Lumu device reads the light that falls on your subject. It gives you the power to measure light before it is reflected and scattered. It does it with incident metering.
All types of light meters are designed to measure light in a consistent way. They cannot see a subject and interpret it as a photographer can. A light meter cannot distinguish a black cat from a white cat, a red balloon from a blue balloon, nor powdery white snow from a shiny white auto paint finish.
Light meters assume that all subjects are of average 18% reflectance, or neutral gray.
The incident meter is placed at the subject of your image and aimed at the light source. It measures the light falling directly on a scene. It gives an accurate and consistent rendition of the tonality and contrast regardless of reflectance, background, color or shape. Subjects that appear lighter than gray will appear lighter. Subjects that are darker than gray will appear darker. Colors will be rendered accurately. Highlight and shadow areas will fall naturally into place.
Hand-held reflected light meters and built-in camera meters read the intensity of light reflecting off a subject and measurements are taken from the cameras position.
These meters read the intensity of light reflecting off the subject and readings may vary according to variances in tonality, color, contrast, background, surface, or shape. Meters are designed to regard all subjects as 18% neutral gray reflectance.
Reflected measurement of any single toned area will result in a neutral gray rendition. Subjects that appear lighter than gray will reflect more light and result in an exposure that renders it darker. Subjects that are darker than gray will reflect less light and result in an exposure that renders it lighter.