A Prerequisite of Vision
Improper illumination may lead to extensive and time-consuming image processing, or even result in loss of information that cannot be recovered in the software. Lighting is a fundamental element in all vision systems, which ensure effective and reliable results.
We know that many parameters need to be considered to make the right choice in regards to lighting, including considerations regarding illumination geometry, types, techniques, wavelength, costs, etc.
At Qtechnology, we have developed a range of external and integrated lighting solutions on various applications. When determining the ideal illumination solution for an application, we always consider the systems’ environment. A detailed examination with different light settings is often the best way to find the right solution, reveal the features of interest such as colours or contrast and minimise the influence of unwanted lighting variations from the object inspected and its surroundings.
Lighting in Vision Systems
We can supply customised LED modules, area light, spots, backlight, etc. – to fit each client’s application.
When designing a lighting system, we establish whether the front or backlight is more suitable and find the ideal angle at which the light falls on the object surface. This may be the most important task, as it governs how a captured image will appear. In this regard, the two important subgroups of front and backlight illumination – bright field and dark field illumination – is considered.
The lighting is either driven directly from the camera or using our external LED controller box that supports up to 8 individual LED strings and up to 1 A configurable per string – in a constant current drive up to 75 V per string.
Each channel can be programmed for different settings, to a minimum and a maximum LED on times and maximum duty cycle. Moreover, overlaying pulsing of the LEDs can be activated to remove unwanted flickering of light.
The luminous flux produced by an LED increases almost linearly with the current. Hence, we recommend using a current controlled LED controller to control the current through the LED’s and not the voltage, so that the light is stable, tightly controlled and repeatable.
Colours and Wavelength
LED modules with different colours or combinations can be tailored to various applications. Many applications require very specific light wavelength to emphasise only certain specific features of the object.
If the colour of the feature of interest is known, then using a similar wavelength of colour LED will highlight this specific feature. Greenlight, for instance, makes green features appear brighter while red light will make green features appear darker.
Other vision solutions might require integration of lasers or other special lighting. For instance, when used for measuring dimensions or to excite certain features in an object. An example is live-cell fluorescence measurement with a dye agent or line laser for object profile measurement. Qtechnology has experience in working with these technologies as well.
LEDs can be switched on/off rapidly, which gives some advantages – including lower power and longer lifetime. It is also possible to drive the LEDs to higher than nominal intensities with a higher current for shorter pulses, also called strobing.
Strobing is useful whenever an increased amount of light is needed, whenever the objects inspected are moving fast and exposure time needs to be low, to minimise motion blur or to eliminate influence from ambient light.
When used as a strobe light, the LED needs to be synchronised to the camera exposure and also it needs to follow the manufacturer’s specification for maximum duty cycle when strobed.
The duty cycle is determined as D = ton / (ton+toff), the duty cycle gives the fraction in per cent.
Advantages of LED Lighting
LED lights are the most commonly used illumination in machine vision systems due to the following advantages:
Extended Life Span and Low Maintenance Requirements
High Output Stability
Suitable for Pulsed Operations
Mechanically Easy to Integrate
Easy to Customise to Different Light Geometries
Wide Range of Individual Colours