In remote sensing, the device used to acquire data i.e. to measure the radiation arriving to the satellite instrument, is usually referred to as a “sensor”. The word sensor is preferred because it refers to a broader way of getting information than a camera. A camera usually refers to information that can only be seen by the eye (and in some case to infrared, depending on the film and camera that are used), whereas remote sensing involves many different types of radiation in the electromagnetic spectrum.
In remote sensing, many different sensors are used that have varying sensitivities to radiations at different wavelengths in the electromagnetic spectrum. For example, some sensors are designed to receive all “green” wavelengths, whilst others are more targeted towards infrared wavelengths.
The infrared viewer, for example, is specially made to “see” objects emitting infrared radiation (even in the dark). In general terms, sensors that use external energy sources to “observe” an object (e.g., the sun light to observe the Earth) are called “passive sensors”.