A light-emitting diode (LED) is an optoelectronic device that produces light through an electroluminescent mechanism. It contains a p-n junction through which current is passed. At the heterojunction, the current produces electrons and holes, which compound to emit integer multiples of photon energy. Although the basic light-emitting process is similar to that of a laser diode, there is no laser generation in a light-emitting diode, i.e., a light-emitting diode does not use excited radiation. They also have a wider spectrum.
1.1 Comparison of LED lighting and traditional lighting
(1) Shape Degrees of Freedom
LED is very small, each unit of LED small piece is 3~5mm square, An LED light source is a combination of many individual LED light-emitting diodes and thus can be made into more shapes than other light sources, it is easier to design the shape and size of the light source for the user's situation.LED according to the characteristics of the light-emitting tube out of the surface of the light is divided into a round lamp, a square lamp, a rectangle, a lateral tube, and so on. The volume of the LED components can be The volume of LED components can be very small, more convenient for a variety of equipment layout and design.
(2) Long service life
For the image processing unit to obtain accurate and reproducible measurement results, the illumination system must ensure that a stable image input is available for a considerable period. Studies have shown that after 100,000 hours of LED operation, the attenuation of its light is only about 50% of the initial LED light source in a continuous long period, its brightness begins to decay, but far less than other forms of light source attenuation. In addition, if you choose to use the control system to make the LED intermittent work, can inhibit the heat of the light-emitting tube, and will also double its life.
(3) Fast response time
LED light-emitting tube response time is very short, compared with the ordinary incandescent lamp response time milliseconds, LED lamp response time is only nanoseconds. The real meaning of the response time is to be able to guarantee the work switching between multiple light sources or between different areas of a light source as required. When a dedicated power supply is used to power the LED light source, the time to reach maximum illumination is less than 10ms.
(4) Free choice of colors
In addition to the shape of the light source, another way to get a stable image is to choose the color of the light source. Even for the same shape of light source, the color of the light source can make a big difference in the image. How to make use of the technical characteristics of the color of the light source to get the best contrast image effect has been the main direction of the development of the light source. LED can achieve a variety of colors of illumination by changing the amount of current can be changed, light-emitting diodes are convenient through the chemical modification of the method, to achieve multi-color light-emitting red, yellow, blue, green, and so on.
(5) Good environmental performance
LED because of its small size, small energy consumption, in energy saving has received good results. LED spectrum in the absence of ultraviolet and infrared, neither heat nor radiation, no pollution, does not contain the element of mercury, cold light source, can be safe to touch, and belongs to the typical green lighting light source.
1.1.2 LED Lighting Characteristics
Each light source has its characteristics. Whether it is incandescent, fluorescent lamps, or induction fluorescent lamps, have their own unique photoelectric performance, safety performance, environmental performance, and cost-effectiveness. LED has a strong potential advantage, applications and market share will rapidly expand, but can not be the one-sided view that the LED in the future will completely replace the traditional light source. With the development of lighting science and technology, the evaluation of future lighting light sources is not only focusing on the scope of light efficiency but should also emphasize the lighting effect, light comfort, light biological effect, light safety evaluation, as well as environmental performance, and resource consumption evaluation.
The performance of LED mainly includes three aspects such as electrical characteristics, optical characteristics, and light safety performance. Similar to other light sources, LED light characteristics mainly include luminous flux, luminous efficiency, radiant flux, radiant efficiency, spectral parameters, and so on.
1.1.3 Lighting design for LED light sources
Currently, LED light sources applied in the field of machine vision can be divided into two main categories: front illumination and back illumination. Front-side illumination is used to detect the surface features of an object, and back-side illumination is used to detect the contours of an object or the purity of a transparent object.
（1）Ring LED light source
A ring-shaped light source is a light source method in which no shadows will be seen. According to the angle, it can be divided into completely direct ring shape, low angle ring shape, and so on. Direct ring for high-speed detection of long-distance irradiation; diffuse reflection ring for detection of reflective objects detection; Dome light that is, the dome-type diffuse light source is mainly used for the detection of spherical or curved objects detection, it is not direct irradiation, but through several diffuse reflections illuminated to the object under test, the light effect is similar to cloudy day sunlight.
（2）LED strip light source
The strip light source is shown in the figure, and it has two more applications: wide detection, which means that 2 or 4 strips are used in combination, and the other is the illumination of idle scanning.
In combined use of strip lights, its most important feature is that each direction of the light source irradiation angle is adjustable, because the light source irradiation angle of the final image effect has a great impact on the flexibility is relatively large, but debugging is also relatively troublesome.
Line scan illumination, also known as a spotlight source, is used in conjunction with line scan cameras. Line scan cameras capture one line at a time and have a high requirement for light source brightness due to the exposure period. The effective working area is a narrow strip for both the light source and the camera. That is, to ensure that the light source shines in this brightest narrow strip and the camera chip is completely parallel, otherwise only a bright spot can be captured across each other. So the line light source has two special requirements, is uniformity and linearity.
This is because the difference between the brightness and darkness of different positions of the line light source will directly affect the brightness level of the image. The light source needs to use a spotting mirror so that the light is concentrated on a line so that the stability of the image can be accurately controlled. The straightness of the light output part depends on the consistency of the LED light-emitting angle, the straightness of the spotting lens, and the straightness of the line light source housing.
1.1.4 Choice of Lighting Method
In general, there are five technical aspects to consider when selecting a light source: direction, spectrum, polarizability, intensity, and uniformity. Then based on the understanding of the relevant prerequisite information, including:
1. Inspection contents, such as appearance inspection, size measurement, and positioning;
2. the form, color, material, and motion status of the object to be tested;
3. corresponding constraints, such as working distance, working conditions, working environment camera type, etc., to select a suitable type of light source. Finally, the same or similar types of products of different brands will be compared and selected according to the budget, usage habits, brand awareness, etc.
When choosing a light source, it is customary to have one assumption: this light source is stable and reliable. This is because light sources, cameras, lenses, and other front-end products of machine vision systems are usually used in near-ideal clean, or good environments. However, light sources and other products on production lines in industrial sites often must operate in toxic, high-temperature, highly humid, and dirty environments, and must withstand vibration and thermal stress. Therefore, light sources and other products used in industrial applications need to be designed, selected, and manufactured with the special use environment in mind to enable their products to be used in applications in such environments.
RZ Optics Co., Ltd. is committed to producing professional optical components, from design to production to sales integration, the main products include spherical lenses, including optical domes, plano-convex lenses, optical windows, optical prisms, mirrors, and so on. We also could provide OEM service, and use many kinds of substrates such as sapphire, fused silica, silicon, etc. Our optical domes, optical prisms, mirrors, etc. can be used for the LED lighting technology applications mentioned in this article. Please contact us if you have any needs!
Previous: Concentrator & Spotlight
