Light source

The role of the light source

In the field of industrial vision, the primary functions of using light sources are as follows:

1. Distinguish the detected region from the background area.

2. Enhance the clarity of the edges of the target to be detected.

3. Eliminate shadows

4. Cancel out noise

Light Source Types and Characteristics

Light source types can be categorized by type: ring light, strip light, surface light (with aperture, parallel, side, side-parallel), coaxial (high-brightness, parallel), shadowless (flat shadowless, ring shadowless, dome shadowless, dome light, square shadowless), line light (high-brightness, light-concentrating type, coaxial, high-brightness coaxial), and point light.

Light Source Selection

1.Based on shape differences, it can be divided into:

2.According to roughness, surfaces can be classified as smooth surfaces and rough surfaces.

BrightfieldThis refers to the process in which a light source directly illuminates the surface of an object, and the reflected light enters the camera to form an image. In this illumination method, light strikes the object at a high angle (from 45° to 90°), allowing a large amount of reflected light to enter the lens and thus producing a clear image.

Features and Advantages:

High contrast: Bright-field illumination helps create high-contrast images, making the outlines and details of objects more distinct.

Full-field illumination: Full-field illumination can illuminate an object from multiple directions, producing diffused and soft reflected light that is free of directional shadows, making it ideal for observing the overall surface texture of an object.

Flexibility: Partial brightfield illumination involves illuminating the object from a specific angle, creating contrast between the damaged area and the background, making it suitable for detecting minute defects on the object’s surface.

Application scenarios:

Observe the overall shape and dimensions of the object.

Detects tiny protrusions and damages on object surfaces.

Suitable for non-mirrored objects or objects with relatively rough surfaces.

Dark fieldIt is a lighting method that illuminates an object from a low angle (less than 45°). In this approach, the reflected light from smooth surfaces scatters in all directions and cannot directly enter the lens; however, in recessed or raised areas, the reflected light strikes at a larger angle, allowing more light to enter the lens and producing a brighter image.

Features and Advantages:

Edge enhancement: Dark-field illumination can highlight the edges and contours of objects, making details stand out more prominently.

Defect Detection: For surfaces with textures or significant height variations, dark-field illumination can clearly reveal defects and damage.

High contrast: Although the overall image is relatively dark, the high contrast between the defect and the background makes the defect easier to identify.

Application scenarios:

Observe microstructures such as crystal defects, dislocations, and twins.

Detects tiny scratches and dents on object surfaces.

Suitable for mirror-like objects or objects with smooth surfaces.

Summary: In bright-field illumination, the flat surfaces of an object appear brighter, while uneven surfaces appear darker; in dark-field illumination, the opposite is true. Diagram showing the position of the light source:

Calculate the size of the light source

Parameter description: WD represents the working distance of the lens, wd represents the working distance of the light source, FOV represents the field of view size, and θ represents the field of view angle.

Meaning of L: Bright-field illumination and backlight: L refers to the outer diameter (length and width) of the illuminated area, with the inner diameter limited by the field-of-view angle range. Dark-field illumination: L refers to the inner diameter (length and width) of the illuminated area, with the outer diameter constrained by the available installation space of the device. Shadow-free illumination: The inner diameter is larger than the field-of-view angle range; the larger the outer diameter, the better (subject to constraints imposed by the device’s installation space).

Lighting Techniques

In order of decreasing wavelength—from red, orange, yellow, green, cyan, blue, to violet—colors that are similar in hue should be lit with white light, while colors that are significantly different should be lit with black light. The shorter the wavelength, the weaker the penetrating power and the higher the diffusion rate; conversely, for more details, please refer to the “Visual” lighting tips.

Camera

Camera Type

Line arrayEncoder + Trigger, line-by-line scanning, and stepwise stitching to form a complete image (typically requiring motion of either the object or the camera). It offers high precision and a wide field of view. The frame rate is calculated as: Frame Rate = Line Frequency (F) / Line Height (H), where the Line Frequency (F) is given by: (Horizontal Resolution (W) × Object’s Motion Speed (V)) / Horizontal Field of View (L). If the scene falls into any of the following conditions, the line frequency needs to be adjusted.

