Handling microfluidics and evaluating samples based on light is a precise science. And that precision comes from the light source, not the actual detection method. But too many times we see standard LEDs being used in these sensing and evaluation applications. Standard LEDs are typically developed for lighting and illumination applications and require too many ancillary components to achieve a minimum level of acceptability. Fortunately, there is an alternate technology.
First, let’s look at a standard LED. Figure 1 shows a typical red LED. You can see the light emission surface is cluttered with the anode pad (square in the middle) and its bond wire. These elements are fine for applications like long-range sensing, lighting and indications, but for precise, up-close applications they cause disturbances.
Most notable is the square hole in the middle of the emission pattern. There are two typical methods to reduce the effect of the hole: lensing and apertures. An aperture essentially restricts the emitted light to a corner of the die, substantially reducing the light energy causing difficulties with low-contrast detections. Using a lens only will maintain the light energy, but the beam will have a fixed focused point that is not acceptable for many applications. But even the bond wire produces reflections and causes spurious emissions. These cannot be tolerated with microfluidics as adjacent channels will become involved in the measurement. An additional aperture is typically used to suppress the spurious emissions.
Fortunately, there is an alternative with MicroSpot LEDs. Basically, the anode and emission areas are inverted as shown in the Figure 2 comparison.
This eliminates the need for the anode and bond wire to interfere with the emitted light. This produces a clean, powerful and collimated emission that will produce consistent results without additional components. This level of beam control is typically reserved for lasers. However, lasers also require more components, are much larger and cost more. The MicroSpot LED is the best choice for demanding life science applications.
Try the MicroSpot for yourself in select Balluff MICROmote miniature photoelectric sensors.
Miniaturization is one of the essential requirements for medical instruments and laboratory equipment used in the life science industry. As instruments get smaller and smaller, the sensor components must also become smaller, lighter and more flexible. The photoelectric sensors that were commonly used in general automation and applied in life science applications have met their limitations in size and performance.
Sensors used in these complex applications require numerous special characteristics such as high-quality optics, unique housing designs, precise LEDs with the best suited wavelength and the ability to be extremely flexible to fit in the extremely small space available. Sensors have been developed to meet the smallest possible installation footprint with the highest optical precision and enough flexibility to be installed where they are needed. These use integrated micro-precision optics that shape and focus the light beam exactly on the object without any undesirable side-effects to achieve the reliability demanded in today’s applications.
Previously many life science applications used conventional plastic fiber optic cables that were often too large and not flexible enough to be routed through the instruments. An alternative to the classic fiber cables is a “wired” fiber with precision micro-optics and extremely flexible cables with essentially no minimum bending radius and no significant coupling losses. Similar to a conventional fiber optic sensor, an external amplifier is required to provide a wide variety of functionalities to solve the demanding applications.
These sensors can be used in applications such as:
Precise detection of liquid levels using either attenuation or refraction with a small footprint
Reliable detection of transparent objects such as microscope slides or coverslips having various edge shapes
Detection of transparent liquids in micro-channels or capillaries
Reliable detection of individual droplets
Recognition of free-floating micro-bubbles in a tube that are smaller than the tube diameter and that cannot be seen by the human eye
Recognition of macro-bubbles that are the diameter of small tubes
For more information on photoelectric sensors that have the capability to meet the demands of today’s life science applications visit www.balluff.com.