**2.1. Solid angle (ω)**

radiations with wavelengths from approximately 100 to 2500 nm. We describe the principles used to measure photometry and radiometry quantities such as total flux, intensity, illuminance, luminance, radiance, exitance and irradiance. Measurement results should be expressed in terms of estimated value and an associated uncertainty, we provide an explanation how to estimate and build the uncertainty budget of measurements. Metrology is based on measurements and comparisons. We measure each quantity in its own units, by comparison with a standard. The unit is a unique name we assign to the measures of that quantity. Base standards must be both accessible and invariable. The metrological traceability chain is the sequence of measurement standards and calibrations that were used to

Optical radiations bathe the world in which we live [1]. Very early in our history, it was observed that light can be produced by different means depending upon the daily rotation of the earth. New ways of using light were discovered to effect changes upon many of the materials we found in the world around us. In this chapter, we discuss some of the procedures and equipment necessary to obtain accurate measurements of the amount of optical radiation that will act on our activities. The principal purpose of the science of photometry is to evaluate visible radiation or light, so the results match as closely as possible with a normal human observer exposed to that radiation. In order to achieve this aim, one must take into account the light stimulus, the radiation entering the eye and the characteristics of the visual organ that produce the relevant sensation of light [2]. Light is essential for vision, the world is only visible when light reflected or emitted by objects reaches our eye. Light is a kind of energy and is portion of a broad range of the electromagnetic spectrum. The visible light spectrum is a little part of this spectrum, between 380 and 760 nm (see **Figure 1**). The total light energy emitted from a source or falling on a surface can be measured. This total energy can cover a part of the visible spectrum, including ultraviolet and infrared energy. The branch of science in which we study light measurement is known as photometry and a subset of the broader

relate the measurement result to the reference.

field is radiometry.

76 Metrology

**Figure 1.** Electromagnetic radiation spectrum [3].

A solid angle (ω) is defined by the surface area of a sphere subtended by the lines and by the radius of that sphere, as shown in **Figure 2**. The dimensionless unit of solid angle is the steradian, with 4π steradians in a full sphere [4].
