Fundamental knowledge of illumination and light measurement is key when specifying LED lighting for industrial automation
When choosing an LED light, designers of machine vision systems must fully understand the nature of the part that needs to be illuminated. To allow the system’s camera to capture an image with the highest contrast, developers can choose from a number of different lighting products. These range from line lights, ring-lights, spotlights and backlights – all of which may be used in on- or off-axis configurations and/or multiple wavelengths ranging from UV, visible to IR/wavelengths.
One of the most important considerations in choosing any type of lighting, however, is the amount of light required for any given application. Backlighting a part, for example, to perform dimensional measurements, may not require an extremely bright backlight. Alternatively, for high-speed line-scan applications where parts are moving at high-speed and camera exposure times are fast, an extremely bright light may be required.
For system integrators tasked with comparing lights from different manufacturers, discerning the amount of light emitted from LED lights that may at first seem comparable may be a difficult task since light output can be specified in a number of different ways.
When a part is illuminated by an LED light, luminance provides a measure of the amount of light reflected from a surface and indicates the brightness of light emitted or reflected from a surface. This can be measured in candelas/square meter (cds/m2) or foot-lamberts (fLs).
Illuminance, on the other hand, describes the measurement of the amount of light illuminating the surface area and is measured in lux or foot-candles and correlates with how humans perceive the brightness of illuminated areas.
While photometric measurements such as luminance and illuminance provide a measurement of light in terms of its perceived brightness to the human eye, radiometric measurements provides information amount the amount of light power (or energy) at all wavelengths. Photometric measurements are often used to determine the power from UV or IR lights and are not commonly used in machine vision applications. Such photometric measurements include irradiance and radiance.
While, irradiance provides a measure of the radiant power received by a surface per unit area and is measured in Watts per square meter (W/m2), radiance is the radiant power emitted by a surface, per unit solid angle per unit projected area which is measured in Watts/steradian/m2.
For the machine vision system designer working in the visible spectrum, the most useful of these measurements is illuminance. Illuminance meters can be used to perform this measurement with lights used in constant operation and those that are strobed.
Measuring the illuminance of a light in constant operation is relatively easy. However, strobed light illuminance can also be calculated using a light meter. If, for example, the if the light is strobed for 10ms and the LED turned off for 100ms before the next strobe is activated, then the actual intensity is approximately 1/10 of what it would be if the light was on constantly.
Inverse square law
Often, a systems integrator will choose a light – for example, a spotlight – and place it a specific distance from the part to be illuminated. If more light is required, one of the most useful rules of thumb in determining how this can be achieved is the inverse square law (Figure 1). Since the intensity of the light decreases as the inverse square of the distance, the amount of light drops as 1/(distance from the part)2. Thus, a light placed 2ft from a part will have ¼ of the visible light placed 1ft away. Obviously then, placing a light closer to the object to be illuminated an increase the amount of light considerably.
Placing a light closer to a part can increase the illuminance level but in cases where this cannot be accomplished, developers should also consider how the amount light used to illuminate the object can be maximized. In the case of a spotlight used to illuminate an object, for example, properly focusing the light over a given field of view and distance can increase the amount of illumination. For example, a 100mm diameter spotlight at a distance of 1m requires a 5.8o lens on the LED to maximize the illumination level at that distance (Figure 2).
To date, it is difficult to compare lighting products. Because of this, the AIA (www.a3automate.org), the EMVA (www.emva.org) and JIIA (www.jiia.org) are developing a standard to allow machine vision systems developers to compare different lights from different manufacturers from a practical rather than a theoretical standpoint. It is hoped that this standard approach will allow effective lighting performance comparisons across manufacturers and within manufacturers’ product lines based primarily on factors as the light intensity at a specified working distance, light pattern uniformity, size/shape (FOV) and the projected light beam spread.