A charged particle energy analyser comprising irradiation means for irradiating a sample for causing the sample to emit charged particles for energy analysis, an electrode structure having a longitudinal axis, the electrode structure comprising coaxial, inner and outer electrodes (14, 15) having inner and outer electrode surfaces (IS, OS) respectively, an entrance opening through which charged particles emitted from said sample can enter a space between said inner and outer electrode surfaces (IS, OS) for energy analysis and an exit opening through which charged particles can exit said space, the inner and outer electrode surfaces (IS, OS) each having end portions, and detection means for detecting charged particles that exit said space through said exit opening, wherein, apart from the respective end portions, said inner and outer electrode surfaces (IS, OS) are surfaces having meridonal planes of symmetry orthogonal to said longitudinal axis, said inner and outer electrode surfaces (IS, OS) being generated by rotation, about said longitudinal axis, of arcs of two non-concentric circles having different radii, R 2 and R 1 respectively, R 2 being always more than R 1, the distance of said outer electrode surface from said longitudinal axis in the respective meridonal plane being R 01 and the distance of said inner electrode surface from said longitudinal axis in the respective meridonal plane being R 02, and wherein said radii R 1 and R 2 and said distance R 02 satisfy the conditions: R 1 = K 1 R 12, R 2 = K 2 R 12 and R 02 = K 3 R 12, where R 12 = R 01 - R 02 and K 1, K 2 and K 3 are dimensionless parameters for which 1 < K 1 < ∞, 1 < K 2 < ∞ and, 0 < K 3 < ∞, where any selected set of the parameters satisfy K 1 ≠ 1 + K 2 and K 1 < K 2 and K 3 < K 2. irradiation means for irradiating a sample for causing the sample to emit charged particles for energy analysis, an electrode structure having a longitudinal axis, the electrode structure comprising coaxial, inner and outer electrodes (14, 15) having inner and outer electrode surfaces (IS, OS) respectively, an entrance opening through which charged particles emitted from said sample can enter a space between said inner and outer electrode surfaces (IS, OS) for energy analysis and an exit opening through which charged particles can exit said space, the inner and outer electrode surfaces (IS, OS) each having end portions, and detection means for detecting charged particles that exit said space through said exit opening, wherein, apart from the respective end portions, said inner and outer electrode surfaces (IS, OS) are surfaces having meridonal planes of symmetry orthogonal to said longitudinal axis, said inner and outer electrode surfaces (IS, OS) being generated by rotation, about said longitudinal axis, of arcs of two non-concentric circles having different radii, R 2 and R 1 respectively, R 2 being always more than R 1, the distance of said outer electrode surface from said longitudinal axis in the respective meridonal plane being R 01 and the distance of said inner electrode surface from said longitudinal axis in the respective meridonal plane being R 02, and wherein said radii R 1 and R 2 and said distance R 02 satisfy the conditions: R 1 = K 1 R 12, R 2 = K 2 R 12 and R 02 = K 3 R 12, where R 12 = R 01 - R 02 and K 1, K 2 and K 3 are dimensionless parameters for which 1 < K 1 < ∞, 1 < K 2 < ∞ and, 0 < K 3 < ∞, where any selected set of the parameters satisfy K 1 ≠ 1 + K 2 and K 1 < K 2 and K 3 < K 2. R 1 = K 1 R 12, R 2 = K 2 R 12 and R 02 = K 3 R 12, where R 12 = R 01 - R 02 and K 1, K 2 and K 3 are dimensionless parameters for which 1 < K 1 < ∞, 1 < K 2 < ∞ and, 0 < K 3 < ∞, where any selected set of the parameters satisfy K 1 ≠ 1 + K 2 and K 1 < K 2 and K 3 < K 2.