Photon reducing agents for fluorescence assays

A system for fluorescence assays, comprising:

• a) a source of excitation light, for fluorescent excitation;
• b) a detector, for measuring emission;
• and
• c) an aqueous reagent mixture, comprising;
• i) a plurality of living cells in contact with a solid surface,
• ii) a first reagent, comprising a photon producing agent,
• iii) a second reagent comprising a photon reducing agent,
• wherein said photon reducing agent is substantially impermeant to a plasma membrane of a living mammalian cell,
• wherein said photon reducing agent does not specifically bind to said membrane compartment,
• wherein said photon reducing agent has an absorption spectrum that overlaps with the absorption, emission or excitation spectrum of said photon producing agent, and
• wherein said photon reducing agent is present in said aqueous reagent mixture at an amount sufficient to reduce light emitted from said aqueous reagent mixture by at least 10% compared to the light emitted from said aqueous reagent mixture in the absence of said photon reducing agent.