Spectral overlap in a simple three-colour panel

Three colours are enough to show how emission spectra share a detector. This glossary defines the words you need before a simple panel is trusted, and the decision it supports is whether a double-positive pair in that panel is overlap, spreading, or two real signals. The instrument path is in how a flow cytometer reads a cell. The imaging version of a shared filter is in fluorescence microscopy without the myths.

Detector layouts are compared in the scientific instruments catalogue. The panel's experimental home is the molecular biology pathway. Filter bands belong in a quote request.

Emission spectrum

An emission spectrum is the curve of wavelengths a fluorophore gives off after it is excited. The curve has a peak and a tail. The tail is the part that causes trouble in a three-colour panel, because it continues into the next filter even though the peak sits neatly in its own. Spectra are physical properties of the dye in a given solvent and chemical environment. Conjugation, pH, fixation and tandem uncoupling can move them a little or, for a broken tandem, a lot.

Excitation and the Stokes shift

Excitation is the band the dye absorbs. The Stokes shift is the gap between the excitation peak and the emission peak. A useful dye emits at a longer wavelength than it absorbs, so a filter can block the laser and pass the emission. In a three-colour panel you may have one laser feeding two dyes and a second laser feeding the third, or three detectors on one laser. The shift does not guarantee that dye two ignores dye one's laser. Cross-excitation is still excitation. It shows up as signal in a detector you thought was private.

Filter window

A filter window is the band of wavelengths a detector is allowed to see. Band-pass filters have a centre and a width. Long-pass filters have a cut-on and then stay open toward the red. The window is an object in the cytometer, not a property of the antibody. Two laboratories with the same three vials and different windows do not have the same overlap. Ask for the windows before you assign colours. A public primer on how fluorescence windows are chosen sits in the MicroscopyU fluorescence section, and a related imaging primer is the Evident fluorescence microscopy overview. Those pages describe optics. They do not calculate your matrix.

Primary detector

The primary detector for a dye is the window where that dye's emission is strongest relative to the others. You assign the dye to that detector, compensate the leftovers, and name the parameter after the dye or the marker. Calling a detector by a colour ("the green channel") is acceptable in conversation and weak in a notebook. Record the filter and the fluorophore.

Spillover

Spillover is the fraction of the primary signal that appears in another detector. In a three-colour panel each dye has two spillover coefficients, one into each neighbour. You measure them with a single stain at least as bright as the sample. The compensation matrix holds the coefficients and subtracts them. Spillover of zero is rare. Spillover you did not measure is still in the file.

Spreading error

Spreading error is the widened distribution left in a spill channel after the median has been corrected. It is largest where a bright dye spills into a detector you hoped to use for a dim dye. Three colours arranged so that the brightest antigen uses the dye with the longest tail into the dimmest antigen will look compensated and still be unresolvable. Swap those two assignments before you call the panel limited by biology.

Tandem dye

A tandem is a pair of fluorophores in which the first absorbs and passes energy to the second, which emits. The intended window is the second dye's emission. If the tandem uncouples, the first dye emits in its own, bluer window, and yesterday's spillover into the three detectors is wrong. Treat a new lot as a new spectrum. A three-colour panel that includes one tandem still needs a single stain of that lot.

Fluorescence-minus-one

A fluorescence-minus-one control contains two of the three stains and omits the dye you are gating. It shows spreading plus autofluorescence in the empty channel. It is not a substitute for the three single stains. It is the control that tells you where a positive gate may sit once the matrix has done what it can.

Unstained and autofluorescence

Unstained cells, processed like the samples, show intrinsic fluorescence in all three windows. Cells with more granules, more porphyrins, or more fixation-induced glow sit higher. A matrix does not erase a difference in autofluorescence between a control cell line and a primary cell. If the three-colour panel will be applied to both, unstained tubes of both belong in the glossary of the experiment, not as an optional extra.

A worked shape, without a recipe

Imagine three dyes on one blue laser and one red laser: a green-emitting dye, a yellow-orange dye, and a red dye. The green dye's tail enters the yellow window. The yellow dye's tail enters the red window. The red dye, if it is also slightly excited by the blue laser, can appear in the greener windows as cross-excitation. You measure six spillover directions, not three. You then look at spreading in the dimmest biologically important channel with a fluorescence-minus-one tube. If that tube's negative is wider than the separation you need, you reassign dyes. Nothing in that paragraph tells you a volume to pipette. The antibody sheet and the instrument manual still own the procedure.

When the words are being misused

People say "it needs more compensation" when the medians already align and the problem is spreading. More subtraction digs a hole under zero and does not narrow the cloud. People say "the colours are compatible" because the vial caps look different. Compatibility is a statement about these filters. People say the unstained was "negative" when it was negative only in a lymphocyte gate and the experimental cells are myeloid and brighter. Match the word to the tube.

Safety and research limits

Defining overlap does not authorise a sample. Biosafety of the tubes remains institutional, with the WHO Laboratory biosafety manual as a reference. A three-colour research panel is not a clinical assay. The International Society for Advancement of Cytometry is where cytometry reporting standards are discussed. This glossary does not certify a panel.

What to send with an enquiry

Name the three fluorophores, the lasers you need, and the filter windows you hope match them. Ask the quotation to list those windows. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. Three colour adjectives are not a specification.