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In addition to a very bright 2 watt LED light with 5 different modes, the NiteRider Sentinel 40 projects 2 laser lines on the ground. These FDA compliant lasers either sit nicely to the left and the right of the bike and give the cyclist a virtual lane. Depending on where you mount the light this virtual bike lane is shifted more forward or backward.
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In order to see the laser lines it has to be reasonably dark, otherwise they are rather difficult to see. But as soon as the sun starts to set this tail light shines in two different ways. The reasonably bright LED and the attention grabbing laser lines, and the 40 Lumen LED light is very bright during the day too. When I ride with the Sentinel in the evening I often notice how folks at intersections press their faces against the car windows to investigate these curious red lines on the ground. Some folks even have pulled up next to me to either make a complimentary comment or to inquire what light I am using. That is something I had not experienced with any other light prior.
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The Sentinel 40 offers a group ride mode among the different settings and it can be charged with a USB cable. Running time is anywhere from 7 to 36 hours depending on the mode chosen. I have ridden in the rain with the Sentinel and it has not shown any trouble despite a heavy downpour during one of the rides.
The Sentinel 40 comes with a seat post mount, but that one is not big enough for larger aero ones, that however is not unique to NiteRider. That is a common and understandable issue. Weight of the Sentinel is 73 grams and it takes about 4 hours to charge it.