Although thermal imagers tend to be judged on the detector technology employed, the detected image must first be formed by an infra-red lens, and the quality and performance of this lens will be an important factor in the overall performance of the imager. For critical applications, it is important to test the lens independently to ensure it meets the required specification before it is fitted to a thermal imaging system.
Testing any kind of lens involves setting up a test object, for example, a narrow slit or a periodic bar pattern, and then analyzing the image formed using a suitable detector. For lenses which operate in the visible region, some assessment can be made by the human eye. In the case of infra-red lenses, however, the image must be analyzed using an infra-red image scanner. This usually takes the form of a small sampling aperture, which is imaged onto a single-element infra-red detector (Figure 1). The aperture is mechanically scanned across the image to be sampled, and then the intensity profile is analyzed by computer.
MTF (Modulation Transfer Function)
In visible lens testing, square-wave targets, which consist of a periodic pattern of light and dark bars, are sometimes used to assess lens performance. Such targets are characterized by a spatial frequency (the number of cycles per millimeter) and a contrast (the ratio of the variation amplitude to the average level of illumination). When a target is imaged by a lens, the image pattern will also be a periodic pattern of light and dark bars, but the contrast will be reduced; some of the light in the bright bars will have “leaked” into the dark areas. If we present a series of targets with a range of line-spacings, we will generally find that the image contrast will reduce as the line-spacing reduces — that is, as spatial-frequency increases. A curve of the contrast reduction factor (image contrast/object contrast) can then be plotted as a function of spatial frequency. This is known as the Contrast Transfer Function, or CTF.
The Modulation Transfer Function is defined in a very similar way, except that a sine-wave target (Figure 2) is used as the test pattern instead of a square-wave target. The target still consists of light and dark bars, but now the intensity varies from dark to light in a sinusoidal manner. A sine-wave MTF can be easily measured by analyzing the image of a pin-hole, line, or edge target. A lens can then be thought of as a low pass filter, imaging the low-frequency components with little reduction in contrast, but progressively attenuating the higher frequency components (see Figure 3).
A Standard Test Bench
A typical infra-red MTF test bench (Figures 4 and 5) consists of these main modules: A target generator. The target is usually a very narrow slit, and is illuminated by a glow-bar or black-body source. The target can be rotated through 90 degrees to allow measurements to be made in two orthogonal directions. A reflecting collimator. The target is placed at the focus of a reflecting parabolic collimator so that rays which diverge from a point on the target approach the lens as a parallel bundle. With this arrangement the lens effectively sees the target as if it were at an infinite distance. A beam-steering mirror. The collimated beam is reflected from a flat beamsteering mirror into the lens under test. When the mirror is at 45 degrees, the lens sees the target in the center of its field-of-view, and the image is formed “on axis.” When the mirror is rotated, the target and the corresponding image move away from the axis, allowing measurements to be made at different positions in the field. A lens mount. The mount holds the lens in the correct position for testing, and may also allow the lens to be rotated, so that the image may be evaluated at any position in the two-dimensional image plane of the lens. An image analyzer. The image analyzer scans the intensity profile of the image formed by the lens — this is the Line Spread Function. The lens under test views the target in the collimator mirror, and the intensity profile of the resulting image is then scanned. The LSF is the intensity variation across the image of the target slit. It is directly measured by moving the single-element detector’s scanning aperture across the image in a series of steps, beginning on one side, where there is minimum energy, and moving toward the center, where energy is at its maximum (see Figure 6). Typically, a range of 100 to 200 sample points are taken. The sine-wave MTF can then be derived from this profile by taking the Fourier Transform of the LSF.