Industrial facilities, automotive and home lighting and, last but not least, microscopy, all benefit from the advantages of LED technology, the development of which is rapidly advancing. Over the past few years, the light yield of white LEDs has risen to a substantial 130 lumen per watt and more.
LEDs are semiconductor crystals that emit light when a current flows through them. The light is red, green, yellow, or blue, depending on the composition of the crystal compounds. White light is obtained through luminescence conversion, i.e. by adding a yellow phosphor coating to blue LEDs.
LEDs are available in a wide range of white light colors from extremely warm white 2,700 K through neutral white 4,000 K to cold white 6,500 K and higher, which is similar to daylight. Incidentally, the physical effect of the electroluminescence on which LED technology is based was discovered more than 100 years ago.
Long Service Life and High Energy Efficiency
LED technology offers myriad benefits. The two main benefits are a long service life and energy efficiency. With a service life of approx. 50,000 hours, LEDs essentially do not need to be replaced during the lifetime of the microscope, so there’s no need to worry about bulb changing causing microscope downtime – a particular concern in a manufacturing environment. In the long term, there is a noticeable reduction in energy consumption: the LED illumination for microscopes of Leica Microsystems uses up to 90% less electricity than comparable illumination with halogen lamps. The results of large-scale energy saving can be seen in the USA, where the government has set up a Green Labs Initiative directed at reducing energy consumption in laboratories and putting LEDs into widespread use. Also the idea of a 2000 Watt society was introduced in Switzerland back in 1998 and is recommended to save energy by using LEDs.
Constant Color Temperature
For microscopy, however, the other benefits of LED technology are even more interesting: LED illumination offers a constant color temperature across all brightness levels. This means that users can view the sample in the same color temperature at any microscope setting. “For the LED illumination in Leica microscopes we use color-neutral LEDs with a color temperature of just under 6,000 K, which is roughly equivalent to daylight,” explains Heinrich Bürgers, Product Manager for Stereo and Digital Microscopes at Leica Microsystems. “To ensure high quality, we only use selected LEDs of reputed manufacturers. That means we can guarantee the LEDs we use have the desired Kelvin band.” A constant color temperature is particularly important for documentation: the camera does not need resetting every time the brightness is altered and there is no need for white balance adjustment either. This property of LEDs means high reproducibility and low error rates.
Stable Conditions for Reproducible Recordings
Not only the color temperature remains constant. The DC components of the LED illumination ensure flicker-free light, which is especially important for recording microscope images. LEDs do not emit any heat, enabling users to view samples over long periods of time under the same temperature conditions. As the lamp housing does not need cooling, the LED illumination is also silent and vibration-free. LEDs are insensitive to voltage fluctuations in the electricity grid.
Compact Construction Enables New Designs
LEDs do not require a light source of their own: Without the need for an external lamp housing, fiber optics, and cables, LEDs are compact devices opening up new perspectives for innovative microscope illumination design. “When developing our LED products for stereo microscopes, we exploited this advantage,“ says Bürgers. “In the Leica LED5000 and Leica LED3000 product family, the illumination is totally integrated in the overall system of Leica Microsystems’ instruments and accessories.” For the user, this means that illumination settings can be controlled by software, saved with the image and recalled whenever required. Moreover, the illumination is perfectly matched to the specific microscope optics. Last, but not least, the compact LED illumination allows easy access to the sample and in a neat, space-saving microscope design.
Applications for Halogen Illumination
Are there still applications where halogen illumination is preferable in spite of the many advantages of LEDs? “There are some applications for which we recommend halogen lamps,“ Bürgers confirms. For instance, some metal samples are easier to view with a halogen illumination, as standard LEDs are often too bright. Halogen illumination may also offer advantages in forensics, as its Color Rendering Index (CRI) is better than that of LEDs. Finally, there are applications requiring infrared light or the measurement of color spectra. Halogen or xenon lamps are still used here as well. “Because of its many advantages, however, the future belongs to LED technology in microscopy illumination, as elsewhere,” says Bürgers. “With its broad range of LED products, Leica Microsystems can already offer the optimal illumination for nearly every application.”