Basics of Lighting Selection

Logical Steps for Lighting Selection

Image processing is the process of detecting changes in pixel density data through calculation. As such, stable detection requires projection of a clear image. The lighting selection plays an important role in determining the performance of image processing–based inspection. This section introduces the basic knowledge required for selecting the correct lighting.

Three steps for selecting Lighting

  1. Determine the type of lighting (specular reflection/diffuse reflection/transmitted light).
    Confirm the characteristics of the inspection (flaw, shape, presence/absence, etc.).
    Check if the surface is flat, curved, or uneven.
  2. Determine the shape and size of the necessary light.
    Check the dimensions of the target and installation conditions.
    Examples: ring, low-angle, coaxial, dome.
  3. Determine the color (wavelength) of lighting
    Check the material and color of the target and background.
    Examples: red, white, blue.

Shapes of typical Lights (LED Lighting)

  • Coaxial vertical

  • Low-angle

  • Direct ring

  • Backlight

  • Dome

  • Bar

Lighting selection: Step1 (Specular reflection, diffuse reflection, transmitted light)

LED Lights can be roughly divided into the following three types:

  • Specular reflection type:
    Light is applied to the target and the lens receives the direct reflection.
  • Diffuse reflection type:
    Light is applied to the target and the lens receives uniform ambient light.
  • Transmitted light type:
    Light is applied from behind the target and the lens receives the transmitted silhouette.

(1)Sample image of specular reflection

Inspecting for the presence or absence of inscriptions on metal surfaces

  1. The inscription is unclear.
  2. The inscription is clear.

It is necessary to bring out the contrast between the flat metal surface and depressions of the inscription

Since a metal surface reflects light easily and the inscription does not, the optimum method is to use specular reflection to enhance the difference between the surface and inscription.

(2)Sample image of diffuse reflection

Inspecting the print on a chip through transparent film

  1. The Lighting reflects on the film surface.
  2. The image is not affected by the film.

It is necessary to bring out the contrast between the surface of the chip and the print by eliminating the reflection from the transparent film (halation).

The optimum method is to use diffuse reflection to prevent specular reflection on the transparent tape.

(3)Sample image of transmitted light

Inspecting foreign matter on nonwoven fabric

  1. The Lighting reflects on the film surface.
  2. The silhouette of the foreign matter is clearly recognized.

It is necessary to bring out the contrast between the target surface and the foreign matter, which is difficult to recognize because of the subtle difference in color.

Even when no difference can be detected with reflected light, applying transmitted light from behind the target will show foreign matter as a black silhouette.


The first step of light selection is to select the lighting method, specular reflection, diffuse reflection, or backlighting, according to the shape of the target and the inspection purpose. Then, select the size and color of light that allows you to capture an optimum image for processing.

Lighting selection: Step2 (Lighting method and shape)

(1)Sample image of specular lighting
Detecting chips in the edge of a glass plate

Simple reflected light

Reflected light is reflected randomly on the glass surface. 

【Selecting lighting according to the work piece characteristics and detection details】
  • The light reflects on the glass surface.
  • It is necessary to enhance the difference between the glass plate and background.
  • It is best to apply lighting vertically to the work piece.
  • A space can be provided above the work piece.

The best selection is coaxial-vertical lighting.

coaxial-vertical illumination
The entire glass surface can be illuminated uniformly.

(2)Detection example of diffuse reflection
Inspecting chips in rubber packing

Simple reflected light

The chips on the outer-circumference cannot be recognized.

【Selecting lighting according to the work piece characteristics and detection details】
  • The work piece is black rubber which does not reflect specular light.
  • The chipping is also black and will not reflect specular light.
  • Illuminating the work piece from an angle to reflect the specular light from the chipped area proves effective.
  • A light can be installed close to the target.

The best selection is low-angle light.

(3)Detection example of transmitted light
Inspecting lead shapes

Simple reflected light

The edges show little contrast.

【Selecting lighting according to the work piece characteristics and detection details】
  • The target is a metal object with projections and depressions, resulting in irregular specular reflection.
  • By using a transmitted light, the edge of the target can be detected without the influence of the projections and depressions.
  • A light can be installed behind the work piece.

The best selection is a backlight.

With a backlight
The complicated outline can be recognized clearly.


After you choose the lighting method, select the type of light based on the detection purpose, background, and surrounding environment.
Basic selections are: coaxial illumination, ring illumination, or bar lights for specular reflection; low angle lights, ring illumination, or bar lights for diffuse reflection; and area illumination or bar lights for backlighting. Ring illumination and bar lights are generally used in particular because they can be used for various purposes by adjusting the installation distance.

Lighting selection: Step 3 (Color and wavelength of lighting)

The last step is to determine the color of illumination according to the target and background. When a color camera is used, the normal selection is white. When a monochrome camera is used, the following knowledge is required.

Detection using complementary colors

A red candy wrapper is in a cardboard box. The following is a comparison of the contrast when LED illumination is used to detect the presence or absence of the candy.

Work piece
  • With a white LED
    The brightness is uniform for the entire image and there is almost no contrast between the work piece and background.
  • With a red LED
    The red work piece is shown brighter, but the contrast is still insufficient.
  • With a blue LED
    The red target appears black, allowing for stable detection. A blue LED is optimum
Color wheel : Green / Blue / Purple / Red / Orange / Yellow

What is a complementary color?
A complementary color is the opposite color in the hue circle. When a light of the complementary color is applied to an object, the object will appear nearly black.

Detection using wavelength

The following is an image comparison of print on a chip in carrier tape taken through a transparent film. The contrast is higher with red illumination than with blue illumination, because of its higher transmittance (lower scattering rate).

[ Invisible light: Ultraviolet light][ Visible light : Purple 380 / Blue 430 / Green-blue 480 / Blue-green 490 / Green 500 / Yellow-green 560 / Yellow 580 / Orange 595 / Red 650 ][ Invisible light : Infrared light 780 ](Unit: nm)

Lights of different wavelength appear as different colors. The wavelength determines the characteristics of a particular color such as being transmitted easily (red light - long wavelength) or being scattered easily (blue light -short wavelength).

Color camera image White illumination / Color camera image Red illumination / Color camera image Blue illumination / Gray camera image The contrast between the print and chip appears clearly through the film.[Red is the best] / Gray camera image Blue illumination

Summary of logical steps for lighting selection

Light selection determines the conditions of captured images, which is most important for image processing. Instead of trying every light without consideration, you can select the right one efficiently by following the procedure below:

  • Decide which of specular reflection, diffuse reflection, or backlighting is most appropriate.
  • Decide on the type and size of the light.
  • Decide on the color of the light.

The next volume will cover“ the effects of color cameras and image enhancement”. It is an important factor for processing the image to the optimum application such as appearance inspection and position detection. The next guide will outline points for selecting the optimum color extraction and image enhancement.



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