Lidar (Light Detection And Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses. Like the similar radar technology, which uses radio waves, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal.

Ceilometer is an automatic, active, remote-sensing instrument for detecting the presence of clouds overhead and measuring the height of their bases. For optically thin clouds, such as most cirrus, more than one layer may be detected, but when optically thick clouds, such as liquid water stratus, are present, the light beam is unlikely to penetrate much beyond the base of the lowest liquid layer. Laser ceilometers use intense pulses of light in a very narrowly collimated, vertically directed beam, and have collocated transmitter and receiver systems. The cloud base heights may be displayed in a variety of time-height section images or backscatter intensity profile plots. Some older ceilometers use separated transmitter and receiver units. The instruments are designed to work during the day or night.

Lidars and ceilometers are based on the same principle, but significantly different from the engineering point of view, as they are meant for different applications. Lidars are more complex in terms of laser power, number of detection channels and therefore of the optics and electronics. This helps to obtain more quantitative data products (e.g. backscatter and extinction profiles at several wavelengths, lidar ratios, Angstrom exponents and particle depolarization ratios), however with an uncertainty that has to be well documented. An important contribution to the data uncertainty comes from the instrument’s imperfections, which are generally difficult to quantify. Each optical or electronic component has an impact, but they also interact at a certain level, and depend on the environmental conditions. For example, the transitivity of the interference filters at the center wavelength changes with temperature, while the dynamic range of the whole system varies with the laser-telescope alignment (this also depending on the temperature).