Scale bars and cross-polarised light

A scale bar is a ruler or printed bar in the same plane as the specimen surface, at the same distance from the lens. Coins, hands and pencils fail because they have no agreed size and sit at a different distance. Print a set of bars (10, 20, 50 mm, black on white and white on black) on card, or buy a photomacrographic scale; the ones with a grey step-wedge and colour patches (the “colour checker” type) also let you correct colour later.

For identification photos, lay the bar next to the specimen, not on it, and keep the camera perpendicular so the bar is not foreshortened. Note the bar length on the record: Stratum's image caption carries it, and the measurement tools can calibrate to it.

Cross-polarised light removes specular reflection. Put a linear polarising sheet over each light and a circular polariser on the lens; rotate the lens filter until the glare vanishes. Wet specimens, glossy shell, polished ammonites and any fossil under a consolidant film photograph without the hot-spots that hide detail, and colours saturate because you are seeing the surface rather than the reflection of the lamp. The trade-off is 2–3 stops of light, so use a tripod.

Raking light (one lamp at 10–20° to the surface) is the opposite trick: it exaggerates relief for low-relief fossils and trace fossils and shows tool marks and restoration boundaries. Take one flat-lit, one raking, one cross-polarised, and the buyer or the forum has everything.

Ammonium chloride whitening — the classic monograph technique of coating a specimen with a sublimated white film to show pure relief — is reversible with water but a nuisance to apply safely; for collectors, cross-polarisation and raking light do most of the same work.