Lumu has made light reading an ease. The Lumu is incredibly compact and pairs to an iPhone and/or iPad.  Lumu uses two sensors, for accurate light reading. Lumu has a free app that allows customization of the reading, allowing for the most tailored results. The Lumu will come calibrated, with a calibration certificate.
Suitable for IPhone: X, XS, 7, 8, XR
Available in three variations:

LITE: Ambient Exposure, Flash Exposure, Cine/Video
POWER: Ambient Exposure, Flash Exposure, Cine/Video, Illuminance, Chromacity, Colour Temperature
PRO: Ambient Exposure, Flash Exposure, Cine/Video, Illuminance, Chromacity, Colour Temperature, Flash Colour Temperature