Control Advanced Illumination Strobe Lighting with LabVIEW Publish Date: Sep 06, 2006

Overview A typical PC-based vision system consists of a camera, optics, lighting, image acquisition device, software, and a host PC. Optimizing lighting is important in machine vision inspection because achieving the maximum contrast is essential for accurate image processing. LED lighting often fits well into an inspection system. Small size, low power consumption, and fast response times give LED lights a distinct advantage in a variety of inspection applications. In addition, using Advanced Illumination’s patented EvenLite™ technology, illumination is uniform throughout the focal region. This application note shows how to integrate Advanced Illumination (Ai) strobe controllers with LabVIEW, IMAQ Vision, and image acquisition devices to create a complete machine vision system for product inspection of manufactured goods.

Table of Contents 1. LED Basics 2. Lighting Basics 3. Strobing Basics 4. LabVIEW and Strobing Applications 5. Conclusion

1. LED Basics Unlike standard lights, LEDs have a long lifetime due to solid-state design, high efficiency, and the reduction of localized heating. LED lifetimes are often rated in hundreds of thousands of hours, compared to hundreds of hours for standard lighting. Another advantage of LEDs is that they are very fast. A typical LED can switch fully on and off as much as a million times per second, with some LEDs exceeding this number ten-fold. This makes LED lighting ideal for high-speed strobing and stop-motion types of image acquisition. LEDs are available in a variety of wavelengths, from ultraviolet (330 nm) to extended infrared (1500 nm). A typical monochrome camera inspection station uses a light with 660 nm LEDs due to their low cost, but many wavelengths from blue (470 nm) to red (660 nm) are available, including 520 nm (green), 590 nm (yellow), and 625 nm (orange). Advanced Illumination uses a patented technique called EvenLite™ to aim the individual LED cells within a lighthead to provide maximum intensity and evenness across the focal region for a particular light, all the while reducing cost by minimizing the number of LEDs required.

2. Lighting Basics Several configurations of LED lighting give you the best results in your application. Advanced Illumination lights fall into five basic categories: darkfield illuminators, coaxial illuminators, diffuselights, backlights, and direct lights (spotlight or ringlight). Each of these is explained below: Darkfield Illuminators Angled light causes variations on a surface to deflect light up into the camera, creating light spots on a dark background or field. If there are no aberrations in the surface, nothing is seen.

Direct Light Light is projected directly at the object, much like a theatrical spotlight aims a beam of light at a performer.

Coaxial Illuminators

1/3

www.ni.com

A perpendicular wall of light is aimed at an angled beam splitter that reflects the light down. The object is viewed through the beam splitter.

Diffuselite Reflected light provides a non-directional and softer illumination that doesn’t create harsh shadows. This is often called cloudy day illumination because it simulates light on an overcast day.

Backlight An even area of illumination projects from behind an object. The object is seen as a silhouette by the camera.

3. Strobing Basics In applications where the exposure time is short, such as imaging fast-moving objects, strobing LED lights is a popular way to take advantage of LED ability for short cycles with high light output. The increase in light output comes from LED ability to be driven for short periods with currents exceeding normal steady state values, followed by a relatively long cool-off time. The ratio of on-time to off-time is typically 1 to 100; pulse durations are typically below 100 microseconds. Advanced Illumination’s LED strobe controllers have several programmable parameters that ultimately determine total on-time and synchronization with the associated camera system. The heart of the strobe source is the trigger detector, which connects to an image acquisition device or an external sensor, and reads a trigger when the strobe event occurs. Once the trigger detector detects a signal, a strobe event begins. A strobe event (Figure 2) is explained below. The IMAQ device detects a strobe pulse, which activates a three to four microsecond hardware timer. When this timer expires, the programmable Delay Timer starts. When the Delay Timer expires, the light turns on and remains on for the amount of time contained within the programmable On Time parameter. After the On Time ends, the light turns off, and the final timer, the Hold Timer, starts and eventually expires, ending the strobe event. Strobe Event

4. LabVIEW and Strobing Applications

2/3

www.ni.com

You can use LabVIEW to initialize and dynamically control the Advanced Illumination strobe controller. Functionality and flexibility for controlling numerous strobe controllers are exposed in the VIs; all of the strobe controller VIs have the ability to specify the com port (i.e., “COM1”) and an output number, either 1 or 2 (for the second output of a dual output controller). Additionally, you can modify current values for all three of the mode VIs. For all lights, except those populated as RGB (where red, green, and blue channels mix to form a new color), the current should be the same for all four channels. To protect internal mechanisms of the strobe light, it is often necessary for the controller to reduce the total energy consumed by the LEDs. The controller can do this by either reducing the current or by reducing the pulse width (On Time). With a protection input, you can specify either favor timing (reducing current), or favor current (reducing timing). The Test Mode VI requires two parameters, On Time and Off Time. These two numbers in combination determine both the frequency and the duty cycle of the strobe controller oscillation. The Strobe Mode VI requires a timing cluster with three additional numerics corresponding to the Delay Time and Hold Time parameters, as well as the Stop After trigger multiplier. In cases where the Stop After is greater than one, the Off Time parameter acts as the dead time between trigger repetitions.

5. Conclusion It is very easy to set up a complete machine vision system and incorporate LED lighting. LabVIEW makes it easy to precisely control all of the components of the machine vision system, including the image acquisition, camera, and lighting.

3/3

www.ni.com