# Tachometer Tool

The tachometer tool measures intensity fluctuations within a region of interest (ROI) to estimate rotation speed.  Suitable subjects might be a motor shaft with visible marker or a propeller rotating face on to the camera.

To start the tachometer measurement select the **Tachometer** tool and check **Enable**.  Scrub or play to the time in the clip where the measurement should begin and stop playback.  Press **Set ROI** and drag the rectangular ROI over the area of the image with the most relevant intensity fluctuations.  The ROI size can be changed by dragging the corners or edges.  Press **Set ROI** again and the ROI should turn solid yellow to indicate that rotation tracking is now active.  

If the object being measured has more than one pulse per revolution (e.g. a multibladed propeller) then the **Divider** should be set to indicate the number of pulses per revolution.

Click on the **Intensity** chart and press play to start the RPM calculation.  Use **Autoscale** for best visibility of the intensity fluctations.  After two cycles the RPM will be calculated and will appear in the overlay "**RPM (by cross)"**.  Once the chart is fully filled with data a second RPM estimate will be computed from the FFT of the intensity data, and this will appear as "**RPM (by FFT)"** in the overlay.  Check **Intensity FFT** to monitor the FFT computation.

Press stop at the end of the measurement and use scrubbing and/or stepping to examine the intensity (luminance) history in the chart and RPM history in the overlay text.  Playback may be resumed from anywhere within the current scrub range and will extend past the last tracked end frame seamlessly.  If a new start frame is desired then press **Set ROI** and scrub to the desired start time.  Reposition the ROI if required and press **Set ROI** again to fix the new start time.

The **Span** slider determines the span of the intensity chart and the length of the FFT computation.  Higher spans will give better FFT resolution.