NONIONIZING ELECTROMAGNETIC RADIATION · 255 5.3.1. RESPONSIVITY Responsivity is important because it is the parameter that transforms the output of the radiometer (photometer) into the corresponding value of the radiometric (photometric) unit being measured. It is the main parameter measured during calibration of the instrument (CIE, 1984a). However, for comparing the performances of different radiometric and photometric sensors, responsivity is not a particularly useful parameter because it represents the slope of the instrument output versus input plot. This can be made to have almost any value by adding appropriate electrical amplification at the output of the photodetection system incorporated in the sensor. The photodetector also produces electrical noise, and this too is amplified by any subsequent amplification stages. This means that the addition of an extra amplification stage will improve the responsivity of a radiometer significantly, yet it is unlikely to benefit the minimum resolvable change in the radiometric quantity being measured. Thus any claims relating to the responsivity of a radiometer by a manufacturer should be treated with caution. Sometimes the term sensitivity is used to describe the performance of instruments including radi- ometers and photometers. This is a term that has two possible meanings. It is sometimes used to denote the responsivity of a radiometer, as described previously, but it is also frequently used to denote the ability of the radiometer to detect and resolve small input signals. However, the correct parameter used to describe the ability of a sensor to measure small input signals is the NEP (Theocharous & Birch, 2002). Noise equivalent irradiance (NEI) and noise equivalent radiance (NER) are used to denote the corresponding parameters for irradiance and radiance. In this chapter the term sensitivity is avoided to prevent this potential source of confusion.