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Microscopy Applications

Advantages of LED Illumination

LEDs do not physically emit heat like halogen lamps, where light is generated by the illumination of a sensitive filament. LED light is generated by recombining electrons within a specific semi-conductor material, making it resistant to shock and vibration.

White LEDs emits light very similar to natural daylight (5600 – 6000 K). Halogen lamps have to have a conversion filter fitted in the light path to achieve this effect, reducing the maximum luminous flux by approx. 50%.

When the brightness of LED light sources is regulated, as a rule the color temperature remains almost constant, meaning that for digital image processing the white balance does not require adjustment after the brightness has been adjusted.

Furthermore, there are no interference effects (flickering) due to the fact that LEDs are operated with continuous current.

The main advantages of LED illumination are its long lifetime and low energy consumption.

In contrast to halogen lamps, which fail after around 50 hours when operated at full power, LED manufacturers specify a lifetime of over 100,000 hours. If there is sufficient cooling, after approx. 50,000 hours the intensity is only reduced to 70%. This corresponds, for example, to around 10 years in two-shift operation. This means the user saves considerably on operating costs as no lamp change is necessary. Additional costs can be saved, particularly in manufacturing facilities, as there are no downtimes caused by lamp failure.

Due to the low energy consumption, changing over to an LED light source can result in potential cost savings of up to 80%.

According to the application, lifetime and energy efficiency savings can amount to several hundred euros per year.

Combination of LED light source with fiber optic light guides
As previously mentioned LED light sources have enormous advantages compared with halogen light sources. Now what are the strengths of using light guides for illumination compared with “direct” LED illumination?

First and foremost more light energy can be generated in a small spot, precisely of importance in stereomicroscopy applications requiring strong magnification. The separate housing means there is more room to optimally cool the illuminant and couple the light into a certain light guide diameter using suitable optics. When using one or several high performance LEDs a great amount of heat is produced that has to be directly transported away by appropriate means. Otherwise the aging process is accelerated in the form of a loss in intensity.

With ringlights, transmitted light bases or spots which illuminate the object or specimen directly, there is limited space available for the cooling. Although heat conducting adhesive and the aluminum housing provide sufficient cooling of the LEDs, compared with “external” light sources cooling efficiency is limited. It is possible to operate “external” light sources with higher power by using larger heat sinks and ventilators, in turn generating a higher light intensity.

To achieve sufficient cooling the “direct” LED illumination units are larger or bulkier. This is not desirable in stereomicroscopy as it limits handling and the field of view. In fact, slimmer and thinner housings are required, both features very easy to realize with fiber optics.

A further advantage of illumination with light guides is complete flexibility with regard to all possible illumination techniques, such as “incident light brightfield”, “incident light darkfield“, “transmitted light brightfield” etc. In this way, users who prefer or require this flexibility, while still wanting to exploit the advantages of LED light, merely have to exchange the light source. All available light guides can still be used.

Comparison of brightness of luminous spot

Left: VisiLED S80-55 ringlight (with 80 LEDs), working distance 75 mm

Right: Gooseneck, Ø 5 mm, KL 1500 LED light source

Units for direct LED illumination

Ringlights have become more and more established over the past five years, above all those ringlights that directly illuminate specimens in stereomicroscopy using several “radial” LEDs. SCHOTT is also represented in this field with its “EasyLED” series for the routine sector and its “VisiLED” series for the high end sector.

Direct LED illumination units stand out, above all, due to their ability to control individual LEDs in segments, that means in small groups of LEDs. In this way, an oblique illumination can be emulated using a fiber optic spot illumination (goose-neck) or light lines for contrasting. The advantage here is that the segments can be quickly and variably controlled, so that the active segment is rotated around the specimen being illuminated. This means the test object no longer has to be manually turned to provide illumination from different directions. For example, this can considerably simplify or speed up quality control in industrial manufacturing.

 
Face of coin illuminated from different directions   by segmented LED groups in a VisiLED ringlight

Conclusion

The future belongs to LED technology. The long lifetime and high energy efficiency form the main arguments for changing over to this technology for illumination applications in stereomicroscopy.

The user has the choice to contrast his specimen directly with LEDs or with fiber optic light guides. Depending on what the requirements are in terms of price, performance and flexibility, the user can find the best solution in the market using this state-of-the-art technology.