A sensor for measuring turbidity or fluorescence comprising: a distal sensing end having: a vertex region; a first surface extending from said vertex region and ending at a first surface end point; a second surface extending from said vertex region and ending at a second surface end point, wherein said first surface and said second surface extend in different directions to form a vertex angle at said vertex region; a convex-curved outer surface that extends between said first surface end point and said second surface end point; a distal sensing surface defined by edges of said first surface, said second surface, said vertex region, and said curved outer surface; wherein a separation distance between said first surface end point and said second surface end point defines a maximum straight line distance on said distal sensing surface; a distal sensing end volume having a cross-section formed by said vertex region, said first surface, said second surface and said curved outer surface; an optical system positioned in said distal sensing end volume comprising: an optical source to generate a beam of electromagnetic radiation; an emission window through said distal sensing surface in optical communication with said beam of electromagnetic radiation and configured to pass at least a portion of said beam of electromagnetic radiation from said distal sensing end volume through said distal sensing surface to a sample volume adjacent to said distal sensing surface; a collection window through said distal sensing surface configured to pass at least a portion of an incoming beam of electromagnetic radiation from particles suspended in fluid in the sample volume in a collection direction to said distal sensing end volume; and a signal photodetector configured to detect said beam of incoming electromagnetic radiation in said collection direction; wherein said optical system is positioned in an optical plane that extends substantially perpendicular to said distal sensing surface and that is substantially aligned with a notional line corresponding to said maximum straight line distance. a distal sensing end having: a vertex region; a first surface extending from said vertex region and ending at a first surface end point; a second surface extending from said vertex region and ending at a second surface end point, wherein said first surface and said second surface extend in different directions to form a vertex angle at said vertex region; a convex-curved outer surface that extends between said first surface end point and said second surface end point; a distal sensing surface defined by edges of said first surface, said second surface, said vertex region, and said curved outer surface; wherein a separation distance between said first surface end point and said second surface end point defines a maximum straight line distance on said distal sensing surface; a distal sensing end volume having a cross-section formed by said vertex region, said first surface, said second surface and said curved outer surface; a vertex region; a first surface extending from said vertex region and ending at a first surface end point; a second surface extending from said vertex region and ending at a second surface end point, wherein said first surface and said second surface extend in different directions to form a vertex angle at said vertex region; a convex-curved outer surface that extends between said first surface end point and said second surface end point; a distal sensing surface defined by edges of said first surface, said second surface, said vertex region, and said curved outer surface; wherein a separation distance between said first surface end point and said second surface end point defines a maximum straight line distance on said distal sensing surface; a distal sensing end volume having a cross-section formed by said vertex region, said first surface, said second surface and said curved outer surface; an optical system positioned in said distal sensing end volume comprising: an optical source to generate a beam of electromagnetic radiation; an emission window through said distal sensing surface in optical communication with said beam of electromagnetic radiation and configured to pass at least a portion of said beam of electromagnetic radiation from said distal sensing end volume through said distal sensing surface to a sample volume adjacent to said distal sensing surface; a collection window through said distal sensing surface configured to pass at least a portion of an incoming beam of electromagnetic radiation from particles suspended in fluid in the sample volume in a collection direction to said distal sensing end volume; and a signal photodetector configured to detect said beam of incoming electromagnetic radiation in said collection direction; an optical source to generate a beam of electromagnetic radiation; an emission window through said distal sensing surface in optical communication with said beam of electromagnetic radiation and configured to pass at least a portion of said beam of electromagnetic radiation from said distal sensing end volume through said distal sensing surface to a sample volume adjacent to said distal sensing surface; a collection window through said distal sensing surface configured to pass at least a portion of an incoming beam of electromagnetic radiation from particles suspended in fluid in the sample volume in a collection direction to said distal sensing end volume; and a signal photodetector configured to detect said beam of incoming electromagnetic radiation in said collection direction;