Lumen (lm): Measures the total visible light emitted by a source. Candela (cd): Focuses on the intensity of light in a specific direction, measured as lumens . From the measurement of light in the electromagnetic spectrum, to understanding perceived brightness to the human eye, light intensity and the tools used to measure light, this guide covers it all. Radiometry is important when physical metrics must be preserved, such as in simulation tasks, and photometry is important in subjective. We speak of light energy as 'flux' and luminous flux is a measure of the flow of light energy emitted by a source, or received by a surface. Measure a known good sample and compare the measurements to your test samples.