Brightness is the visual perception of how light or dark a source or illuminated surface appears. It is subjective and is not measured directly in lumens. Use luminous flux (lumens), luminous intensity (candela), or illuminance (lux) when an objective lighting quantity is required. Perceived brightness depends on the observer, adaptation level, contrast, viewing angle, illuminated surface, light distribution, and surrounding conditions. A lamp with more lumens may not make every part of a work area appear brighter if its light pattern differs. Objective product comparisons should therefore use the relevant photometric quantities and evaluate the complete installation.