MTF Testing in Practice
Unfortunately, a lens cannot be characterized by a single MTF curve. The image quality will vary with position in the field, and hence, so will the MTF. Image quality tends to be best at the center of the field, and then decreases towards the edges of the field of view. A test bench must therefore allow the test target to be presented at different field positions, and this can be achieved either by placing the lens and image scanner on a swinging arm assembly, or by using a beam steering mirror to reflect the collimated beam into the lens at different angles. The whole measurement procedure can be automated, collecting sets of MTF curves from several different field points, before the overall quality of the lens can be assessed, and a pass/fail decision can be made.
A good lens will have a high MTF curve — that is, it will stay higher for longer. A typical specification might say that the MTF must be at least 70% at 10 c/mm and 40% at 20 c/mm, for example. Consistency over the field of view, however, is very important. There will typically be one specification for the center of the field, another for the mid-field position, and a third for the edge-of-field position. The specification tends to become more lenient as the distance from the center of the field increases.
Other Test Bench Measurements
The bench can measure the position of the image very accurately for a range of different field angles, and this enables the effective focal length and distortion to be accurately determined. The bench measures MTF as a function of focus position at different field positions, too, providing information on the depth-offield and field curvature of the lens. The total energy in the image can also be measured as a function of the field angle and thus provide a measurement of relative illumination from the center to the edge of field.
Using a suitable detector it is possible to measure the “ensquared energy,” or the fraction of the energy from a point object which falls into a square aperture, expressed as a percentage of the total energy in the image. Ensquared energy is an alternative to MTF as a measure of image quality, and is sometimes preferred since it can tell the designer how much energy from a single point will fall onto a pixel of a detector compared with the amount that falls onto neighboring pixels.
Lastly, the bench can be used to measure the displacement of the on-axis image from the true mechanical axis of the lens mount, and thus provide a measure of the Boresight Error. This quantity measures how well the mechanical and optical axes of the imager are aligned, an important measurement when trying to overlay the images from a thermal camera and a visible camera, which are mounted side by side.
Reasons for MTF Testing
Manufactured lenses often do not behave exactly as the designer intended. The curvatures of the surfaces, the centration of the individual elements, and the distances between them, will all be subject to the inevitable variations that arise in manufacture. There may also be problems arising from flaws in the optical materials used. Along with military and space applications, thermal imagers are now being designed for luxury cars, and since their use may be safety-related, it is important to guarantee a certain minimum performance. For critical applications it is therefore essential to check that a lens performs to a minimum set of specifications before it can be used in practice.
This article was written by Kevin Urben, senior decision maker at Image Science (Oxford, England). For more information, visit http://info.hotims.com/34458-151.