What is the Benefit of Dual Sided Lighting vs. Single Sided Lighting
The real benefit of using a dual-sided lighting system is the extremely large uniform inspection volume. The “useful inspection volume ” is defined as the volume in which the illumination varies by less than 10% of the target value. With a single-sided illumination system the useful inspection volume can be very small. In fact, the height of the inspection volume can be less than the length of the container! Since the Probability of Rejection is directly proportional to the intensity of the light in the inspection volume it is easy see why the inconsistency of Lot Inspection exist when using a single-sided inspection system.
The illustration below shows the light intensity as a function of the distance from a source of two fluorescent lamps positioned above a Black / White background. The wattage and size of the lamps varies between 40 watt (T12) and 15 watt (T5) but the configuration is usually the same with the position somewhere above the Black / White background. This is typical of an older design manual inspection booth for a single-side (top only) lighting system implemented in the industry. This represents a “waterfall” illumination system in which the light intensity decreases dramatically as the inspection site is moved away from the source. The primary problem with this type of lighting system is not that the light intensity decreases but the ability of the user to increase the light intensity on demand. The normal response during human inspection is to “increase” sensitivity when a suspect condition exist. The human inspector will often more the container closer to the illumination source to increase the Probability of Rejection, and thus change the inspection parameters.
This change in the light intensity during the inspection process is the single largest source of error. In order to produce consistent inspection results, the light intensity must remain as constant as possible throughout the inspection volume. The inspection volume is considered the volume in which the container will experience a light intensity variation of not more than ±10%. Under ideal conditions the light intensity within the inspection volume should be held to ±5%. The Dual-Sided Lighting System is designed to keep the total light intensity near a constant. As one moves away from one light source the light intensity will decrease but at the same time the light intensity from the second light source will increase, thus A(side) + B(side) = Constant. Dual Sided Lighting can be implemented in a Top / Bottom format or a Left / Right Side format. The selection of the format is determined by several important inspection criteria. Small containers usually have a small radius of curvature and a small light obscuration zone (light source reflection on exterior sidewalls of the container). As the radius increases the size of the light reflection zone will increase and reduce the volume of fluid that can be observed during the inspection process.
Why this is important. When using a single sided lighting system the light intensity falls off rapidily (see diagram above). Depending on the length of the container being inspected the light intensity may change more than 10% from one end tothe other. This is especially important when inspecting large volume containers, i.e., greater than 20 ml. The Probability of Rejection is directly proportional to the the light intensity used for inspection. The Probability of Rejection will change from the top to bottom of large containers.
Want more consistent inspection results? Then you need a more consistent inspection environment. The latest generation of the Phoenix Imaging Dual Sided Lighting Systems are based on several new precision lighting controllers designed specifically for the latest generation LED lighting modules. The new designs incorporate photo-diode feedback which is temperature insensitive and LED lighting modules used have matched LED for consistent illumination across the light plates.
The illustration above is the MIB-200 Light Intensity Map which represents the light intensity as a function of position within the inspection volume. The first thing that you will notice is that total variation in light intensity is minimized. The inspection volume has a subtle “saddle shape” with a very flat (consistent) light intensity region in the center. The light intensity in this region has a total variation of less than 10%.
The real benefit of using a dual-sided lighting system is the extremely large uniform inspection volume. The “useful inspection volume ” is defined as the volume in which the illumination varies by less than 10% of the target value. With a single-sided illumination system the useful inspection volume can be very small. In fact, the effective inspection volume can be less than the overall length of the container! Since the Probability of Rejection is directly proportional to the intensity of the light in the inspection volume it is easy see why the inconsistency of Lot Inspection exist when using a single-sided inspection system.
The Dual-Sided Lighting Systems offered by Phoenix Imaging provide the most consistent inspection environment available in any manual inspection booth. The light sources used in the MIB product are precision balanced and intensity controlled with active feedback circuitry. The Dual-Sided lighting systems are available with either the sources mounted top/bottom or left/right sides. The choice of the lighting direction is dependent on customer preference and the size of the container that is to be inspected.