The following are suggestions for standardizing digital and film-based photomicroscopy of diatoms. There can and should be no compulsion, but when people send images to each other for identification and discussion, it makes life a lot simpler if the images are comparable, i.e. in the same 'style'. Most of the suggestions made below follow common practice, as will be apparent from inspecting standard floras and guides [e.g. K. Krammer & H. Lange-Bertalot (1986-1991). Bacillariophyceae, vols 1-4. in Süsswasserflora von Mitteleuropa. G. Fischer, Stuttgart & Jena]

General principles:

• the image should convey as much information as possible. So, because black-spot focus images of diatoms mounted in high refractive index mountant generally show the valve outline more clearly than white-spot focus images and also show the striae (pores), black-spot is usually preferable
• but where the image needs to combine information on solid structures as well as holes, intuition dictates that solid is dark and holes are light
• where the valve structure is coarse and/or the holes through the valve are large chambers (diameter > 1 µm), rather than simple, small pores, use the white-spot focus

Application of principles

• Centric diatoms
  • those lying in valve view are often shallow and may have spines as well as pores: use white-spot focus
  • those lying in girdle view, e.g. Aulacoseira, Melosira: use black-spot focus
• Araphid pennate diatoms
  • generally use black-spot focus
  • but for genera with strongly silicified transverse bars (Diatoma, Tabellaria, Tetracyclus, Meridion, Odontidium), use the white-spot focus for the striae, with the bars black.
• Raphid pennate diatoms
  • for all genera that lack fibulae (small bars beneath the raphe slits), use the black-spot focus, except for
    • at least the larger species of Diploneis, in which the valve structure is coarse and also complex, with canals and chambers within the valve wall. In many cases, multiple images are needed to describe structure (e.g. see Diploneis biremiformis)
    • Gyrosigma, Pleurosigma, Donkinia, Rhoicosigma,Toxonidea; here the structure consists of oblong or hexagonal meshwork, which is perhaps best imaged in white-spot focus
    • large Pinnularia species. In Pinnularia, the striae are transverse tubes running from raphe to valve margin. In large species these tubes are too wide to image either as single white lines or single dark lines. Focus with the edge of the tube crisp and dark ,but with the stria centre pale.
  • fibulate diatoms
    • Epithemia and Rhopalodia: white-spot focus
    • Nitzschia, Hantzschia, Bacillaria, Denticula: generally a black-spot focus on the striae will not allow the fibulae to be imaged well. Image the fibulae as dark structures (especial care is needed in those species, e.g. Hantzschia amphioxys and its allies, where the fibulae are larger than the spaces between them) and the striae will usually be in white-spot focus at the same time.
    • Tryblionella, Psammodictyon (formerly Nitzschia sects Tryblionella and Panduriformes): the valves of most of these are transversely undulate. A compromise fous will have the top of the wave in black-spot focus and the bottom in white-spot focus. Often no truly satiisfactory single image can be obtained.
    • Surirella, Cymatopleura, Campylodiscus, Stenopterobia. Often, no truly satisfactory single image is possible. The simpler, flatter Surirella (e.g. S. ovalis, brebissonii, peisonis, angusta) are best imaged with the fibulae and transverse ribs dark.