Scale bars and what an image can claim

A scale bar is a claim about distance. A lookup table is a claim about brightness. Mixing them carelessly makes both untrustworthy. The decision this comparison supports is which presentation can carry a distance, which can carry an intensity comparison, and which only looks like evidence. Population measurements without a picture stay with the cytometer, described in how a flow cytometer reads a cell. The optical reasons a bright pixel can lie are in fluorescence microscopy without the myths.

Cameras and microscopes are in the scientific instruments catalogue. The figure often belongs to the molecular biology pathway. Pixel size and file metadata belong in a quote request.

Distance claims

The honest distance mark is a bar whose length you computed from the pixel size at the specimen. That pixel size comes from a calibration: a stage micrometer, or the microscope software's record of objective, camera pixel and any extra lens. It changes when you change objective, binning or zoom. A bar copied from a previous figure is a claim about a different optical path. Recalculate.

Write the length on the bar in ordinary units. Keep the bar long enough to read and short enough to sit inside the field it describes. If you crop, leave the bar inside the crop or add a new one computed for the same pixel size. Cropping away empty space is fine. Cropping to a single cell and implying it was the whole story is a different problem: the scale can be right while the sampling of cells is wrong. Do not let a correct bar launder a selected field into a percentage.

A magnification string can stay in the methods. It does not replace the bar on the figure, because the reader is not looking through your eyepiece. The MicroscopyU fluorescence section and the Evident fluorescence primer are useful on image formation. They do not know your camera's pixel size. You have to write that down.

Brightness claims

A lookup table maps stored intensities to displayed greys or colours. A linear table preserves order and proportion. A gamma other than one bends the mid-tones. A pseudocolour table can make a dim gradient look like a new structure, and it can make a green cell look green when the fluorophore was not a green fluorescent protein. Colour is a display choice. Say which table you used.

For a comparison between treated and control, use the same table, the same minimum and maximum, and the same exposure and gain at acquisition. State those numbers in the legend or the methods. If one image needed a different maximum to be visible, show both the matched display and, if you must, a separately labelled enhanced display. Do not quietly enhance only the interesting panel.

Saturated pixels sit at the maximum value. They cannot be compared. The histogram of the image shows a pile at the right edge when this has happened. Acquire again below saturation if intensity is the claim. If only location is the claim, you may still be able to use a non-saturated channel, or a shorter exposure, and you should say the bright channel was clipped if you show it.

What the image can claim, side by side

Overlays, crops and selections

An overlay of two lookup tables shows both channels in one rectangle. It supports a claim of apparent overlap at the resolution of the objective, and only if each channel was controlled with a single-label image. It does not support contact, binding or a ratio, unless you calibrated a ratio method and said so. A merged RGB file that has thrown away the original grey values cannot be re-quantified. Keep the original channels.

A crop can make a scale bar look optional. It is not. A crop can also remove the ugly cell that contradicted the story. The ethical and scientific rule here is ordinary: the bar must be true, the display must be declared, and the selection of fields must be described. This article is not a misconduct procedure. It is a reminder that the figure is an argument.

Comparing an image to a cytometer percentage in one legend requires both kinds of scale: a bar and a calibrated pixel size on one side, axis scales and a gate hierarchy on the other. The International Society for Advancement of Cytometry community cares about the file side of that pair. The image side is your pixel size and your lookup table.

Failure modes

A bar that changed length when the figure was resized in a graphics program was drawn as a line in the wrong layer. Compute the bar in pixels from the calibration, then keep it with the image. A bar labelled with a round number you liked, rather than the number the pixels support, is fiction. A gamma of something other than one applied only to the treated panel invents a difference. A saturated nuclear stain used as a counterstain is acceptable for location of the nucleus and unacceptable as a measure of DNA amount.

If the software does not know the objective you used, the pixel size is a guess. Calibrate, or write the missing metadata, before you draw the bar. Do not publish a figure and plan to "add the scale later" from memory.

Pseudocolour deserves a named choice. A thermal table can show a gradient the eye misses in grey, and it can also invent edges where the stored values change smoothly. If you use one, name it and show the range of values beside it. Two panels that share a thermal table and hide different ranges are not the same colour. Set the range from the data, or set it explicitly and write the numbers down. A reader should be able to tell a dim cell from a harsh stretch of the lookup table without asking you for the project file.

Safety and research limits

A careful figure is still research communication. It is not a diagnosis, and it does not change the biosafety of the specimen that was imaged. Unfixed human material remains an institutional decision under the WHO Laboratory biosafety manual. This comparison does not approve that imaging. Display choices do not inactivate a hazard.

What to send with an enquiry

Ask whether the camera and software write micrometres per pixel, bit depth, exposure, gain and the lookup display into a file you can reopen. State the objectives you will use. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A megapixel count without pixel size at the specimen cannot support a scale bar.