LED Basics
Everyone asks. so why use LED's? There are many reasons to consider LED's.
Written By Jon Chouinard, Nerlite
First, a quick overview:
LED's (Light Emitting) are a solid state electronic device that emit light when a current is applied to the junction of the LED. Depending on the composition of materials in the LED, it will emit a specific wavelength, or white light when used with phosphors.
As LED's are current devices, control of current through the LED junction is critical. It is critical that the device is driven at a steady current, but more importantly regulating how much power is put through the junction over time. Power over time equals heat, which is the biggest killer in LED's. Therefore, it becomes important to follow the manufacturer's guidelines in maximum ambient temperature for LED lighting devices.
In turn, there are many good reasons to use LED's over fluorescent and incandescent lighting.
The following are a few good reasons to go down the LED path:
- Life & Degradation of light - LED's when run properly last a long time. A red LED will last over 100K hours when run in ideal conditions. When compared to fluorescents and incandescent, life is extremely long. Life of a 150 Watt Halogen reflector lamps only last 200-500 hours when at full power, fluorescent lamps last about 7000 hours. This is based on reaching 50% of the initial intensity. When used in a machine vision application, you must ask yourself, "at what point does my application start to falsely fail due to change in light intensity". Ideally you want to setup your system and only periodically adjust parameters. Using LED's will allow you to accomplish all of this.
- Monochrome Light - Due to the nature of LED's, they emit a narrow band of the light spectrum. This is call monochrome light. When used in conjunction with a CCD camera, accuracy will increase. The increase in accuracy is due to how light travels thought a medium such as a glass element in your lens. If you use one band of light, the light will always focus at the same point. Using white light, it is broken down into discrete wavelengths focusing at different points. In turn, when relating to imaging a part onto a CCD sensor, a specific point using monochromatic light will always focus at the same oint. White light or broad band light will not allow this.
- Controls - LED's can be turned on and off in nanoseconds, in turn controlling LED's can be accomplished using LED controllers. The fast response time can allow the LED lights to be strobed for high speed applications allowing you to stop motion. Controls allow the LED's to be sync'd with the camera in strobe mode, but can also be run in intermittent mode. In intermittent mode, the LED can be turned when needed to capture an image. In turn, life can be extended further or made not to interfere with other applications. Higher brightness levels can also be achieved when strobing LED's due to a low duty cycle. In some cases, LED's can be driven 10X more then when run in continuous mode.
- Compactness - LED's are available in various package styles. Some packages allow dense packing of LED's to put many LED die in a small area. In turn, space is minimized, allowing a more compact system design and low weight.
- Low Energy Consumption - LED's typically require very little current (approx. 20-30 mA) and run at low voltage (12 or 24 VDC). In turn, total power dissipation is low.