherence　signal.　The　IRS　measurement　model　was　developed　by　designing　the　profile　of　the　OPD-modulated　cam　and　the　optical　coherence　displacement　measurement　(OCDM)　system,　followed　by　an　analysis　of　the　influence　of　the　cam　profile　on　IRS　measurement　performance.　Experiments　were　performed　on　both　constant-speed　and　variable-speed　over　a　rotational　speed　range　of　1　rpm　to　1500　rpm.　The　results　demonstrate　that　the　proposed　method　enables　real-time,　accurate　measurement　of　the　IRS　across　the　full　scale,　with　a　repeatability　error　below　2.5　%,　nonlinearity　error　within　1　%,　and　indication　error　not　exceeding　2.55　%.　Notably,　as　the　rotational　speed　increases,　the　measurement　error　progressively　decreases.　This　continuous　sensing　principle　enables　real-time,　precise　measurement　at　ultra-low　rotational　speeds.　It　effectively　eliminates　measurement　dead　zones　and　slow　dynamic　response,　which　are　inherent　in　traditional　encoder-based　methods　that　rely　on　discrete　pulse　signals　with　insufficient　counts　at　such　speeds.　©　2025