11. Testing Issues in LED Manufacturing

One needs to make the distinction between two kinds of efficacy. ILV, the International Lighting Vocabulary published by CIE [1] defines two terms. The one used to compare electric light sources (term 17-729 of ILV) is the usual (luminous) efficacy of light source defined as the ratio of the emitted luminous flux and the input electrical power. Throughout this chapter, the word efficacy refers to this quantity. The second type of efficacy is called (luminous) efficacy of source of radiation (term 17-730 of ILV) defined as the ratio of the emitted luminous flux and the emitted radiant flux. Both quantities are measured in lm/W. The third term related to efficiency to characterize LEDs is called radiant efficiency (term 17-1018 of ILV) defined as emitted radiant power divided by the supplied input electric power. See also the JEDEC JESD51-53 [2] document for some of these definitions.

Chapter 4 on thermal testing and Chap. 6 on standardization provided definitions of LED temperature. These definitions are based on repeatability considerations of measurements. In daily practice, convenience of LED users is the major factor which determines how LED vendors define thermal reference points for their products.

Usually, the purpose of measuring exactly the case temperature is to determine the R _thJC junction-to-case thermal resistance.

In testing standards of light sources and luminaries.

For a physically sound definition of thermal resistance between two locations along a heat-flow path refer to Sect. 6.​1.​4 of Chap. 6.

One should wait with starting the optical measurement at least as much after the electrical switching or temperature change as the thermal time-constant of the system containing the test LED, see, e.g., the timing diagram shown in Fig. 6.​20 of Chap. 6.

According to the JEDEC LED thermal testing standards (JESD51-51 [3] and JESD 51-52 [4]), the temperature of the LED under test has to be controlled. For details refer to Chap. 4 and Chap. 6.

In case of forward emitting LEDs attached to a cold plate at the side of an integrating sphere it is sufficient to control the cold plate temperature which in such a setup is used as reference temperature. This is the requirement of the JEDEC JESD51-52 standard [4].

See Sect. 5.​1.​2 and Eq. 5.​9 in Chap. 5.

Note: this is not the “Partial LED flux” quantity defined by CIE, but is instead a simple expression that not all the flux is measured.

The spectral distribution of the sensitivity of human eye under photopic conditions is described by the V(λ) function—see also Chap. 5.

See the definition of luminous flux in Chap. 5.