**5.4 Optical radiation measurement standard sources**

The standard radiation sources discussed in the above examples are generally incandescent lamps. Since their output cannot be calculated directly from the properties of the lamps, these sources must be calibrated from primary measurement standards traceable to the SI as indicated in Section 3 above and discussed in Section 7 below. However, with care, incandescent lamps are excellent secondary, reference and working measurement standards that can be used to transfer calibrations from the primary standards realised in NMIs to the working standards used in many calibration laboratories. The choice and use of these lamps as standards requires consideration of their spectral, geometrical, electrical, mechanical and recalibration characteristics (CIE 149:2002).

### **5.4.1 Spectral characteristics**

The output spectral distributions of incandescent lamps are particularly suitable for use as standards because the spectral distribution is a smooth and continuous function of the wavelength such as shown in Figure 8. This reduces the errors and uncertainties associated with measurements of their spectral distributions in narrow wavelength bandwidths. There are three main types of incandescent lamps characterized by the different gas fillings. Bulbs with no filling, or vacuum bulbs, can be used up to a maximum CCT of approximately 2400 K before the effects of filament evaporation become unacceptable. General lighting service bulbs filled with nitrogen and an inert gas at approximately atmospheric pressure may be used from approximately 2000 K to 2900 K CCT before the evaporation becomes excessive. Tungsten-halogen lamps, which operate with a gas filling pressure between 7 to 10 times atmospheric pressure and with a hot quartz envelope, may be used up to CCTs of approximately 3400 K.

### **5.4.2 Geometrical characteristics**

The construction of the bulb and the filament characteristics of these lamps can be somewhat adjusted to facilitate their application to the different geometrical quantities discussed.

The lamps used for total flux in integrating spheres are usually designed with spherical bulbs and circular or distributed filament shapes to provide a uniform spatial output for a uniform illumination of the sphere walls.

The lamps used for intensity and illuminance are designed with planar filaments that enables the distance between the lamp and detector to be determined reproducibly and accurately. The bulb of the lamp is also shaped, such as a triangular shape, to reduce the inter-reflections inside the bulb that will cause scattered light errors in the measured output of the lamp. Spectral irradiance lamps are usually of the tungsten-halogen design operating with a CCT of approximately 3200 K to increase the amount of UV radiation. The deviation of the irradiance from these lamps from the inverse-square-law should be measured if they are to be used at different distances from that at which they were calibrated.

Sources used for luminance and radiance standards are often the irradiance/diffuser combination described in Section 5.3. If a higher radiance is required, ribbon or stripfilament lamps can be used. Present lamps of this type are limited to approximately 2900 K.
