Sunlight vs UV Lamp for Cyanotype

Expose a cyanotype at noon in high summer and it can be finished in three minutes. Expose the identical negative under a grey sky and you may stand there for an hour. Printers who log their times report everything from two-minute prints in Texas summer sun to winter exposures in Scandinavia that take a full day or simply never finish. Nothing about the chemistry changed between those prints — only the sky did.

That is the real difference between the sun and a UV lamp. Sunlight makes prints every bit as good as a lamp does, and plenty of experienced printers prefer working outdoors. What sunlight cannot do is deliver the same dose of ultraviolet twice. The paper responds to UV, not to visible brightness, and your eyes are a poor UV meter: a sky that still looks bright can carry a fraction of the ultraviolet it did an hour earlier. Once you want the same result twice — a second copy, a calibrated negative, a small edition — that variance stops being charming.

In strong midday summer sun, most printers land somewhere between 2 and 20 minutes, with the majority done inside 15. Morning and late-afternoon light stretches that noticeably, and the same print in winter can take anywhere from 30 minutes to several hours; at high latitudes, printers report winter exposures of four to six hours, and some stop printing outdoors entirely until spring.

Overcast changes more than beginners expect. A grey sky typically pushes a print that needed a few minutes into the 45-to-60-minute range, because what the paper counts is the UV dose, and a sky that still looks pleasant can carry very little of it. Season, latitude and the hour of day drive the exposure — experienced printers check the UV index, not the brightness. Tilting the frame so it faces the sun square-on also shortens the exposure measurably.

The coating shifts from yellow-green toward a bronzed grey-green as it finishes, and when in doubt, go longer: underexposed prints wash out pale in the rinse, while a moderately overexposed print usually survives. Misjudging exposure is one of the most common cyanotype beginner mistakes, and the sun makes it an easy one to make.

A UV exposure unit delivers the same dose at nine in the evening in January as it does in June. Most modern LED units and exposure boxes produce a well-exposed cyanotype in about 3 to 10 minutes, and once you have run a single test strip at a fixed lamp height, that number stays valid print after print. Fix the distance, write it down, and exposure stops being a variable at all — which is exactly what you need when you start calibrating digital negatives or printing an edition.

Two honest caveats. First, LED output fades: printers report exposure times creeping to two or three times their original length after a year or so of regular use, so a fresh test strip every few months keeps the number honest. Second, a lamp only fixes exposure — streaky coating, poor negative contact and over-washing ruin prints under a lamp exactly as they do in the sun.

Cyanotype chemistry responds to a band that runs from roughly 280 nm up to about 370 nm, and its sensitivity falls off sharply beyond that. A lamp specified at 365 nm sits right at the top of that band. The violet floodlights sold simply as blacklights usually emit at 395–405 nm instead — past the drop-off — where the chemistry reacts at roughly half the rate per watt. They still work; they just take appreciably longer, which is why printers who have tried both call 395 nm the slower second choice.

Wattage, the number lamp listings shout loudest, predicts surprisingly little. Printers report similar exposure times from 20 W and 150 W units, and identically rated lamps that behave completely differently, because optics, distance and true UV output vary so much between models. Wavelength first, then distance to the print — watts last. Ordinary household bulbs are out of the running entirely: they emit almost no UV, and a 40 W halogen has been left on a print for twelve hours without producing an image.

Almost every cyanotype is exposed through glass — it is what presses the negative flat against the paper. Plain, uncoated glass from a cheap picture frame passes most of the ultraviolet the process uses, and that is exactly what most printers reach for. But glass is never fully transparent to UV: it absorbs the shortest wavelengths of the band cyanotype responds to, so a print under glass always needs a little more time than one without.

The real trap is treated glazing. Conservation framing glass, car glass and many acrylic sheets are deliberately manufactured to block UV, and a UV-filtering sheet can slow an exposure dramatically or stop it working altogether — printers have bought glass from a window shop only to watch prints fail under it. The same logic applies to printing indoors on a sunny windowsill: the windowpane adds one more UV filter between the sun and your paper, modern coated or double-glazed units especially, so expect longer and less predictable exposures than outdoors on the same day.

Everything above sounds like an argument for buying a lamp on day one. It isn't. Your first dozen prints will succeed or fail on coating, negative contact and washing — not on exposure — and the sun lets you practice all of that for free. Sunlight also fails loudly and instructively: a print that needed an hour under a grey sky teaches you what the process responds to faster than any spec sheet.

The lamp earns its place the day you want the same result twice. When you start matching a print you made last week, calibrating digital negatives, or printing through a northern winter, a 365 nm unit removes the one variable you cannot control. Printers typically budget under about 50 dollars for one — roughly what the rest of a starter setup costs together. Until then, the sun is the better teacher, and the cyanotype beginner setup lists the exact chemistry, paper and frame to start with; none of it needs a lamp.