Stray light, also referred to as flare, is any light that reaches the focal plane via a non-design optical path (P2020). It causes contrast loss (decrease in dynamic range), false features (seeing ghost images), and changes in color; all of which can degrade the quality of the images captured and even interfere with the ability to detect objects in the scene.
Stray light testing requires capturing images of a small, bright light source in a dark room. The light emanating from the light source should overfill the front of camera device under test, again including any front surface that could affect the optical path, and the camera should be well-aligned with the projected beam of light emanating from the light source. To build field coverage, the camera can be rotated or the light source can be moved in order to change the angle of the light source with respect to the camera. The captured images are later processed into “normalized stray light” metric images (Point Source Rejection Ratio) which are the primary output from Imatest 22.2 Stray Light (Flare) analysis.
In summary, to test for stray light, you need:
A dark (black) room
A small, bright light source
A way to rotate the camera under test (or a way to move the light source around the camera under test)
Imatest offers both hardware and software for testing stray light, including:
Imatest Motorized Gimbal
The Imatest Motorized Gimbal allows for programmatic control over camera roll and yaw (or alternatively, pitch and yaw) which effectively provides control over the field angle and azimuth angleof a light source when properly set up and aligned. The Motorized Gimbal includes 2 axes of automated rotational positioning and one axis of manual rotational positioning, allowing users to create automated test plans with many camera “poses”. The Imatest Motorized Gimbal is available on the Imatest Store.
Broadband Halogen Fiber Optic Illuminator
The Broadband Halogen Fiber Optic Illuminator is a relatively cheap multi-purpose light source that is usable for basic stray light testing. The halogen bulb covers both the visible and near-infrared spectrum, and the fiber is easy to maneuver for simple and flexible lab setup.
Imatest Configurable Stray Light Source (in development)
Imatest is developing a configurable light source designed for stray light testing. The design includes a 4” integrating sphere with a radiance monitor detector for highly uniform and controlled output. Attached to the integrating sphere are three ports for swappable LED engines. The integrating sphere acts as a diffusor that effectively blends the light from the separate LED engines into one uniform light source. This hardware component is still in development and is estimated to be ready in Q1 2023.
Imatest Motorized Test Stand (in development)
Imatest is developing a Motorized Test Stand that can be used for a variety of automated image quality testing purposes, including stray light testing. The Motorized Test Stand provides three axes of automated translational movement for sub-millimeter precision. When paired with a Motorized Gimbal and light source, the Motorized Test Stand can significantly aid with set up,camera-to-source alignment, and automated testing. This hardware component is still in development and is expected to be ready in 2023.
Ideally, stray light testing should be done in a large, dark room to eliminate any potential for reflection. However, a blackout curtain may be used if a large, dark room is not available. The blackout curtains should not immediately surround the setup, but rather, there should be some space between the curtains and the setup (camera and light source) to further prevent unwanted reflections. The larger the enclosure, the better. Additional baffles may be used throughout the setup to further reduce extraneous reflections and
Imatest Version 22.2
Imatest version 22.2 includes the stray light (flare) module for analyzing stray light images. Import images into Imatest to run the analysis, where Imatest will process the images into normalized stray light metric images using the point source rejection ratio (PSRR) method. Various output file types are produced, and the stray light module is available in both Master and IT.