Direct Radiation

Direct solar radiation quantifies unscattered beam sunlight traveling directly from the sun to the surface without atmospheric or cloud interference, measuring the harsh, high-contrast lighting component that creates defined shadows, specular highlights, and challenging exposure latitude. Unlike diffuse radiation which scatters from all sky directions, direct radiation originates from the small solar disk, producing parallel rays that cast sharp shadows behind every object and create the strong directional lighting photographers either embrace for dramatic effect or avoid for softer results.

Maximum direct radiation under clear skies reaches 800-1000 W/m² at solar noon depending on atmospheric clarity, producing harsh midday conditions with deep shadows and bright highlights that exceed most cameras' dynamic range for single-exposure capture. Moderate direct radiation of 400-700 W/m² occurs during morning and evening hours as sun angles lower and atmospheric path length increases, providing directional lighting at more favorable angles suitable for landscape photography. Low direct radiation below 200 W/m² indicates significant cloud interference or very low sun angles, shifting lighting character toward soft diffused as the direct component weakens. Zero direct radiation means complete cloud obstruction where all lighting comes from diffuse scatter.

The critical photography insight comes from the direct-to-diffuse ratio: when direct radiation dominates total radiation (>70% direct), lighting becomes harsh and contrasty requiring careful exposure management, while situations where diffuse exceeds direct (<30% direct) produce the soft even lighting ideal for portraits, forest scenes, and subtle tonal subjects. Photographers targeting soft portrait lighting specifically seek conditions where direct radiation falls below 100 W/m² while total radiation remains 300-500 W/m², indicating thick high clouds creating giant natural softboxes. Conversely, dramatic landscape photographers may seek high direct radiation of 600-900 W/m² at golden hour sun angles, combining strong directional light with favorable color temperature. PhotoWeather tracks direct radiation to identify harsh lighting periods requiring contrast management or subject selection favoring bold graphical forms, forecast soft-lighting windows where diffuse dominates, and predict dramatic directional lighting at optimal sun angles. Understanding direct radiation patterns helps photographers distinguish between clear harsh conditions, filtered softness, and complete overcast. For soft even lighting with minimal shadows, seek direct radiation below 150 W/m². For dramatic directional lighting at favorable angles, target 500-800 W/m² during golden hour. For harsh challenging midday conditions, expect 800-1000 W/m² requiring selective subjects or HDR techniques.