A MALDI TOF mass spectrometer having an ion source and a drift region, said ion source including: a repelling plate to which a voltage can be applied to repel ions away from said plate, wherein the repelling plate is a sample plate on which a sample is deposited prior to ionisation; and at least one extraction lens to which a voltage can be applied to accelerate ions towards the drift region, wherein the ion source is configured to extract ions from an extraction region defined as an area between the repelling plate and the extraction lens by pulsing an electric field defined by the repelling plate and the extraction lens, said extraction lens including: an element having an aperture, said aperture extending through the element so as to form a through channel, such that, in use, ions may pass from one side of the element to the opposite side of the element by passing through said through channel; wherein an axis of the through channel of the extraction lens is substantially perpendicular to the plane of the repelling plate and substantially co-linear with an ion optical axis of the spectrometer; wherein the aperture in the extraction lens has a width that is 0.5 to 2 times the distance between the extraction lens and the repelling plate; and wherein said through channel has a length equal to or greater than 8/10 of the diameter of said aperture; wherein a laser source is configured to direct a laser light beam onto the sample plate after passing through the aperture in the extraction lens along a path at a small angle to or substantially coincident with the ion optical axis of the spectrometer, wherein the MALDI ion source is configured to direct light into the extraction region through the aperture in the extraction lens to allow viewing of the sample. a repelling plate to which a voltage can be applied to repel ions away from said plate, wherein the repelling plate is a sample plate on which a sample is deposited prior to ionisation; and at least one extraction lens to which a voltage can be applied to accelerate ions towards the drift region, wherein the ion source is configured to extract ions from an extraction region defined as an area between the repelling plate and the extraction lens by pulsing an electric field defined by the repelling plate and the extraction lens, said extraction lens including: an element having an aperture, said aperture extending through the element so as to form a through channel, such that, in use, ions may pass from one side of the element to the opposite side of the element by passing through said through channel; wherein an axis of the through channel of the extraction lens is substantially perpendicular to the plane of the repelling plate and substantially co-linear with an ion optical axis of the spectrometer; wherein the aperture in the extraction lens has a width that is 0.5 to 2 times the distance between the extraction lens and the repelling plate; and wherein said through channel has a length equal to or greater than 8/10 of the diameter of said aperture; an element having an aperture, said aperture extending through the element so as to form a through channel, such that, in use, ions may pass from one side of the element to the opposite side of the element by passing through said through channel; wherein an axis of the through channel of the extraction lens is substantially perpendicular to the plane of the repelling plate and substantially co-linear with an ion optical axis of the spectrometer; wherein the aperture in the extraction lens has a width that is 0.5 to 2 times the distance between the extraction lens and the repelling plate; and wherein said through channel has a length equal to or greater than 8/10 of the diameter of said aperture; wherein a laser source is configured to direct a laser light beam onto the sample plate after passing through the aperture in the extraction lens along a path at a small angle to or substantially coincident with the ion optical axis of the spectrometer, wherein the MALDI ion source is configured to direct light into the extraction region through the aperture in the extraction lens to allow viewing of the sample.