The Importance of Lighting Control
Appropriate lighting is crucial to the success of a project. When engineers begin building a new machine vision system, they typically carefully consider factors like motion speed, optics, and image processing software. Lighting is often the next concern because suitable lighting can significantly reduce software development time and even be critical to the success or failure of a project. The purpose of lighting is to project a uniform illuminated area onto the field of view to obtain clear images and separate desired features from the background.
Types of Machine Vision Lighting Control
Most machine vision systems use one of the following three driving methods for their light sources: constant voltage drive, voltage drive with pulse width modulation (PWM), or constant current drive. The earliest lighting controllers were voltage-driven, which was sufficient for ordinary applications. However, there are significant drawbacks to using voltage-driven light sources: the power supply voltage of the light source is constant, and brightness is determined by current rather than voltage. As the temperature of the light source changes during use, the same voltage can result in different brightness levels.
Figure 1 shows the linear relationship between illuminance and forward current for a typical light source. It demonstrates that the relationship between forward current and voltage is highly nonlinear, with a small change in power supply voltage causing a significant change in current and, consequently, brightness. This effect means that voltage drive cannot achieve precise and accurate adjustment of lighting intensity. Voltage drivers are also susceptible to slight fluctuations in system supply voltage, which can lead to significant changes in illumination level.
Despite the limitations of voltage drive, some lighting controllers leverage this type of power supply to save costs. Since voltage drive cannot directly control lighting brightness, pulse width modulation (PWM) can be used instead. PWM switches on and off several times during each exposure to achieve average brightness. PWM pulses are controlled by an internal clock, and a faster internal clock is required for precise brightness control. The clock limits the number of illumination levels PWM can provide. A typical voltage-driven lighting controller provides 100 intensity levels, while a typical current-driven lighting controller typically provides several thousand intensity levels and better control levels.
The design of constant current lighting controllers directly controls illumination intensity through controllable current. Constant current drive may be the most precise and reliable choice in LED lighting control because illumination intensity is directly proportional to current, and current drive is unaffected by power supply variations. Constant current controllers also have significant advantages in high-speed systems and applications that require pulsed light.
High-Speed Lighting Control
An important challenge of lighting control is creating repeatable illumination intensity for each exposure. This is achieved with precise square pulses with stable rising and falling edges. Sharp pulse edges are difficult to achieve with voltage drive, especially when there is a long cable between the lighting controller and the lamp. This can result in variations in brightness over time and different exposures. Constant current lighting controllers can use higher voltage levels to quickly establish accurate lighting without causing EMC interference. This makes constant current drive 10 times faster than voltage drive.
In PWM systems, the accuracy of lighting intensity depends on having a large number of voltage pulses for each exposure. High-speed lighting systems may have exposure times of only a few microseconds, with only a few PWM pulses per exposure. In such high-speed lighting systems, differences in PWM cycle lengths can have a measurable impact on lighting intensity. As shown in Figure 2, the timing of camera exposure and PWM pulses results in the first camera exposure receiving over 30% more light than the second exposure. A faster internal PWM clock can reduce this effect, but these high-speed voltage pulses can cause EMC interference in neighboring systems. More advanced PWM systems may offer the ability to synchronize PWM cycles with camera shutters, but this may be difficult to achieve, especially with rolling shutter camera systems.
Advantages of Strobing
One significant benefit of current-stage lighting is that the light source can produce illumination higher than the maximum brightness specified by the device manufacturer. This is achieved through a technique called strobing, which involves a brief but high current passing through the light source. When diode internal heating is properly managed, strobing does not damage LEDs, so it never exceeds safety levels, which are precisely calculated based on the LED's rated power, current through the light source, and duty cycle. Voltage-based lighting controllers providing 24V cannot create a controllable strobe level, and those providing dual voltage can result in unpredictable brightness discontinuities between the two voltage levels. Only constant current controllers can safely and accurately control strobing.
FALenses Technology specializes in providing machine vision core hardware. You can go to the official website of FALenses Technology at https://www.falenses.com/ for more information.