Detecting dust on sensors using collimated light vs. diffuse light

Flatfield analysis in Imatest can be used to measure nonuniformities in flat images, including:

Dust particles
Hot or dead pixels
Illumination falloff (e.g., lens vignetting)

When attempting to analyze dust particles on a bare sensor (without a lens), the geometry of the lighting is critical, as it influences how light interacts with the sensor surface and can affect the visibility of smaller particles. Choosing between collimated light (for example, from the Imatest Stray Light LED Source) or diffuse light (for example, from an Imatest LED Lightbox) can significantly impact results.

Diffuse light

Diffuse light may not reveal smaller particles on a bare sensor due to the scattered nature of the illumination. However, it is still effective in identifying hot pixels and other general nonuniformities.

A diagram illustrating how diffuse light scatters in multiple directions, allowing it to illuminate the area behind a dust particle if there is a gap between the particle and the sensor pixels. The scattering reduces the visibility of the dust, making smaller particles harder to detect.

A camera (without a lens) imaging the diffuse lighting from an Imatest LED Lightbox. This setup was used to capture the image shown below.

An image of a diffuse light source from a monochrome camera sensor without any lens.

Collimated light

Collimated light is particularly useful for detecting dust particles on a bare sensor. The directional illumination causes the dust to cast visible shadows, making even smaller particles detectable.

A diagram illustrating how collimated light travels in parallel rays, causing a dust particle to cast a well-defined shadow on the sensor pixels beneath it. The directional illumination enhances the visibility of dust, making smaller particles easier to detect.

The Imatest Stray Light LED Source is used to illuminate a camera (without a lens) with a uniform collimated beam of light. This setup was used to capture the image shown below.

An image of a collimated light source from a monochrome camera sensor without any lens.

The image captured with the collimated light source reveals the effects of dust on the sensor, as the particles produce visible diffraction patterns and, for larger particles, distinct shadows. The minimum detectable dust size will depend on the distance between the dust and the sensor (i.e., the thickness of the protective glass layer) as well other aspects of the sensor's design and sensitivity.

A magnified crop of the image of the collimated light source above, showing diffraction patterns and shadows caused by light interacting with dust particles on the sensor's protective layer.

Imatest Flatfield blemish analysis

Flatfield analysis in Imatest can be used to detect the artifacts in the image of the collimated light source. The blemish detection settings provide control over certain filtering and thresholding, which can be tuned to identify and isolate the dust-induced artifacts. The number of detected blemishes, as well as their location and size in the image, are included in Imatest’s standard output files (CSV, JSON).

A display from Imatest Flatfield analysis highlighting blemishes caused by dust in the image of the collimated light source.