7.8 LED Measurements

Presently, a fast and steady large scale technological change is taking place: The traditional incandescent bulb is increasingly being replaced by special semiconductor devices called light emitting diodes or LEDs. Over the past decade, LEDs have caught up in efficiency and now offer an economical alternative to incandescent bulbs even for bright signal lamps such as traffic and automotive lighting.

An LED can be designed to emit light of only the desired color. Since the emission color depends on the spectral distribution of the LED, there is a certain demand for LEDs with identical specifications. LEDs therefore have to be selected according to their specifications due to the tolerances in the production. This procedure is called binning. In this process, very fast and precise measurements have to be performed. These are achieved through optimized spectral radiometers that use diode arrays.

A typical LED has a lifetime of 100,000 hours (compared to about 1000 hours for incandescent bulbs) thus drastically reducing the need for maintenance, and often leading to significant overall cost reduction when LEDs are used in the place of traditional lighting. As an example, in the late 1990’s the city of Denver had replaced some 20000 incandescent bulbs in traffic light signals with LED devices. Calculated for the lifetime of a LED device, they estimated total savings of about $ 300 per signal.

Polychromatic vs. Monochromatic

The laser is the most commonly encountered monochromatic source. Because of its monochromatic and coherent radiation, high power intensity, fast modulation frequency, and beam orientated emission characteristics, the laser is the primary source used in fiber optic communication systems, range finders, interferometers, alignment systems, profile scanners, laser scanning microscopes, and many other optical systems.

Traditional monochromatic radiometric applications are found in the range of optical spectroscopy with narrow band-pass filtered detectors and scanning monochromators used as monochromatic detection systems or monochromatic light sources.

Optical radiation describes the segment of electromagnetic radiation from λ = 100 nm to λ = 1 mm. Most lasers used in measurement equipment and fiber optic telecommunication systems work predominantly in the 200 nm to 1800 nm wavelength range.

Because of the monochromatic emission spectrum and fixed output wavelength, detectors used to measure laser power do not need a radiometric broadband characteristic. This means that the typical spectral sensitivity characteristic of Si or InGaAs photodiodes can be used without requiring spectral correction.

For absolute power measurements, the bare detector’s spectral response can be calibrated at a single wavelength or over its complete spectral range (typically done in 10 nm increments).

The corresponding calibration factor for that specific wavelength is selected when making the laser power measurement. Some meters offer the capability of selecting a wavelength by menu on the display. The meter then calculates the reading by applying the calibration factor for the wavelength selected and displays the measurement result.

There are two typical measurement strategies for laser power detection:

• Lasers with collimated (parallel) beams are typically measured with a flat-field detector whose active size is larger than the laser beam diameter. Because of the high power of lasers, the responsivity of the detector may have to be reduced by an attenuation filter. However, there is a risk of measurement errors due to polarization effects, surface reflections from optical surfaces in the light path and misalignment of the beam on the detector.

• Lasers with non-collimated (divergent) beams cannot be measured with a flatfield detector because of the different angles of incidence. The power output of these lasers is typically measured using detectors combined with an integrating sphere that collects all incoming radiation independent of the angle of incidence. The following are more unique features offered by the integrating sphere:

• Through multiple internal reflections, the sphere offers high attenuation for high power measurements. The maximum power is limited by the sphere’s upper operating temperature limit.

• In addition, the multiple internal reflections prevent measurement errors caused by polarization effects with flat-field detectors.

• The sphere port diameter can be enlarged by increasing the sphere diameter thus enabling measurement of larger diameter beams

• Laser Stray-light: Although very useful, laser radiation can pose a health risk to the human eye. Even stray-light from lasers may be hazardous due to the typically high power levels. The EN 60825 standard describes the risk and measurement methods for risk classification. Laser stray-light can be assessed using a detector head with a 7 mm diameter free aperture to mimic the open pupil.