Diffuse Radiation

Diffuse solar radiation quantifies scattered sunlight reaching the surface from all sky directions rather than directly from the solar disk, measuring the soft, multidirectional lighting component that eliminates harsh shadows and reduces contrast to create the even illumination photographers seek for portraits, product photography, and subtle tonal subjects. Unlike direct radiation which originates as parallel rays from the small sun, diffuse radiation has been scattered by atmospheric molecules, aerosols, and cloud particles, arriving from the entire sky hemisphere to wrap around subjects with soft, flattering light lacking sharp shadow edges.

Maximum diffuse radiation of 400-600 W/m² occurs under bright overcast conditions where optically thick clouds scatter abundant solar energy evenly across the sky, creating the conditions photographers call "giant natural softbox" or "perfect portrait light." Moderate diffuse radiation of 200-400 W/m² occurs under partly cloudy conditions or thin overcast, providing substantial softening while allowing some directional quality. Low diffuse radiation below 100 W/m² indicates clear skies where atmospheric scattering alone provides minimal softening, resulting in harsh direct lighting.

The photography magic happens when diffuse radiation dominates total radiation: conditions where diffuse exceeds 300 W/m² while direct remains below 100 W/m² produce flawless even lighting where shadows barely exist, skin tones render smoothly, and exposure latitude expands to accommodate both highlights and shadows easily. Portrait photographers specifically seek high diffuse low-direct conditions, while landscape photographers may find such lighting flat and uninteresting, preferring lower diffuse radiation that permits directional quality and atmospheric perspective. Diffuse radiation also affects color rendering: high diffuse conditions under overcast produce cool color temperatures around 6500-7500K and muted saturation, while low diffuse clear-sky conditions allow warm direct light to dominate color character. PhotoWeather tracks diffuse radiation to identify optimal portrait and macro photography lighting when diffuse dominates, forecast harsh conditions where direct overwhelms diffuse, and predict balanced conditions offering both gentle shadows from moderate direct and fill light from moderate diffuse. Understanding the direct-to-diffuse ratio transforms lighting assessment: total radiation of 500 W/m² means harsh midday if 80% is direct, but perfect softbox conditions if 80% is diffuse. For portrait photography and soft even lighting, target diffuse radiation 300-500 W/m² with direct below 100 W/m². For landscape drama, accept lower diffuse 100-200 W/m² allowing directional quality. For ultimate softbox conditions, seek diffuse above 400 W/m² under bright overcast.