A simple portable light meter for use with fiber-optic microprobes was developed. The meter has a flat spectral quantum responsivity for 400 to 700 nm light (photosynthetically available radiation, PAR). With scalar irradiance microprobes connected to the meter, it was possible to directly measure photosynthetically available quantum scalar irradiance, E₀(PAR), at <100 μm spatial resolution and over a dynamic range from <1 to >1300 μmol photons m^-2 s^-1. We used the new instrument for scalar irradiance measurements in microbial mats from a freshwater lake (Lake Stigsholm, Denmark) and from a hypersaline pond (Eilat, Israel). Combined measurements of quantum scalar irradiance by fiber-optic microprobes and oxygenic photosynthesis by oxygen microelectrodes made it possible to measure gross photosynthesis as a function of the prevailing scalar irradiance (P vs E₀ curves) at distinct depths within an undisturbed hypersaline microbial mat of immotile unicellular cyanobacteria (Aphanothece spp.). Intense photosynthesis by the cyanobacteria resulted in oxygen supersaturation and a 10-fold increase of oxygen penetration in the illuminated mat (z_max = 2.5 mm) as compared to the oxygen penetration in dark incubated mats (z_max = 0.2 to 0.3 mm). The mat changed from a net oxygen consuming to a net oxygen producing community at an oxygen compensation irradiance of 14 to 26 μmol photons m^-2 s^-1. The photic zone of the microbial mat was only 0.6 mm deep due to a high attenuation of PAR. The diffuse vertical attenuation coefficient of E₀(PAR) was K₀(PAR) = 6.3 mm^-1. In the upper 0.2 mm of the microbial mat photosynthesis was photoinhibited at scalar irradiance above 200 μmol photons m^-2 s^-1. At 0.3 mm the strong light attenuation prevented inhibition in deeper layers of the microbial mat and photosynthesis approached saturation at 35 μmol photons m^-2 s^-1. In the lower part of the photic zone, photosynthesis increased linearly with E₀(PAR). Areal gross photosynthesis exhibited no photoinhibition at high irradiance and started to approach saturation above a downwelling quantum irradiance of 97 μmol photons m^-2 s^-1.