Wavelength and alignment tests for confocal spectral imaging systems.

Confocal spectral imaging (CSI) microscope systems now on the market delineate multiple fluorescent proteins, labels, or dyes within biological specimens by performing spectral characterizations. However, we find that some CSI present inconsistent spectral profiles of reference spectra within a particular system as well as between related and unrelated instruments. We also find evidence of instability that, if not diagnosed, could lead to inconsistent data. This variability confirms the need for diagnostic tools to provide a standardized, objective means of characterizing instability, evidence of misalignment, as well as performing calibration and validation functions. Our protocol uses an inexpensive multi-ion discharge lamp (MIDL) that contains Hg+, Ar+, and inorganic fluorophores that emit distinct, stable spectral features, in place of a sample. An MIDL characterization verifies the accuracy and consistency of a CSI system and validates acquisitions of biological samples. We examined a total of 10 CSI systems, all of which displayed spectral inconsistencies, enabling us to identify malfunctioning subsystems. Only one of the 10 instruments met its optimal performance expectations. We have found that using a primary light source that emits an absolute standard "reference spectrum" enabled us to diagnose instrument errors and measure accuracy and reproducibility under normalized conditions. Using this information, a CSI operator can determine whether a CSI system is working optimally and make objective comparisons with the performance of other CSI systems. It is evident that if CSI systems of a similar make and model were standardized to reveal the same spectral profile from a standard light source, then researchers could be confident that real-life experimental findings would be repeatable on any similar system.