Irradiance detector, scalar is an irradiance sensor with equal sensitivity to light from all directions. If the sensor is exposed to sunlight, scalar irradiance is often referred to as actinic flux. Scalar spectral irradiance is abbreviated E_o(λ). Scalar irradiance collectors often look like spheres. Chloroplasts frequently resemble small scalar collectors, and researchers concerned with photosynthesis or phytoplankton ecology often measure irradiance with scalar sensors. An above-water irradiance reference sensor equipped with a scalar collector often features a plate-like cutoff to help eliminate surface reflection.  These sensors are called “hemispherical scalar collectors.” Also see kPAR for the “special” properties of the attenuation coefficient of PAR.