A glossary of optical measurements

An optical specification is a list of measurements a later user can check, not a brand sentence. The decision this selection guide supports is which measurements to name when you compare a microscope or a cytometer, so the light path can be repeated. How a cytometer turns that path into events is in how a flow cytometer reads a cell. How a fluorescence microscope can still mislead after the numbers are right is in fluorescence microscopy without the myths.

Catalogue classes live in the scientific instruments catalogue. The work may sit in the molecular biology pathway. The measurements below are what a quote request should carry.

Numerical aperture and magnification

Numerical aperture is the cone of light the objective accepts. It sets resolution and photon collection together. Magnification is the enlargement of the intermediate image. Specify both. A purchase compared only on magnification drifts toward empty enlargement. Working distance, the gap between the front lens and the focus, decides whether that aperture can reach a sample in a dish. Specify it when the vessel is not a thin slide.

Immersion class and coverslip correction

Air, water, oil and silicone are immersion classes. The objective is built for one of them. Coverslip correction is the glass thickness the design expects, often about 0.17 mm for high-aperture lenses, or a collar that covers a range. A specification that says "oil immersion 100×" and omits aperture and coverslip mark is incomplete. State the vessel you will use so the reply can confirm the match.

Wavelength, excitation and emission band

Wavelength is the number that has to match a fluorophore. For a microscope, specify the excitation lines or the lamp or LED bands, and the emission filter centres and widths. For a cytometer, specify laser lines and each detector's filter. Two instruments with a "blue laser" can still differ in power and in the filters downstream. The MicroscopyU fluorescence section and the Evident fluorescence primer explain why bands matter. Your specification should name the bands anyway, because a primer cannot see your dyes.

Pixel size and sampling

Pixel size at the specimen is the camera pixel divided by the magnification in the image path, after any relay or zoom. It decides whether you are sampling the resolution the aperture provides or merely enlarging blur. Specify the camera pixel size and that the software writes micrometres per pixel into the file. A cytometer does not have a field pixel. Its analogous specification is the pulse parameters: height, area and width, plus the number of bits and the range of each parameter.

Bit depth, gain and offset

Bit depth is the number of grey levels. Gain scales the signal into those levels. Offset sets the floor. Specify the bit depth you need for the intensity claim, and require that gain, offset, exposure or dwell time, and binning are stored with the image. A cytometer equivalent is detector voltage or gain, the threshold, and the parameter range. A file that stores the picture and not the gain cannot be compared with next month's file.

Field, section and format

Field of view decides how many cells one microscope frame contains. Optical section thickness, set by aperture and by a confocal pinhole if present, decides the depth claim. Specify whether you need widefield, a scanning confocal class, or a disk class, and whether z-step size is recorded. On a cytometer the format question is tube versus plate, and analysis versus sorting. Sorting adds nozzle class and aerosol controls, which are a facilities specification as much as an optical one.

A comparison you can actually check

Suppose two microscopes are offered for the same fluorescent protein in live medium. One quotes a high magnification and a camera. The other quotes numerical aperture, water immersion, a correction for a dish bottom, and a pixel size at the specimen. The second quotation can be checked against the Abbe idea: the resolvable distance tracks wavelength and aperture, and the pixel size tells you whether you will sample that distance or merely enlarge it. The first quotation cannot be checked. The same split appears on cytometers. An offer that names a laser count and a colour count does not say whether area and width are stored, so you cannot know whether doublet gating is possible. Ask for those parameters by name.

Bit depth belongs in the same sentence as gain. A deep digitiser does nothing useful if the gain parks every event in the lowest slice of the scale, and it does nothing if the bright population saturates. Specify that the software can show a saturation count, and that you will reject a configuration you cannot keep on scale. Working distance and transmission deserve their own line when the dye is far red or the sample sits in a thick dish. Coatings that look fine in green can cut the red, so a high aperture engraved for visible light may still be a dim objective at your emission. Ask for confirmation that the objective is intended for that band. On the cytometer, ask whether the detectors on the red laser are the sensitive class a dim antigen needs, not only whether the laser exists. These are the measurements that stop a purchase being decided by the largest magnification number on the page.

Failure modes in a specification

If two quotations cannot be lined up in a table of these measurements, you are comparing adjectives. Ask again for the missing bands and apertures. If a cytometer quotation lists lasers and not filters, the spillover of your panel is unknowable. If a microscope quotation lists a camera and not pixel size, the scale bar of future figures is unknowable. If sorting is in scope and aerosol containment is not mentioned, the optical specification has outrun the room. Those are reasons to pause, not reasons to invent the missing number.

A copied voltage sheet or a copied exposure from another laboratory is not a specification. It is someone else's gain on someone else's filters. Keep it out of the purchase document.

Safety and research limits

An optical specification does not assign a biosafety level. If the sample is unfixed human or infectious material, say so in the enquiry so the reply can address containment, and let the institution decide, using the WHO Laboratory biosafety manual as a reference. This guide does not approve the sample. It does not make the instrument a diagnostic device. Cytometer file standards are part of the work of the International Society for Advancement of Cytometry.

Heat, dust and unstable power belong in the same enquiry when they are true of the room. A high-aperture objective and a laser both dislike a hot, unsteady bench. Ask about operating conditions the maker states. Do not invent a room specification you have not measured.

What to send with an enquiry

Send the table: aperture, immersion, coverslip or vessel, wavelengths, pixel size or pulse parameters, bit depth, and tube versus plate or widefield versus confocal. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. Leave brand preference out until the measurements match the experiment.