Estimate your exposure time in 30 seconds, without calibration.

Three variables determine a UV exposure time: the chemical sensitivity of the process, the UV intensity of the source, and the maximum density of the negative. The calculation above combines these three factors as: process base time × source multiplier × (density % / 50).

The base time corresponds to a classic practitioner reference: mid-season sun (spring or early autumn, solar noon, clear sky) and a medium-density negative. Gum bichromate is the fastest (very reactive sensitiser), cyanotype the slowest of the common processes — platinum/palladium and Van Dyke sit in the middle, carbon between the two.

The source multipliers are derived from field observations: summer sun delivers about 30% more UV-A than mid-season, BLB tubes are slower than sunlight (narrower spectral output), the Luminograph approaches mid-season solar performance.

These values are estimates. Real variations — paper humidity, particular rig, chemistry freshness, altitude, source distance — can shift the result by 30% or more either way.