Getting the best from industrial LEDs relies on getting some important factors right. Jools Hudson from Gardasoft explains how to achieve this with the aid of an intelligent lighting platform.
Producing high quality images is a key requirement in machine vision applications to ensure that all subsequent actions such as measurement and classification can be performed reliably and reproducibly. While cameras, lenses and environmental conditions can all influence the image quality at source, the lighting used is fundamental to generating the image contrast and signal-to-noise ratio needed for the particular imaging task. The LEDs used for machine vision illumination should deliver light with the required intensity, but most importantly, the intensity must be consistent from image to image. Failure to achieve consistency is likely to lead to significant underperformance from the machine vision system. Machine vision applications are frequently short of light, particularly those that require the acquisition of images at high speed. A dedicated lighting controller can be used to safely overdrive LEDs and achieve an intensity brighter than 100 per cent of the manufacturer’s specification, which is a powerful technique.
LEDs are current driven devices and their brightness is approximately proportional to the amount of current flowing through them. They have a current rating which is closely controlled. They also have a voltage rating which can vary widely from batch to batch. LEDs produce heat as well as light and this needs to be conducted away from the device, since operating the LED at elevated temperatures affects its lifetime. Nevertheless, the typical lifetime is around 50,000 hours when run continuously at the rated current providing it is not overheated. LEDs will output 100 per cent brightness when operated continuously at their current rating. However, since LEDs can be switched on and off with virtually no delay in reaching full intensity, a lighting controller can be used to overdrive the LED for short pulses with more than its standard current rating (see figure below). More than 100 per cent brightness is achieved during these pulses, with the controller providing precise control of pulse frequency, duration and intensity to ensure that the pulse coincides with camera exposure.
Safety first for overdriving
Overdriving temporarily heats up the LED and it is necessary to allow the heat to dissipate between pulses, otherwise the heat generated by multiple pulses will combine to cause accelerated aging or damage the device. Therefore, steps must be taken to regulate the overdriving conditions. The longer the time between the pulses, the higher the overdrive intensity that is possible, but the average current during overdriving must be kept below the current rating for the LED. This is achieved by limiting the duty cycle of the output pulse. Since the brightness of an LED is actually determined by the current through it and not the supply voltage, accurate control of light intensity requires good current control. Thus, when overdriving, a constant current lighting controller is even more important than for continuous output. Constant voltage controllers will not be able to provide high stability for brightness and cannot provide predictable overdriving. Lighting controllers from Gardasoft are all current controlled and Gardasoft’s SafeSense™ technology imposes safe working limits on overdrive. The current rating of the light is automatically sensed when a light is connected or can be preset into the controller. Once the current rating is known, the controller can ensure that only safe brightness and pulse widths can be set. It also limits the trigger rate to ensure that the duty cycle is not too high, according to the settings shown in the table.
However, these limits are based on generalized parameters, designed to be safe for a variety of LED types and so are frequently lower than is possible for a specific light. If data for the actual light being used are available, overdriving parameters can be set to the safe limits specific to that particular model, ensuring the generation of maximum brightness.
Intelligent solutions to overdrive control
LED overdrive limits are a function of a number of parameters including the actual operating temperature of the light and they vary from product to product and manufacturer. The values shown in the table allow safe overdriving for LEDs but may not deliver the optimum performance for any given light. If the actual parameters for a light are known they could be programmed into a controller, but an alternative solution is that of intelligent lighting. Here, a chip in the light or lighting cable could be used to hold fixed and variable LED data that could be accessed directly by the controller. For example, the chip could provide measurements from sensors within the light such as the temperature of the light and the light intensity. Using this data, the controller can then optimize overdriving for the specific light. Intelligent lighting systems, such as triniti™ from Gardasoft are already available. This approach takes lighting control to a new level by the networking of LED lighting, camera and imaging software to provide an integrated application with a single graphical interface for set up and control. The intelligent lighting platform consists of three key elements:
- Light identification and operational data – special chips mounted into LED lights provide information on model data, electrical characteristics, optical characteristics and usage information for individual lights
- Integration of lights into software – these specially-enabled LED lights can be seamlessly integrated into machine vision networks via an SDK or through an API in major imaging processing packages. This provides users with a single graphical interface to set up the camera and lighting, visualize the timing and captured images, and save the settings to the camera and controller
- Expert light control- the systems incorporate all the functionality of LED light controller technology
This seamless link between imaging software, cameras, lights and lighting controllers means that the current illumination level within the system is known at any time. This not only allows overdriving to be optimized but brings a number of other benefits as well. One of these is the ability to monitor light intensity in a feedback loop to ensure consistent light intensity over time. This is important in ensuring reliable, repeatable measurements, especially for applications where subtle variations in the image such as surface detail, colour variations or contrast levels are critical.