UV torch or lamp for minerals?
A torch lights 10 to 62 square centimetres at 20 cm; a 25 cm tube about 625. Coverage, not power, separates the two, and in the field a torch always wins.
Coverage decides it. A torch with a 10 to 25 degree beam lights a specimen over 9.6 to 61.8 square centimetres at 20 cm; a 25 cm tube lamp lights about 625 square centimetres at the same distance — 10 to 65 times the area. The torch wins on irradiance per square centimetre and loses on area by roughly the same factor.
In short
- The arithmetic is geometry, not wattage. A torch beam is a cone: at 20 cm a 10 degree beam lights a circle 3.5 cm across, or 9.6 cm2; a 25 degree beam lights 61.8 cm2. A 25 cm tube held 20 cm above a tray lights roughly 25 by 25 cm, or 625 cm2.
- So the question is how many specimens you need lit at once. One specimen in the hand, or a face of rock: torch. A tray of thumbnails, a drawer, a comparison across a suite: lamp. Beyond about ten specimens at a time the torch stops being a tool and becomes a nuisance.
- In the field there is no choice to make. A mains lamp needs mains. On a dump at night the only shortwave source is a battery one, which settles the question regardless of the arithmetic.
- This page does not answer which band to buy. That is shortwave versus longwave and it is answered on the best UV lamp page, together with the inverse-square argument for working closer rather than spending more.
- Shortwave ultraviolet is a genuine eye and skin hazard at either form factor, and a torch is the more dangerous of the two precisely because it is handheld and pointed. See ICNIRP's ultraviolet guidance; nothing here is safety advice.
| Task | Specimens lit at once | Better form factor | Why |
|---|---|---|---|
| Testing one specimen in the hand | 1 | Torch | High irradiance on a small area, and you can turn the specimen rather than move the lamp |
| Checking a dump or a rock face at night | 1 to a few | Torch | No mains. This is the whole argument |
| Sorting a tray of thumbnails | 10 to 40 | Lamp | 625 cm2 against 10 to 62 cm2; a torch means sweeping, and sweeping means missing |
| Comparing a suite side by side | 5 to 20 | Lamp | Even illumination across the group. A torch makes the nearest specimen look brightest |
| Photographing fluorescence | 1 to 3 | Lamp, on a stand | Fixed geometry and a long exposure. A hand-held torch will not hold still |
| Showing a cabinet to visitors | Whole shelf | Lamp, fixed | Coverage again, plus nobody is pointing a shortwave source at anyone |
| First purchase, general collecting | Mostly 1 | Torch | Cheaper, portable, and covers the commonest task. Add a lamp when the tray sorting starts |
The coverage calculation, with the assumptions on the table
Manufacturers compete on wattage, and wattage is the wrong axis. A torch and a mains lamp differ mainly in the shape of what they illuminate.
The torch. A torch beam is a cone with some full angle θ. At a working distance d, the lit circle has radius d tan(θ/2) and area πd²tan²(θ/2). At d = 20 cm:
• a 10° beam gives radius 1.75 cm and area 9.6 cm²;
• a 15° beam gives radius 2.63 cm and area 21.8 cm²;
• a 25° beam gives radius 4.43 cm and area 61.8 cm².
The tube lamp. A 25 cm tube held 20 cm above a tray illuminates a band roughly as long as the tube and about as wide as the working distance allows — call it 25 by 25 cm, or 625 cm².
The ratio is between 10 and 65. For the same total ultraviolet output, the torch puts 10 to 65 times as much on each square centimetre it lights, and lights 10 to 65 times less of the tray.
The assumptions, stated so you can substitute your own. Beam angles are taken as 10 to 25 degrees, which spans the common designs; the tube is 25 cm; the working distance is 20 cm in both cases; and the tube is treated as a line source illuminating a band, which understates its edge falloff. Change the distance and the torch figure scales as d² while the tube's stays closer to constant along its length — which makes the gap wider at short range, not narrower.
What the arithmetic means in practice
One specimen at a time: the torch is better, and not marginally. High irradiance on a small area is exactly what you want when you can pick the specimen up and turn it. You also get to work at 10 cm rather than 20, which the inverse-square law turns into four times the ultraviolet at the surface — the argument already made on the best UV lamp page.
A tray at a time: the lamp is better, and the failure mode of the torch is subtle. Sweeping a torch across forty thumbnails does light them all eventually, but you are comparing responses seen at different moments, at different angles and at different distances. A weak fluorescer at the edge of the beam looks like a non-fluorescer. Under a fixed lamp the whole tray is lit at once and the comparison is real.
The crossover is somewhere near ten specimens. Below that the sweeping is quick and the geometry does not matter much. Above it, the torch stops being a tool for surveying a group and becomes one for examining members of it.
For photography the lamp wins for a different reason. Fluorescence photography needs long exposures, and a long exposure needs a light source that does not move. A torch on a clamp is a lamp; a torch in a hand is not. The technique is a separate problem — see how to photograph a mineral specimen honestly.
The field, where the arithmetic does not get a vote
Everything above assumes a bench and a socket. On a dump at midnight neither exists.
Field fluorescence work is battery work, full stop. That is not a preference; a mains tube lamp without mains is an ornament. Battery-powered tube lamps exist and are heavy, awkward and comparatively fragile; a torch is the tool that actually gets carried.
Three field-specific points that do not follow from the coverage arithmetic.
Dark adaptation matters more than output. Twenty minutes without a white light gets more out of a modest torch than a bigger torch will get out of an unadapted eye. A red head torch for walking preserves it; a white one destroys it instantly.
You are looking at a surface, not a specimen. On a dump the useful response is from broken faces and fresh material, so the small bright spot of a torch is well matched to the task: you are examining a patch of rock at a time, not surveying a tray.
A shortwave source in the hand points wherever the hand points. Outdoors, in the dark, with other people about and uneven ground underfoot, that is the situation this equipment is least forgiving of. ICNIRP's ultraviolet guidance publishes the exposure guidance; the practical rules are on do I need a UV lamp, and the ground-safety side is on is it safe to collect on old mine dumps.
What to buy first, and what to add
First purchase: a torch, in the band this site's lamp page recommends. It covers the commonest task — one specimen at a time — it is the only thing usable in the field, and it costs a fraction of a mains unit. The form factor question and the band question have different answers, and buying the wrong band is the expensive mistake rather than buying the wrong form factor.
Second purchase, when you notice the symptom: a fixed lamp. The symptom is specific. You find yourself sweeping a torch over a tray, or you cannot tell whether specimen A is brighter than specimen B, or you want to photograph something. That is the point at which coverage has become the binding constraint.
What not to buy: a 395 nm torch sold as a UV light, which is violet light rather than usable ultraviolet, and which the do I need a UV lamp page deals with directly. Also not: a higher wattage as a substitute for working closer. Moving from 20 cm to 10 cm is free and quadruples what reaches the specimen.
The honest cost of doing this properly is a torch, a lamp, a viewing box or a dark room, eye protection, and enough fluorescent material to make it worth the trouble. That is real money for a capability that applies to a minority of specimens — the site's own survey of 74 Handbook entries found only two species recorded as shortwave-only. Whether it is worth it is a question the do I need a UV lamp page answers honestly, including the case for not buying one at all.
We do not sell equipment or specimens and there is no catalogue on this site. The rest of this section is indexed at collecting guides, and the only commercial route is the wanted list.