Luminous flux emitted by LED depends directly on forward current (If) magnitude. Input power needed to reach typical luminous flux is expressed by following equation:
P = If · Vf ,
where Vf is LED forward voltage (voltage drop) at nominal If . All these values (typical luminous flux, If , and Vf ) are listed for junction temperature Tj = 25 °C. In this case we are talking about “cold lumens”. But in real luminaire, junction (active area) of LED is heated up significantly by dissipative heat, which becomes from energy conversion within the LED die.
Luminous flux decreases with increasing Tj (Figure 4.1.1). Therefore it is necessary to keep Tj on the lowest possible value. Term “hot lumens” is used when speaking about luminous flux emitted by LED at operational Tj (70 – 120 °C) after steady-state conditions were reached.
Figure 4.1.1: Relative luminous flux as a function of junction temperature.