Image stretching (in the direction of motion)—by reducing the line frequency: for example, if you keep the horizontal resolution and field of view constant while slightly decreasing the object’s motion speed, you can effectively prevent image stretching. Image compression (in the left-right direction)—by increasing the line frequency: keeping other factors unchanged and appropriately increasing the object’s motion speed is ideal for highly dynamic motion scenarios.

Surface arrayExposure + multi-line: Suitable for scenes with low speed requirements or static scenes (capturing the entire image in one shot, just like a conventional camera). Schematic diagrams of area-array cameras and line-scan cameras.

Camera parameters

• Global shutter: Achieved by exposing the entire scene at the same time. Under a strobe light source, if the light sources don't match, the brightness of consecutive frames will be inconsistent.

• Roller shutter: This type of shutter operates by exposing the sensor line by line. When shooting fast-moving objects, motion blur may occur. Under a strobe light source, mismatched lighting can result in alternating bright and dark stripes in the image.

Calculate the short and long sides of the sensor’s diagonal based on resolution and pixel size: Sensor long side = (Sensor long-side pixels × Pixel size) / 1000 (mm); Sensor short side = (Sensor short-side pixels × Pixel size) / 1000 (mm). From these long and short sides, you can determine the target area dimensions. Conversely, you can also deduce the long and short side values. For example: If the resolution is 1280 × 1024 pixels and the pixel size is 4 microns, then the long side is 1280 × 4 / 1000 = 5.120 mm, and the short side is 1024 × 4 / 1000 = 4.096 mm. The diagonal length (calculated using the Pythagorean theorem) is approximately 6.556 mm. Therefore, its theoretical value is 6.556 / 16 ≈ 0.410, or 1/2.44 inches; the actual value is 1/2.7 inches (industry standardization).

Camera Selection

1. Calculate the field of view based on the target's dimensions:

Field of View (FOV) = (1.2~1.5) × Target Size

2. Calculate camera resolution based on field of view and precision:

Camera resolution R = Field of View (FOV) / Detection accuracy × Number of pixels IxN

The actual camera field of view only needs to be larger than the values mentioned above.

For example: Detection accuracy of 0.05 mm; the target dimensions are 22 mm in length and width; the actual calculated accuracy is 0.02 mm; the field of view must be at least 4.5+22+45=30–32 mm/Using 1.24 × 22, we get approximately 30 (FOV). Taking a step size of 0.02, the camera resolution (R) can be calculated as follows: R = 30 / 0.02 = 1500. Therefore, the camera’s horizontal resolution must be at least greater than 1500. After determining the resolution, select the camera based on whether it’s black-and-white or color. The minimum camera resolution is: (30 / 0.02) × (30 / 0.02) = 225 W. Thus, you can choose a camera with a resolution of at least this value—for example, a 500 W camera. Once the camera is selected, proceed to choose the lens, ensuring that the lens’s field of view covers the entire area of interest and matches the camera’s sensor size. Additionally, the light source’s area must be larger than the actual detection field of view of the object.

Lens

Lens features

In the industrial field, the lenses commonly used are typically standard FA lenses and telecentric lenses. Compared to standard lenses, telecentric lenses have the following distinctive features: high resolution, ultra-wide depth of field, extremely low distortion, and parallel light beams. The differences are as follows:

Lens parameters

Core formula:

1s + 1 / s0 = 1 / fHere, s represents the object distance, s0 represents the image distance, and f is the focal length.

The purpose of adding a circle:

Advantages: Increasing the image distance while reducing the object distance, thereby shortening the working distance and narrowing the field of view, achieving...Zoom in on the imageThe purpose of

Disadvantages: Reduces lens light intensity, decreases depth of field.

δ represents depth of field, and s represents object distance.The function of adding a polarizerEliminate shadows and glare.