Optoelectronic Devices Text, Chapter 11 Sze is a good reference

Spectral Response

Demonstration using nine pieces of the latest white LEDs: a luminous flux of 90 lm was achieved at an input power of 0.6 W. Nichia Corp. has developed a white light emitting diode (LED) with a luminous efficiency of 150 lm/W at a forward current of 20 mA (photo). The efficiency is 1.5 times that of the company's current product. When compared to other light sources in terms of the efficiency alone, it is approximately 1.7 times that of a high-color rendering fluorescent lamp (90 lm/W) and approximately 11.5 times that of an incandescent lamp (13 lm/W). Its efficiency is even higher than that of a high pressure sodium lamp (132 lm/W) which is regarded as the most efficient light source possible. As with the common products, the white LED is a combination of a blue LED chip and a YAG yellow phosphor. It is contained in the same package as the one used for Nichia's NICHIARAIKOH. The output luminous flux at 20 mA is 9.4 lm, when the color temperature is set to 4,600 K. The average color rendering index (Ra) is 95 Nichia Unveils White LED with 150 lm/W Luminous Efficiency 12 21, :39 Satoshi Ookubo, Nikkei Electronics 21, :39 Satoshi Ookubo, Nikkei Electronics

Currently, the degree of luminous flux achieved from the input power, when the light source is placed in lighting equipment, is evaluated for comparison of brightness between the white LED and other light sources, in addition to the luminous efficiency. Nichia made the comparison with this index, i.e. luminaire efficiency, between the latest white LED and an incandescent lamp. Assume that the white LED loses about 20% of the input from the power source; its luminous efficiency lowers by about 25% when the luminescent color is changed to the light bulb color; and all the light emitted from the LED can be taken out of the lighting equipment because of the LED's high directivity. Then, the LED reportedly achieved a luminaire efficiency of 90 lm/W. In the case of the incandescent lamp, only about 70% of the emitted luminous flux could be taken out of the equipment, resulting in a luminaire efficiency of 9.1 l m/W. Nichia claims that the luminaire efficiency of its new white LED is 10 times higher than that of an incandescent lamp. The company says that the high luminous efficiency was achieved through a combination of elemental technologies developed so far by its own staff. Although Nichia did not reveal the details of improvements added to these technologies, "we have revised the light emitting layer as well as the package to enhance light extraction efficiency," says a spokesperson. The white LED is in the prototyping stage and the schedule for its commercial launch is yet to be decided. The company plans to improve manufacturing techniques in order to launch a 150 lm/W product as early as possible.

Lasers