Method and apparatus for increasing the efficiency of a fluorescence measurement cell

A method for estimating a property of a fluid in an earth formation, the method comprising:

• conveying a logging instrument in a borehole penetrating the formation, the logging instrument comprising a plurality of light sources configured to illuminate a sample of the fluid with light that causes the sample to fluoresce light, each of the light sources being configured to provide a light beam with a solid angle and a distance traveled to the sample, the solid angle and the distance being configured to concentrate the beam at an area of the sample that is overlapped substantially a same amount by a light beam from another light source in the plurality;
• illuminating the sample with light from the plurality of light sources;
• detecting light fluorescing from the sample;
• and
• estimating the property from the detected fluorescent light.