Lighting based on the use of Light Emitting Diodes (LEDs) has reached a point at which the brightness and efficiency they now provide an attractive alternative to tungsten incandescent bulbs and halogen-based illumination systems. With a reliability measured in thousands of hours, LEDs provide cost-effective lighting solutions in both commercial lighting and machine vision applications.
Due to the demands of the commercial marketplace, many LED manufacturers have focused their efforts on developing LED products that produce warm, neutral and cool white lights with color temperatures that range from 2700K to 8300K. In industrial applications, cool white lights may be adequate. However, it is often necessary for system integrators to use illumination products outside the visible range in the IR and UV spectrum.
The deployment of Short-wave infrared (SWIR) LED lighting, for example, can enable objects whose colors are almost identical using visible light to be differentiated. Near Infra Red (NIR) LED lighting, on the other hand, will illuminate samples with light that can penetrate into and through materials such as paper and plastic. Illuminating a product with UVA, UVB or UVC LED light, on the other hand, can cause material under inspection to be excited and fluoresce, emitting light at a specific wavelength in the visible spectrum.
By packing hundreds of LEDs together in an Integrated Array, LED vendors offer lighting developers a source of high intensity light in a single unit. A variety of such integrated arrays that contain up to 100 LEDs on a single housing are on offer from LED vendors.
Arrays with a small light emitting surface up to 14mm, for example, can be employed that challenge applications where incandescent, halogen and CFL lighting were previously used, since they can deliver between 500-6500 lumen. Larger integrated arrays with light emitting surfaces greater than 14mm, on the other hand, can be used to create lighting for systems where ceramic and pulse-start metal halide illumination had been employed, since they can deliver light between 3000-18,000 lumen.
Just as the total quantity of visible light emitted by LED integrated arrays has increased dramatically over the past decade, so too has the speed at which such devices can be strobed. Multi-die LED arrays can now operate at over 100,000 strobes per second, producing up to 10 times the light output that would be achieved otherwise in constant operation. However, since such LEDs arrays do not include any controllers, lighting vendors often develop their integrated constant current drivers with built-in strobe inputs, obviating the need for their customers to develop external drivers to control the light.
In certain specialized embedded imaging applications, it may be important for a system developer to illuminate a product with light in a number of spectral bands. This is particularly important in microscopy, where a user may need to choose the wavelength of light that will enable the best image of an object to be imaged. To do so, custom ring based lighting systems can meet these needs (Figure 1).
Like their incandescent rivals, LED arrays emit light in all directions. To shape the output from the LED, designers of industrial lighting systems take the bare LED die and mount their own custom designed primary optics onto the die. While these optics have traditionally been manufactured from glass, injection molded silicon optics can provide a more cost-effective solution. However, the light from such primary optics may still be too broad for certain applications. Hence, secondary optics, such as lenses and reflectors can be mounted on the device to collect the light from the primary optic to direct it to a target.
As cost-effective as LED lighting solutions have become, the diode laser may challenge LED technology in both commercial and industrial lighting applications. While LEDs lose efficiency when driven by high electrical currents, the efficiency of diode lasers increases, providing more light than LEDs. Today, however, diode lasers are more expensive to fabricate than LEDs. However, as time progresses, their cost are likely to decrease, providing another source of inexpensive illumination for machine vision systems designers.