Phase-contrast images are formed by removing the objective aperture entirely or by using a very large objective aperture. This ensures that not only the transmitted beam, but also the diffracted ones are allowed to contribute to the image. Instruments that are specifically designed for phase-contrast imaging are often called HRTEMs (high resolution transmission electron microscopes), and differ from analytical TEMs mainly in the design of the electron beam column. Whereas analytical TEMs employ additional detectors attached to the column for spectroscopic measurements, HRTEMs have little or no additional attachments so as to ensure a uniform electromagnetic environment all the way down the column for each beam leaving the sample (transmitted and diffracted). Because phase-contrast imaging relies on differences in phase between electrons leaving the sample, any additional phase shifts that occur between the sample and the viewing screen can make the image impossible to interpret. Thus, a very low degree of lens aberration is also a requirement for HRTEMs, and advances in spherical aberration (Cs) correction have enabled a new generation of HRTEMs to reach resolutions once thought impossible.