Precautions for Using LED Lamps
Changes in Characteristics Relating to Temperature (Luminosity, Wavelength, Forward Voltage)
The characteristics of an LED lamp change according to the chip temperature (Tj: the temperature of the junction emitting the light). The chip temperature includes both the ambient temperature and the heat emitted by the LED itself.
The following text describes typical changes in the characteristics.
The amount of light emitted by the LED lamp diminishes as Tj rises. This is because of an increase in the recombination of holes and electrons that make no contribution to light emission.
Toshiba's LED lamp technical data shows graphs of the characteristics of the luminous intensity relative to the case temperature (where the luminous intensity is standardized to 1.0 at 25°C - see Appendix 1).
Within the guaranteed operating temperature range, the luminous intensity Varies from 0.7 to 1.2 (based on luminous intensity of 1.0 at 25°C)
Like the luminous intensity, the emitted wavelength also changes, mainly due to changes in the semiconductor energy gap caused by changes in temperature. The extent of the changes in the wavelength varies according to the semiconductor material; in InGaAlP-type LEDs, a rise in temperature causes d to change by about 0.1 nm /°C. For applications where strict wavelength standards are required, any change in wavelength within the guaranteed operating temperature range of equipment must be taken into consideration.
Forward Voltage (Vf)
Except for special cases, changes in the Vf, like changes in the emitted wavelength, are caused by changes in the semiconductor energy gap. As the temperature rises, Vf falls by about 2 mV/°C. Change in Vf is a major consideration in circuit design. Where the LED lamp operates on constant current, changes in Vf do not affect the circuit constants. However, where the LED lamp operates at a roughly constant voltage, Vf falls as the temperature rises and the current increases. An increase in the current causes Tj to rise still further and Vf to fall further. The current continues increasing until a balance is reached. Conversely, at low temperatures Vf rises and the current falls. The luminous intensity required may not be obtained when the circuit is operated at a constant voltage.
Fluctuations in Characteristics
Variation in characteristics values between different LEDs arises at the manufacturing stage. Toshiba specifies a minimum value for the luminosity and a minimum or maximum value for each of the electrical characteristic parameters. Hence, optical circuits must be designed taking these fluctuations in to account. For example, in addition to changes in the temperature, Vf also has a certain distribution. When a circuit has no built-in design margin, devices with a large Vf fluctuation must be checked so as to ensure that the desired characteristics can still be obtained when the temperature changes. Depending on the characteristic of the circuit or equipment, it may be necessary to restrict the amount of fluctuation in the LED lamp characteristic values. In such cases, Toshiba will investigate there is a need for a special standard and decide whether or not a special standard can be applied.
Differences between Visible LEDs and Sensor Light Sources
LED lamps are emitters of visible light. Hence, LED standards are based on light visible to humans. Accordingly, Marktech does not recommend using a visible LED as the light source for an optical sensor. Relative efficiency Figure 6 shows the luminous efficiency curve, featuring the wavelength characteristics of light to which the human eye is sensitive. The human eye is most sensitive to light with a wavelength of approximately 555 nm. When the luminosity of an LED is measured, the value of the luminosity at each wavelength must be corrected according to the luminous efficiency curve shown in Figure 6.
Figure 6 - Luminous efficiency curve
Either use the luminous efficiency curve to correct the LEDs' output for each wavelength to be measured or pass the LEDs' output to be measured through an optical filter with the same transmission characteristics as the luminous efficiency curve. Naturally, it is also necessary to take into account the wavelength characteristics of the photodetector. Photodiodes or CCD image sensors are sometimes used in photodetectors for checking luminosity. In these cases, the difference between the luminosity of visible LEDs is not simply due to the differences in the sensitivity of their photodetectors. For example, a simple comparison between the luminous intensity of a 660-nm GaA As LED and that of a 570-nm InGaAlP LED shows that the latter has a luminous intensity than the former. However, taking into account the wavelength characteristics of the reception sensitivity of photodetectors with photodiodes or CCD image sensors, the 660-nm GaAlAs LED has the higher output. In addition, the wavelength characteristic diagram in the technical data shows only the visible light spectrum and does not indicate that no other light is emitted from the LED. In particular, depending on the type of LED, the emission may have a large peak in the near-infrared area. When using photodiodes to measure luminosity, do not forget to take infrared light into account.