Microscope Illumination: LEDs are the Future
than 100-watt mercury lamps in the blue and red excitation regions, but it is weaker than mercury in the green excitation region [1]. Arrays of LEDs can produce lamp intensities as high as 70 W/cm2 (Figure 2). Te introduction of higher- performing sputter-coated filters can be beneficial where more intensity is desired. Because LEDs are more
Figure 1: Comparison of lifetime and intensity for microscope illumination sources. LEDs last significantly longer and consume less power [4].
devices that emit light directly without using a bulb [5]. Although previously used only as indicator lights, LEDs are now sufficiently intense to be used for illumination. Teir potential in consumer applications is making them more common in markets such as automotive products and lighting for buildings. In fact, LEDs can be up to six times more efficient than a conventional incandescent or discharge lamp. As a result, considerable development has been carried
out by the LED-chip manufacturers to increase brightness and by the end-product manufacturer to engineer specialized packaging, thermal management, and optics dedicated to end-user application. Together these improvements now offer LED illumination systems that provide greater intensity than incandescent lamps in the regions of the spectrum important for microscopy. By actively cooling the LEDs even greater intensity and stability can be achieved [6]. In addition to increased intensity, LEDs offer instant on/off, long lifetime, low running costs, greater efficiency, and less heat generation. Tere are significant performance benefits that make use of the light source simpler and more convenient. When one considers that LEDs can be switched on and off instantly, the amount of actual “on” (viewing or imaging) time in a day could be less than one hour. Furthermore, LED intensity remains broadly the same over its entire life [7]. Lifetimes of 10,000, or even 50,000 hours, are now commercially available. Tis means an LED illuminator can be expected to be usable for 15 to 20 years— the lifetime of the microscope itself. So a significant financial saving can be made against the cost of purchase, replacement, alignment, and disposal of the equivalent conventional bulbs that would have been required over that period. Fluorescence, semiconductor, and art restoration are all
applications where the ability to change the color balance is a noticeable benefit of LEDs. As white light from LEDs can be made up from distinct wavebands, color balancing to provide a high color-rendering index (CRI) can be achieved for true color rendering. LED illumination can allow the user to switch from “white” light to near monochromatic light, thus removing the tendency for chromatic aberration and improving contrast. When light is delivered through the episcopic port,
consideration must be given to the optical filters that are used [3]. Misaligned or poorly performing filters can reduce performance considerably. Also, LED intensity can be greater
20
Figure 2: LEDs can be combined in arrays for superior performance. Intensity can be as high as 70 W/cm2 on the LED surface.
www.microscopy-today.com • 2011 July
efficient, less energy is used. Tere is less heat dissipated, and this can be an important issue if the microscopes are
used in close or cramped conditions. Tey exhibit greater stability and repeatability, which makes comparative tests, or tests against a reference sample, more reliable. Also, with the instant on/off capability and intensity control, LEDs can perform the functions of a shutter and a neutral density (ND) filter, which can save on costs (Figure 3). LEDs can produce white light with a fixed color
temperature. Tis can be achieved using either a blue LED with a phosphor overlay to shiſt the light (as used in most consumer products) or by combining individual LEDs with red, green and blue wavelengths (as in the way a TV picture is generated). Te ability to tune the color is possible [7]. Applications. Microscopy applications requiring
illumination at only one or two discrete wavelengths lend themselves to LED illumination. A simple LED unit is highly cost-effective in these circumstances, although care must be taken to ensure that optical homogenising elements are used so that the LEDs and any artifacts are not imaged on the sample. Fluorescence microscopy lends itself to illumination by a specific color. In fact, many of the common clinical screening stains such as auramine, acridine, and FITC (fluorescein isothiocyanate) can all be illuminated using a single 470-nm LED light source. Tere are now LED colors available for most
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